The Endocrine System Design Hormones Glands BIO 37 by Owen Borville March 29, 2025
Many types of hormones are found throughout the body. Homeostasis is made possible by the release of chemicals in the body called hormones that aid in communication between neighboring cells and between cells and tissues in distant parts of the body. The human body produces over 50 known hormones.
Hormones are released into body fluids (such as blood) and the hormones are carried to their target cells. At the target cells, which are cells that have a receptor for a signal or ligand from a signal cell, the hormones create a response. The cells, tissues, and organs that secrete hormones make up the endocrine system.
Adrenal glands in the endocrine system produce hormones that regulate responses to stress and the thyroid gland produces hormones that regulate metabolic rates.
Hormones can be divided into three classes based on their chemical structure: lipid-derived, amino-acid derived, and peptide (and protein) hormones.
Lipid-derived hormones can diffuse across plasma-membranes while amino-acid derived and peptide hormones cannot. Lipid-derived or lipid-soluble hormones are mostly derived from and structurally similar to cholesterol. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols. Their chemical names end in -ol for alcohols and -one for ketones. Examples of steroid hormones include estrogens (such as estradiol) and androgens (such as testosterone).
Gonadal hormones, produced by the gonads, include both steroid and peptide hormones. Androgens and estrogens resemble one another in chemical structure and originate form the same molecule. Estrogens are important for sexual development in an ovarian reproductive system, while androgens drive development in a testicular reproductive system. The ovaries produce steroid hormones such as estradiol and progesterone. When androgens are produced, some of them are later converted to estrogens. Small amounts of estrogen occur through aromatase actions in adipose, brain, skin, and bone, which convert testosterone to estrogen. The testes and the adrenal cortex both secrete testosterone.
Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water and are transported by proteins in blood and therefore remain in circulation longer than peptide hormones.
Amino-acid derived hormones are relatively small molecules that are derived from the amino acids tyrosine and tryptophan. If a hormone is acid-derived, its chemical name will end in -ine. Epinephrine and norepinephrine are amino acid-derived hormones produced in the medulla of the adrenal glands, and thyroxine, produced in the thyroid gland. The pineal gland in the brain makes and secretes melatonin which regulates sleep cycles.
Peptide hormones have a structure of a polypeptide chain (of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, like growth hormones produced by the pituitary, and large glycoproteins such as follicle-stimulating hormonesproduced by the pituitary.
Secreted peptides like insulin are stored within vesicles in the cells that synthesize them. They are then released in response to stimuli such as high blood glucose levels in the case of insulin. Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells, therefore their receptors are found on the surface of the target cells.
An endocrinologist is a medical doctor that specializes in treating disorders of the endocrine glands, hormone systems, and glucose and lipid metabolic pathways. An endocrine surgeon specializes in the surgical treatment of endocrine diseases and glands.
How Do Hormones Work? Hormones mediate changes in target cells by binding to specific hormone receptors. Therefore, even though hormones circulate throughout the body and come in contact with many different types of cells, hormones only affect cells that possess the necessary receptors. Receptors for a specific hormone may be found on many different cells or may be limited to a small number of specialized cells. Cells can have many receptors for the same hormone but often also possess receptors for different types of hormones. The number of receptors that respond to a hormone determines the cell's sensitivity to that hormone, and the resulting cellular response. Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone and allowing for more cellular activity. When the number of receptors decreases in response to rising hormone levels, it is called down-regulation and cellular activity is reduced.
Receptor binding alters cellular activity and results in an increase or decrease in normal body processes. Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular hormone receptors and modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways.
Lipid-derived (soluble) hormones such as steroid hormones diffuse across membranes of the endocrine cell. Once outside the cell, they bind to transport proteins that keep them soluble in the bloodstream. At the target cell, the hormones are released from the carrier protein and diffuse across the lipid bilayer of the plasma membrane of cells. The steroid hormones pass through the plasma membrane of the target cell and adhere to intracellular receptors residing in the cytoplasm or in the nucleus. The cell signaling pathways induced by the steroid hormones regulate specific genes on the cell's DNA. The hormones and receptor complex act as transcription regulators by increasing or decreasing the synthesis of mRNA molecules of specific genes. This, in turn, determines the amount of corresponding protein that is synthesized by altering gene expression. This protein can be used either to change the structure of the cell or to produce enzymes that catalyze chemical reactions. In this way, the steroid hormone regulates specific cell processes.
Other lipid-soluble hormones that are not steroid hormones, such as vitamin D and thyroxine, have receptors located in the nucleus. While thyroxine is mostly hydrophobic, its passage across the membrane is dependent on transporter protein. Vitamin D diffuses across both the plasma membrane and the nuclear envelope. Once in the cell, both hormones bind to receptors in the nucleus. The hormone-receptor complex stimulates transcription of specific genes.
Amino acid-derived hormones (except thyroxene) and polypeptide hormones are not lipid-derived (lipid-soluble) and therefore cannot diffuse through the plasma membrane of cells. Lipid insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors. Unlike steroid hormones, lipid insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA. Binding of these hormones to a cell surface receptor results in activation of a signaling pathway. This triggers intracellular activity and carries out the specific affects associated with the hormone. In this way, nothing passes through the cell membrane. The hormone that binds at the surface remains at the surface of the cell while the intracellular product remains inside the cell. The hormone that initiates the signaling pathway is called a first messenger, which activates a second messenger in the cytoplasm.
One important second messenger is cyclic AMP (cAMP). When a hormone binds to its membrane receptor, a G-protein that is associated with the receptor is activated. G-proteins are proteins separate from receptors that are found in the cell membrane. When a hormone is not bound to the receptor, the G-protein is inactive and is bound to guanosine diphosphate, or GDP. When a hormone binds to the receptor, the G-protein is activated by binding guanosine triphosphate, or GTP, in place of GDP. After binding, GTP is hydrolysed by the G-protein into GDP and becomes inactive.
The activated G-protein in turn activates a membrane-bound enzyme called adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP, in turn, activates a group of proteins called protein kinases, which transfer a phosphate group from ATP to a substrate molecule in a process called phosphorylation. The phosphorylation of a substrate molecule changes its structural orientation, thereby activating it. These activated molecules can then mediate changes in cellular processes.
The effect of a hormone is amplified as the signaling pathway progresses. The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase. Each molecule of adenylyl cyclase then triggers the formation of many molecules of cAMP. Further amplification occurs as protein kinases, once activated by cAMP, can catalyze many reactions. In this way, a small amount of hormone can trigger the formation of a large amount of cellular product. To stop hormone activity, cAMP is deactivated by the cytoplasmic enzyme phosphodiesterase, or PDE. PDE is always present in the cell and breaks down cAMP to control hormone activity, preventing overproduction of cellular products.
The specific response of a cell to a lipid insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm. Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release.
Hormones and Regulation of Body Processes: Excretory System, Reproductive System, Metabolism, and Diseases
Hormones have a wide range of effects and modulate many different body processes, including the excretory system, the reproductive system, metabolism, blood calcium concentrations, growth, and stress responses.
Water balance in the body prevents dehydration or overhydration. Water concentration in the body is regulated by osmoreceptors in the hypothalamus, which are sensory cells that detect the concentration of electrolytes in the extracellular fluid. The concentration of electrolytes in the blood rises when there is water loss caused by excessive perspiration, inadequate water intake, or low blood volume due to blood loss. An increase in blood electrolyte levels results in a neuronal signal being sent from the osmoreceptors in the hypothalamic nuclei. The pituitary gland has two components: anterior and posterior. The anterior pituitary is composed of glandular cells that secrete protein hormones. The posterior pituitary is an extension of the hypothalamus. It is composed of largely neurons that are continuous with the hypothalamus.
The hypothalamus produces a polypeptide known as antidiuretic hormone (ADH), which is transported to and released from the posterior pituitary gland. The principle action of ADH is to regulate the amount of water excreted by the kidneys. ADH (also known as vasopressin) causes direct water reabsorption from the kidney tubules, and salts and wastes are concentrated in what will eventually be excreted as urine. The hypothalamus controls the mechanisms of ADH secretion, either by regulating blood volume or the concentration of water in the blood. Dehydration or psychological stress can can cause an increase in osmolarity above 300 mOsm/L, which in turn, raises ADH secretion and causes water to be retained, causing an increase in blood pressure. ADH travels in the bloodstream to the kidneys. Once at the kidneys, ADH changes the kidneys to become more permeable to water by temporarily inserting water channels, aquaporins, into the kidney tubules. Water moves out of the kidney tubules through the aquaporins, reducing urine volume. The water is reabsorbed into the capillaries lowering blood osmolarity back toward normal. As blood osmolarity decreases, a negative feedback mechanism reduces osmoreceptor activity in the hypothalamus, and ADH secretion is reduced. ADH release can be reduced by certain substances, including alcohol, which can cause increased urine production and dehydration.
Chronic underproduction of ADH or a mutation in the ADH receptor results in diabetes insipidus. If the posterior pituitary does not release enough ADH, water cannot be retained by the kidneys and is lost as urine. This causes increased thirst, but water taken in is lost again and must be continually consumed. If the condition is not severe, dehydration may not occur, but severe cases can lead to electrolyte imbalances due to dehydration.
Another hormone responsible for maintaining electrolyte concentrations in extracellular fluids is aldosterone, a steroid hormone that is produced by the adrenal cortex. In contrast to ADH, which promotes the reabsorption of water to maintain proper water balance, aldosterone maintains proper water balance by enhancing Na+ reabsorption and K+ secretion from extracellular fluid of the cells in kidney tubules. Because it is produced in the cortex of the adrenal gland and affects the concentrations of minerals Na+ and K+, aldosterone is referred to as mineralocorticoid, a corticosteroid that affects ion and water balance. Aldosterone release is stimulated by a decrease in blood sodium levels, blood volume, or blood pressure, or an increase in blood potassium levels. It also prevents the loss of Na+ from sweat, saliva, and gastric juice. The reabsorption of Na+ also results in the osmotic reabsorption of water, which alters blood volume and blood pressure.
Aldosterone production can be stimulated by low blood pressure, which triggers a sequence of chemical release. When blood pressure drops, the renin-angiotensin-aldosterone system (RAAS) is activated. Cells in the juxtaglomerular apparatus, which regulates the functions of the nephrons of the kidney, detect this and release renin. Renin, an enzyme, circulates in the blood and reacts with a plasma protein produced by the liver called angiotensinogen. When angiotensinogen is cleaved by renin, it produces angiotensin I, which is then converted into angiotensin II in the lungs. Angiotensin II functions as a hormone and then causes the release of the hormone aldosterone by the adrenal cortex, resulting in increased Na+ reabsorption, water retention, and an increase in blood pressure. Angiotensin II in addition to being a potent vasoconstrictor also causes an increase in ADH and increased thirst, both of which help to raise blood pressure.
Hormonal regulation of the reproductive system is a process that requires the action of hormones from the pituitary gland, the adrenal cortex, and the gonads. During puberty in both males and females, the hypothalamus produces gonadotropin-releasing hormone (GnRH), which stimulates the production and release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. These hormones regulate the gonads (testes in males and ovaries in females) and are therefore called gonadotropins. In both males and females, FSH stimulates gamete production and LH stimulates production of hormones by the gonads. An increase in gonad hormone levels inhibits GnRH production through a negative feedback loop.
In the testes, FSH stimulates the maturation of sperm cells. FSH production is inhibited by the hormone inhibin, which is released by the testes. LH stimulates the production of the sex hormones (androgens) by the interstitial cells of the testes and therefore is also called interstitial cell-stimulating hormone.
The most commonly known androgen in males is testosterone, which promotes the production of sperm and growth and development of the testes and reproductive organs, skeletal and muscle growth, larynx growth, body hair growth, and sexual drive. Testosterone secretion is regulated by both the hypothalamus and the anterior pituitary gland. The hypothalamus sends releasing hormones that stimulate the release of gonadotropins from the anterior pituitary gland.
The ovarian reproductive system regulation occurs in the ovaries, as FSH stimulates development of egg cells, called ova, which develop in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production. LH also plays a role in the development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the ovaries (as well as testosterone production by the testes). Estradiol and progesterone are steroid hormones that serve several functions in the human body. Estradiol causes the egg to mature and release during the menstrual cycle, and thickens the uterine lining prior to egg implantation. Estradiol also helps with bone health, nitric oxide production, and brain function. During puberty, estradiol produces increased development of breast tissue, redistribution of fat toward hips, legs, breast, and the maturation of the uterus and vagina. Both estradiol and progesterone regulate the menstrual cycle.
In addition to producing FSH and LH, the anterior portion of the pituitary gland also produces the hormone prolactin (PRL). Prolactin stimulates the production of milk by the mammary glands following childbirth. Prolactin release inhibits the release of GnRH from the hypothalamus, resulting in a loss of FSH and LH release from the anterior pituitary. Prolactin levels are regulated by the hypothalamic hormones prolactin-releasing hormone (PRH) and prolactin-inhibiting hormone (PIH), which is now known to be dopamine. PRH stimulates the release of prolactin and PIH inhibits it.
The posterior pituitary releases the hormone oxytocin, which stimulates uterine contractions during childbirth. The uterine smooth muscles are not very sensitive to oxytocin until late in pregnancy when the number of oxytocin receptors in the uterus peaks. Stretching of tissues in the uterus and cervix stimulates oxytocin release during childbirth. Contractions increase in intensity as blood levels of oxytocin rise via a positive feedback mechanism until the birth is complete. Oxytocin also stimulates the contraction of myoepithelial cells around the milk-producing mammary glands. As these cells contract, milk is forced from the secretory alveoli into milk ducts and is ejected from the breasts in milk ejection (let-down) reflex. Oxytocin release is stimulated by the suckling of an infant, which triggers the synthesis of oxytocin in the hypothalamus and its release into circulation at the posterior pituitary.
Regulation of Metabolism by Hormones
Blood glucose levels vary through out the day as food is consumed and digested. Insulin and glucagon are the two hormones primarily responsible for maintaining homeostasis of blood glucose levels and additional regulation is accomplished with the thyroid hormones.
Managing nutrient intake involves storing excess intake and utilizing reserves when necessary and this is accomplished as the body uses hormones to moderate energy stores. Insulin is produced by the beta cells of the pancreas, which are stimulated to release insulin as blood glucose levels rise such as during food intake. Insulin lowers blood glucose levels by enhancing the rate of glucose uptake and utilization by target cells, which use glucose for ATP production. It also stimulates the liver to convert glucose to glycogen, which is then stored by cells for later use. Insulin also increases glucose transport into certain cells, such as muscle cells and the liver, resulting from an insulin-mediated increase in the number of glucose transporter proteins in the cell membranes, which remove glucose from circulation by facilitated diffusion. As insulin binds to its target cell via insulin receptors and signal transduction, it triggers the cell to incorporate glucose transport proteins into its membrane. This allows glucose to enter the cell, where it can be used as an energy source. However, this does not occur in all cells. Some cells, including those in the kidney and brain, can access glucose without the use of insulin. Insulin also stimulates the conversion of glucose to fat in adipocytes and the synthesis of proteins. These actions mediated by insulin cause blood glucose concentrations to fall, called a hypoglycemic low sugar effect, which inhibits further insulin release from beta cells through a negative feedback loop.
Damaged insulin function can lead to a condition called diabetes mellitus, with main symptoms of lethargy, stupor, excessive thirst and hunger, blurred vision, smell of acetone, weight loss, hyperventilation, nausea, vomiting, abdominal pain, frequent urination, glucose in urine. Diabetes mellitus can be caused by low levels of insulin production by the beta cells of the pancreas, or by reduced sensitivity of tissue cells to insulin. This prevents glucose from being absorbed by cells, causing high levels of blood glucose, or hyperglycemia (high sugar). High blood glucose levels make it difficult for the kidneys to recover all of the glucose from nascent urine, resulting in glucose being lost in urine. High glucose levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced and this can cause dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system. Oversecretion of insulin can cause hypoglycemia, low blood glucose levels. This causes insufficient glucose availability to cells, often leading to muscle weakness, and can sometimes cause unconsciousness or death if left untreated.
When blood glucose levels decline below normal levels (fasting or exercise), the hormone glucagon is released from the alpha cells of the pancreas. Glucagon raises blood glucose levels, eliciting what is called a hyperglycemic effect, by stimulating the breakdown of glycogen to glucose in skeletal muscle cells and liver cells in a process called glycogenolysis. Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells. Glucagon also stimulates absorption of amino acids from the blood by the liver, which then converts them to glucose. This process of glucose synthesis is called gluconeogenesis. Glucagon also stimulates adipose cells to release fatty acids into the blood. These actions mediated by glucagon result in an increase in blood glucose levels to normal homeostatic levels. Rising blood glucose levels inhibit further glucagon release by the pancreas via negative feedback mechanism. Therefore, insulin and glucagon work together to maintain homeostatic glucose levels.
Regulation of blood glucose levels by thyroid hormones
The basal metabolic rate, which is the amount of calories that required by the body at rest, is determined by two hormones produced by the thyroid gland: thyroxene (T4) and triiodothyronine (T3). These hormones affect many cells in the body and are transported across the plasma membrane of target cells and bind to receptors on the mitochondria resulting in increased ATP production. In the nucleus, T3 and T4 activate genes involved in energy production and glucose oxidation, resulting in increased rates of metabolism and body heat production, also known as the calorigenic effect.
T3 and T4 release from the thyroid gland is stimulated by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary. TSH binding at the receptors of the follicle of the thyroid triggers the production of T3 and T4 from a glycoprotein called thyroglobulin. Thyroglobulin is present in the follicles of the thyroid, and is converted to thyroid hormones with the addition of iodine. Iodine is formed from iodide ions that are actively transported into the thyroid follicle from the bloodstream.
The follicular cells of the thyroid require iodides (anions of iodine) in order to synthesize T3 and T4. Most diets in North America produce enough iodine from salts in foods. Diets with inadequate iodine intake that occur in developing countries are not able to produce enough T3 and T4, and the thyroid gland enlarges during a condition called goiter, when TSH is overproduced without the formation of the thyroid hormone.
Disorders in humans can result from underproduction and overproduction of thyroid hormones. Hypothyroidism is the underproduction of thyroid hormones and can cause a low metabolic rate leading to weight gain, sensitivity to cold, and reduced mental activity. Hyperthyroidism is the overproduction of thyroid hormones and can cause an increased metabolic rate, weight loss, excess heat production, sweating, and increased heart rate.
Hormonal control of blood calcium levels are important for generation of muscle contractions and nerve impulses, which are electrically stimulated. High calcium levels can cause a decrease of membrane permeability to sodium and make membranes less responsive. Low calcium levels can cause membrane permeability to sodium increases and cause convulsions or muscle spasms.
Blood calcium levels are regulated by the parathyroid hormone (PTH), which is produced by the parathyroid glands. PTH obtains calcium from bones and releases it into the bloodstream to increase calcium levels. PTH stimulates osteoclasts, causing bone to be reabsorbed and releasing calcium into the blood. PTH also inhibits osteoblasts, reducing calcium deposition into the bone.
PTH also obtains calcium from the kidneys and intestines to release into the bloodstream by reabsorption. Hyperparathyroidism results from the overproduction of the parathyroid hormone (PTH). Hypoparathyroidism is the underproduction of PTH and causes low levels of blood calcium.
The hormone calcitonin is produced by the parafollicular or C cells of the thyroid. Calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and stimulating calcium excretion by the kidneys, which causes calcium to be added to bones for structural integrity.
Hormonal regulation of growth is required for the growth and replication of most cells in the body. Growth hormone (GH) produced by the anterior portion of the pituitary gland, accelerates the rate of protein synthesis, particularly in skeletal muscle and bones. Growth hormone has direct and indirect mechanisms of action. The first direct action of GH is stimulation of triglyceride breakdown (lipolysis) and release into the blood by adipocytes. This results in a switch by most tissues from utilizing glucose as an energy source to utilizing fatty acids. This process is called a glucose-sparing effect.
In another direct mechanism of action, GH stimulates glycogen breakdown in the liver and the glycogen is then released into the blood as glucose. Blood glucose levels increase as most tissues are utilizing fatty acids instead of glucose for their energy needs. The GH mediated increase in blood glucose levels is called a diabetogenic effect because it is similar to the high blood glucose levels seen in diabetes mellitus.
The indirect mechanism of GH action is mediated by insulin-like growth factors (IGFs) or somatomedins, which are a family of growth-promoting proteins produced by the liver, which stimulates tissue growth. IGFs stimulate the uptake of amino acids from the blood, allowing the formation of new proteins, particularly in skeletal muscle cells, cartilage cells, and other target cells. This process is important after a meal, when glucose and amino acid concentration levels are high in the blood. GH levels are regulated by two hormones produced by the hypothalamus. GH release is stimulated by growth hormone-releasing hormone (GHRH) and is inhibited by growth hormone-inhibiting hormone (GHIH), also called somatostatin.
Growth hormone production balance is important for proper growth and development. Underproduction of GH in adults is not very important, however, in children, underproduction of GH can cause pituitary dwarfism, where growth is reduced and children grow to sizes much smaller than normal, but the body is still symmetrical. Oversecretion of growth hormone can cause gigantism in children, which leads to excessive growth. In adults, excessive GH can lead to acromegaly, which causes excessive growth in the bones of the face, hands, and feet.
Hormonal regulation of stress occurs as the body responds to stressful situations and releases hormones that will help the situation. The effects of the stress response include increased heart rate, dry mouth, and hair standing up. The sympathetic nervous system regulates the stress response through the hypothalamus. Stressful stimuli cause the hypothalamus to signal the adrenal medulla (which mediates short-term stress responses) via nerve impulses and the adrenal cortex, which mediates long-term stress responses, through the hormone adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary.
Short-term stress response occurs when the body responds by calling for the release of hormones that provide a burst of energy. The hormones epinephrine (or adrenaline) and norepinephrine (or noradrenaline) are released by the adrenal medulla. Epinephrine and norepinephrine provide a burst of energy by increasing blood glucose levels by stimulating the liver and skeletal muscles to break down glycogen and by stimulating glucose release by liver cells. Additionally, these hormones increase oxygen availability to cells by increasing the heart rate and dilating the bronchioles. The hormones also prioritize body function by increasing blood supply to essential organs such as the heart, brain, and skeletal muscles, while restricting blood flow to organs not in immediate need, such as the skin, digestive system, and kidneys. Epinephrine and norepinephrine are collectively called catecholamines.
Long-term stress response is different from short-term stress response because the body cannot sustain the bursts of energy from epinephrine and norepinephrine for long periods. Therefore, other hormones help out in the stress response. In a long-term stress response, the hypothalamus triggers the release of ACTH from the anterior pituitary gland. The adrenal cortex is stimulated by the ACTH to release steroid hormones called corticosteroids. Corticosteroids turn on transcription of certain genes in the nuclei of target cells. They change enzyme concentrations in the cytoplasm and affect cellular metabolism. There are two main corticosteroids: glucocorticoids such as cortisol, and mineralcorticoids such as aldosterone. These hormones target the breakdown of fat into fatty acids in the adipose tissue. The fatty acids are released into the bloodstream for other tissues to use for ATP production. The glucocorticoids primarily affect glucose metabolism by stimulating glucose synthesis. Glucocorticoids also have anti-inflammatory properties through inhibition of the immune system, however, it cannot be used long term because it suppresses the immune system.
Mineralcorticoids function to regulate ion and water balance of the body. The hormone aldosterone stimulates the reabsorption of water and sodium ions in the kidney, which results in increased blood pressure and volume.
Excessive production of glucocorticoids can cause Cushing's disease, which causes the shifting of fat storage areas of the body and can also cause the accumulation of adipose tissue in the face and neck, and excessive glucose in the blood. Hyposecretion, or excessive production of corticosteroids can cause Addison's disease, which can result in bronze skin, hypoglycemia, and low electrolyte levels in the blood.
The regulation of hormone production and release are primarily controlled by negative feedback systems, where a stimulus causes the release of a substance and once the substance reaches a certain level, it sends a signal that stops the further release of the substance. Therefore, the concentration of hormones in the blood can be regulated in this way.
Three mechanisms cause endocrine glands to be stimulated to synthesize and release hormones: humoral stimuli, hormonal stimuli, and neural stimuli.
Humoral stimuli refer to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. Hormonal stimuli refer to the release of hormones in response to another hormone, such as when endocrine glands release hormones when stimulated by hormones released by other endocrine glands. Neural stimuli occur when the nervous system directly stimulates endocrine glands to release hormones and the nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.
Endocrine glands in the endocrine system work together to maintain homeostasis. The endocrine system and the nervous system use chemical signals to communicate and regulate the body's physiology. The endocrine system releases hormones that act on target cells to regulate development, growth, energy metabolism, reproduction, and other behaviors. The nervous system releases neurotransmitters or neurohormones that regulate neurons, muscle cells, and endocrine cells. Since the neurons can regulate the release of hormones, the nervous system and endocrine systems work together to regulate the body physiology.
The hypothalamus in vertebrates integrates the endocrine and nervous systems. The hypothalamus is an endocrine organ located in the diencephalon of the brain and it receives input from the body and other brain areas and initiates endocrine responses to environmental changes. The hypothalamus synthesizes hormones and transports them along axons to the posterior pituitary gland. It synthesizes and secretes regulatory hormones that control the endocrine cells in the anterior pituitary gland. The hypothalamus contains autonomic centers that control endocrine cells in the adrenal medulla via neural control.
The pituitary gland is known as the hypophysis or master gland and is located at the base of the brain in the sella turcica, a groove of the sphenoid bone of the skull. The pituitary gland is attached to the hypothalamus via a stalk called the pituitary stalk (or infundibulum). The anterior portion of the pituitary gland is regulated by releasing or release-inhibiting hormones produced by the hypothalamus, and the posterior pituitary receives signals via neurosecretory cells to release hormones produced by the hypothalamus. The pituitary has two distinct regions: the anterior pituitary and posterior pituitary. Both of these together secrete nine different peptide or protein hormones. The posterior lobe of the pituitary gland contains axons of the hypothalamic neurons.
The anterior pituitary gland, or adenohypophysis, is surrounded by a capillary network that extends from the hypothalamus, down along the infundibulum, and to the anterior pituitary. This capillary network is part of the hypophyseal portal system that carries substances from the hypothalamus to the anterior pituitary and hormones from the anterior pituitary into the circulatory system. A portal system carries blood from one capillary network to another. Therefore, the hypophyseal portal system allows hormones produced by the hypothalamus to be carried directly to the anterior pituitary without first entering the circulatory system.
The anterior pituitary produces seven hormones: growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), melanin-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Anterior pituitary hormones are sometimes called tropic hormones because they control the functioning of other organs. While these hormones are produced by the anterior pituitary, their production is controlled by regulatory hormones produced by the hypothalamus. These hormones can be releasing or inhibiting hormones regulating the amount of hormones secreted. These hormones travel from the hypothalamus through the hypophyseal portal system to the anterior pituitary where they exert their effect. Negative feedback then regulates the amount of hormone secretion.
The posterior pituitary is significantly different in structure from the anterior pituitary. The posterior pituitary is part of the brain, extending down from the hypothalamus, and contains mostly nerve fibers and neuroglial cells, which support axons that extend from the hypothalamus to the posterior pituitary. The posterior pituitary and the infundibulum together are referred to as the neurohypophysis.
The hormones antidiuretic hormone (ADH), also known as vasopressin, and oxytocin are produced by neurons in the hypothalamus and transported with these axons along the infundibulum to the posterior pituitary. These hormones are released into the circulatory system via neural signaling from the hypothalamus.
The thyroid gland is located in the neck, just below the larynx and in front of the trachea. The thyroid gland is a butterfly-shaped gland with two lobes that are connected by the isthmus. The color is dark red due to its extensive vascular system. Sometimes the thyroid can swell during disfunction.
The thyroid is made up of many spherical thyroid follicles, which are lined with a simple cuboidal epithelium. These follicles contain a viscous fluid, called colloid, which stores the glycoprotein thyroglobulin, the precursor to thyroid hormones. The follicles produce hormones that can be stored in the colloid or released into the surrounding capillary network for transport to the rest of the body via the circulatory system.
Thyroid follicle cells synthesize the hormone thyroxine, (or T4, four atoms of iodine), and triiodothyronine (T3, three atoms of iodine). Follicle cells are stimulated to release stored T3 and T4 by thyroid stimulating hormone (TSH), which is produced by the anterior pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.
A third hormone, calcitonin, is produced by parafollicular cells of the thyroid either releasing hormones or inhibiting hormones. Calcitonin release is not controlled by TSH, but instead is released when calcium ion concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body fluids and acts in the bones to inhibit osteoclast activity and in the kidney to stimulate excretion of calcium, the combination of which lowers body fluid levels of calcium.
Parathyroid glands are located on the posterior surface of the thyroid gland and most people have between two to six of these glands. There is also usually a superior gland and inferior gland associated with the thyroid's two lobes. Each parathyroid gland is covered by connective tissue and contains many secretory cells that are associated with a capillary network. Parathyroid glands produce the parathyroid hormone (PTH). PTH increases blood calcium concentrations when calcium ion levels fall below normal. PTH enhances reabsorption of Ca2+ by the kidneys, stimulates osteoclast activity and inhibits osteoblast activity, and stimulates synthesis and secretion of calcitriol by the kidneys, which enhances Ca2+ absorption by the digestive system. PTH is produced by the chief cells of the parathyroid. PTH and calcitonin work in opposition to one another to maintain homeostatic calcium levels in the body fluids.
Adrenal glands are associated with the kidney, as one gland is located on top of each kidney. The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla and these regions secrete different hormones.
The adrenal cortex is made up of layers of epithelial cells and associated capillary networks. These layers form three distinct regions: an outer zona glomerulosa that produces mineralocorticoids, a middle zona fasciculata that produces glucocorticoids, and an inner zona reticularis that produces androgens.
The main mineralocorticoid is aldosterone, which regulates the concentration of Na+ ions in urine, sweat, pancreas, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.
The three main glucocorticoids are cortisol, corticosterone, and cortisone. The glucocorticoids stimulate the synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells and they promote the release of fatty acids from adipose tissue. These hormones increase blood glucose levels to maintain levels within a normal range between meals. These hormones are secreted in response to ACTH and levels are regulated by negative feedback.
The adrenal cortex also produces small amounts of testosterone precursor, although the role of this additional hormone production is not fully understood. Testosterone is a type of androgen that promotes a suite of characteristics such as the growth and development of the testes and penis, increased skeletal and muscular growth, enlargement of the larynx, increased growth and redistribution of body hair, and increased sexual drive. Testosterone secretion is regulated by both the hypothalamus and the anterior pituitary gland. The hypothalamus sends releasing hormones that stimulate the release of gonadotropins from the anterior pituitary gland. Testosterone produced in small amounts in the adrenal cortex may work with sex hormones released from the gonads.
The Adrenal medulla contains large, irregularly-shaped cells that are closely associated with blood vessels. These cells are innervated by preganglionic autonomic nerve fibers from the central nervous system. The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline) and another that produces norepinephrine (noadrenaline). Epinephrine is the primary adrenal medulla hormone accounting for 75 to 80 percent of its secretions. Epinephrine and norepinephrine increase heart rate, breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels. They also accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue.
The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous system. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic fibers innervating the adrenal medulla. These neural impulses originate from the hypothalamus in response to stress to prepare the body for the fight-or flight response.
The pancreas is an elongated organ that is located between the stomach and the proximal portion of the small intestine. It contains both exocrine cells that excrete digestive enzymes and endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both endocrine and exocrine functions.
The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans. The pancreatic islets contain two primary cell types: alpha cells, which produce the hormone glucagon, and beta cells, which produce the hormone insulin. These hormones regulate blood glucose levels. As blood glucose levels decline, alpha cells release glucagon to raise the blood glucose levels by increasing rates of glycogen breakdown and glucose release by the liver. When blood glucose levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by increasing the rate of glucose uptake in most body cells, and by increasing glycogen synthesis in skeletal muscles and the liver. Together, glucagon and insulin regulate blood glucose levels.
The pineal gland produces the hormone melatonin and the rate of melatonin production is affected by the photoperiod. Collaterals from the visual pathways innervate the pineal gland. During the day photoperiod, little melatonin is produced; however, melatonin production increases during the dark photoperiod (night). In some mammals, melatonin has an inhibitory effect on reproductive functions by decreasing production and maturation of sperm, oocytes, and reproductive organs. Melatonin is an effective antioxidant, protecting the CNS from free radicals such as nitric oxide and hydrogen peroxide. Lastly, melatonin is involved in biological rhythms, particularly circadian rhythms such as the sleep-wake cycle and eating habits.
The gonads include the testes and ovaries and produce steroid hormones. The testes produce androgens, testosterone being the most prominent, which allow for the growth and development of the testes and penis, increased skeletal and muscular growth, enlargement of the larynx, increased growth and redistribution of body hair, and the production of sperm cells. The ovaries produce estradiol and progesterone, which cause secondary sex characteristics and prepare the body for childbirth.
Endocrine glands and their associated hormones and effects include the following:
The hypothalamus endocrine gland and releasing and inhibiting hormones regulate hormone release from the pituitary gland. Also produces oxytocin, produces uterine contractions and milk secretion in breast tissue.
The hypothalamus endocrine gland and antidiuretic hormone (ADH) causes water reabsorption from the kidneys and vasoconstriction to increase blood pressure.
The pituitary (anterior) endocrine gland and the growth hormone (GH) promotes growth of body tissues, protein synthesis, and metabolic functions. The pituitary and prolactin (PRL) promote milk production. The pituitary and thyroid stimulating hormone (TSH) stimulates thyroid hormone release. The pituitary and adrenocorticotropic hormone (ACTH) stimulates hormone release by adrenal cortex and glucocorticoids. The pituitary and follicle-stimulating hormone (FSH) stimulates gamete production (both ova and sperm) and secretion of estradiol. The pituitary and luteinizing hormone (LH) stimulates androgen production by gonads, ovulation and secretion of progesterone. The pituitary and melanocyte-stimulating hormone (MSH) stimulates melanocytes of the skin increasing melanin pigment production.
The pituitary (posterior) endocrine gland and antidiuretic hormone (ADH) stimulates water reabsorption by the kidneys. The posterior pituitary and oxytocin hormone stimulates uterine contractions during childbirth and milk ejection, stimulates ductus deferens and prostate gland contraction during emission.
The thyroid endocrine gland and thyroxine, triiodothyronine hormones stimulate and maintain metabolism, growth, and development. The thyroid and calcitonin hormones reduce blood Ca2+ levels.
The parathyroid endocrine gland and parathyroid hormone (PTH) increases blood Ca2+ levels.
The adrenal cortex endocrine gland and aldosterone hormone increases blood Na+ levels and increases K+ secretion. The adrenal cortex and cortisol, corticosterone, and cortisone hormones increase blood glucose levels and have anti-inflammatory effects.
The adrenal medulla endocrine gland and epinephrine, norepinephrine hormones stimulate fight-or-flight responses, increase blood glucose levels and increase metabolic activities.
The pancreas endocrine gland and insulin hormone reduces blood glucose levels. The pancreas endocrine gland and glucagon hormone increases blood glucose levels.
The pineal endocrine gland and melatonin hormone regulates some biological rhythms and protects CNS from free radicals.
The testes endocrine gland and androgen hormones regulate, promote, increase, or maintain sperm production and include characteristics of growth and development of the testes and penis, increased skeletal and muscular growth, enlargement of the larynx, and increased growth and redistribution of body hair.
The ovaries endocrine gland and estrogen hormones promote uterine lining growth, increased development of breast tissue, redistribution of fat towards hips, legs, and breast and the maturation of the uterus and vagina. The ovaries endocrine gland and the progestins hormone promotes and maintains uterine lining growth.
Several organs in the body exist whose primary functions are not endocrine but also possess endocrine functions. These include the heart, kidneys, intestines, thymus, gonads, and adipose tissue.
The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These cells release the hormone atrial natriuretic peptide (ANP) in response to increased blood volume. High blood volume causes the cells to be stretched, resulting in hormone release. ANP also causes the kidneys to reduce the reabsorption of Na+, causing Na+ and water to be excreted in the urine. ANP also reduces the amount of renin released by the kidneys and aldosterone released by the adrenal cortex, which further prevents the retention of water. Therefore, ANP causes a reduction in blood volume and blood pressure, and reduces the concentration of Na+ in the blood.
The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located in the mucosa of the GI tract throughout the stomach and small intestine. Some of the hormones produced include gastrin, secretin, and cholecystokinin, which are secreted in the presence of food, and some of which act on other organs such as the pancreas, gallbladder, and liver. They trigger the release of gastric juices, which help to break down and digest food in the GI tract.
The kidneys have endocrine function in addition to the adrenal glands. Renin is released in response to decreased blood volume or pressure and is part of a system that leads to the release of aldosterone. Aldosterone then causes the retention of Na+ and water, raising blood volume. The kidneys also release calcitriol, which aids in the absorption of Ca2+ and phosphate ions.
Erythropoietin (EPO) is a protein hormone that triggers the formation of red blood cells in the bone marrow. EPO is released in response to low oxygen levels. Because red blood cells are oxygen carriers, increased production results in greater oxygen delivery throughout the body. High EPO levels can also cause health problems, however.
The thymus is found behind the sternum and it is most prominent in infants, becoming smaller in size through adulthood. The thymus produces hormones called thymosins, which contribute to the development of the immune response.
Adipose tissue is a connective tissue found throughout the body and it produces the hormone leptin in response to food intake. Leptin increases the activity of anorexigenic neurons, producing a feeling of satiety or fullness after eating, and therefore it reduces appetite. Leptin is also associated with improving reproduction fertility and it must be present for GnRH (gonadotropin-releasing hormone) and gonadotropin synthesis to occur. GnRH is the hormone produced by the hypothalamus in the brain that stimulates the pituitary gland to release two other hormones to regulate the gonads: FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
Many types of hormones are found throughout the body. Homeostasis is made possible by the release of chemicals in the body called hormones that aid in communication between neighboring cells and between cells and tissues in distant parts of the body. The human body produces over 50 known hormones.
Hormones are released into body fluids (such as blood) and the hormones are carried to their target cells. At the target cells, which are cells that have a receptor for a signal or ligand from a signal cell, the hormones create a response. The cells, tissues, and organs that secrete hormones make up the endocrine system.
Adrenal glands in the endocrine system produce hormones that regulate responses to stress and the thyroid gland produces hormones that regulate metabolic rates.
Hormones can be divided into three classes based on their chemical structure: lipid-derived, amino-acid derived, and peptide (and protein) hormones.
Lipid-derived hormones can diffuse across plasma-membranes while amino-acid derived and peptide hormones cannot. Lipid-derived or lipid-soluble hormones are mostly derived from and structurally similar to cholesterol. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols. Their chemical names end in -ol for alcohols and -one for ketones. Examples of steroid hormones include estrogens (such as estradiol) and androgens (such as testosterone).
Gonadal hormones, produced by the gonads, include both steroid and peptide hormones. Androgens and estrogens resemble one another in chemical structure and originate form the same molecule. Estrogens are important for sexual development in an ovarian reproductive system, while androgens drive development in a testicular reproductive system. The ovaries produce steroid hormones such as estradiol and progesterone. When androgens are produced, some of them are later converted to estrogens. Small amounts of estrogen occur through aromatase actions in adipose, brain, skin, and bone, which convert testosterone to estrogen. The testes and the adrenal cortex both secrete testosterone.
Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water and are transported by proteins in blood and therefore remain in circulation longer than peptide hormones.
Amino-acid derived hormones are relatively small molecules that are derived from the amino acids tyrosine and tryptophan. If a hormone is acid-derived, its chemical name will end in -ine. Epinephrine and norepinephrine are amino acid-derived hormones produced in the medulla of the adrenal glands, and thyroxine, produced in the thyroid gland. The pineal gland in the brain makes and secretes melatonin which regulates sleep cycles.
Peptide hormones have a structure of a polypeptide chain (of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, like growth hormones produced by the pituitary, and large glycoproteins such as follicle-stimulating hormonesproduced by the pituitary.
Secreted peptides like insulin are stored within vesicles in the cells that synthesize them. They are then released in response to stimuli such as high blood glucose levels in the case of insulin. Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells, therefore their receptors are found on the surface of the target cells.
An endocrinologist is a medical doctor that specializes in treating disorders of the endocrine glands, hormone systems, and glucose and lipid metabolic pathways. An endocrine surgeon specializes in the surgical treatment of endocrine diseases and glands.
How Do Hormones Work? Hormones mediate changes in target cells by binding to specific hormone receptors. Therefore, even though hormones circulate throughout the body and come in contact with many different types of cells, hormones only affect cells that possess the necessary receptors. Receptors for a specific hormone may be found on many different cells or may be limited to a small number of specialized cells. Cells can have many receptors for the same hormone but often also possess receptors for different types of hormones. The number of receptors that respond to a hormone determines the cell's sensitivity to that hormone, and the resulting cellular response. Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone and allowing for more cellular activity. When the number of receptors decreases in response to rising hormone levels, it is called down-regulation and cellular activity is reduced.
Receptor binding alters cellular activity and results in an increase or decrease in normal body processes. Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular hormone receptors and modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways.
Lipid-derived (soluble) hormones such as steroid hormones diffuse across membranes of the endocrine cell. Once outside the cell, they bind to transport proteins that keep them soluble in the bloodstream. At the target cell, the hormones are released from the carrier protein and diffuse across the lipid bilayer of the plasma membrane of cells. The steroid hormones pass through the plasma membrane of the target cell and adhere to intracellular receptors residing in the cytoplasm or in the nucleus. The cell signaling pathways induced by the steroid hormones regulate specific genes on the cell's DNA. The hormones and receptor complex act as transcription regulators by increasing or decreasing the synthesis of mRNA molecules of specific genes. This, in turn, determines the amount of corresponding protein that is synthesized by altering gene expression. This protein can be used either to change the structure of the cell or to produce enzymes that catalyze chemical reactions. In this way, the steroid hormone regulates specific cell processes.
Other lipid-soluble hormones that are not steroid hormones, such as vitamin D and thyroxine, have receptors located in the nucleus. While thyroxine is mostly hydrophobic, its passage across the membrane is dependent on transporter protein. Vitamin D diffuses across both the plasma membrane and the nuclear envelope. Once in the cell, both hormones bind to receptors in the nucleus. The hormone-receptor complex stimulates transcription of specific genes.
Amino acid-derived hormones (except thyroxene) and polypeptide hormones are not lipid-derived (lipid-soluble) and therefore cannot diffuse through the plasma membrane of cells. Lipid insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors. Unlike steroid hormones, lipid insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA. Binding of these hormones to a cell surface receptor results in activation of a signaling pathway. This triggers intracellular activity and carries out the specific affects associated with the hormone. In this way, nothing passes through the cell membrane. The hormone that binds at the surface remains at the surface of the cell while the intracellular product remains inside the cell. The hormone that initiates the signaling pathway is called a first messenger, which activates a second messenger in the cytoplasm.
One important second messenger is cyclic AMP (cAMP). When a hormone binds to its membrane receptor, a G-protein that is associated with the receptor is activated. G-proteins are proteins separate from receptors that are found in the cell membrane. When a hormone is not bound to the receptor, the G-protein is inactive and is bound to guanosine diphosphate, or GDP. When a hormone binds to the receptor, the G-protein is activated by binding guanosine triphosphate, or GTP, in place of GDP. After binding, GTP is hydrolysed by the G-protein into GDP and becomes inactive.
The activated G-protein in turn activates a membrane-bound enzyme called adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP, in turn, activates a group of proteins called protein kinases, which transfer a phosphate group from ATP to a substrate molecule in a process called phosphorylation. The phosphorylation of a substrate molecule changes its structural orientation, thereby activating it. These activated molecules can then mediate changes in cellular processes.
The effect of a hormone is amplified as the signaling pathway progresses. The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase. Each molecule of adenylyl cyclase then triggers the formation of many molecules of cAMP. Further amplification occurs as protein kinases, once activated by cAMP, can catalyze many reactions. In this way, a small amount of hormone can trigger the formation of a large amount of cellular product. To stop hormone activity, cAMP is deactivated by the cytoplasmic enzyme phosphodiesterase, or PDE. PDE is always present in the cell and breaks down cAMP to control hormone activity, preventing overproduction of cellular products.
The specific response of a cell to a lipid insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm. Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release.
Hormones and Regulation of Body Processes: Excretory System, Reproductive System, Metabolism, and Diseases
Hormones have a wide range of effects and modulate many different body processes, including the excretory system, the reproductive system, metabolism, blood calcium concentrations, growth, and stress responses.
Water balance in the body prevents dehydration or overhydration. Water concentration in the body is regulated by osmoreceptors in the hypothalamus, which are sensory cells that detect the concentration of electrolytes in the extracellular fluid. The concentration of electrolytes in the blood rises when there is water loss caused by excessive perspiration, inadequate water intake, or low blood volume due to blood loss. An increase in blood electrolyte levels results in a neuronal signal being sent from the osmoreceptors in the hypothalamic nuclei. The pituitary gland has two components: anterior and posterior. The anterior pituitary is composed of glandular cells that secrete protein hormones. The posterior pituitary is an extension of the hypothalamus. It is composed of largely neurons that are continuous with the hypothalamus.
The hypothalamus produces a polypeptide known as antidiuretic hormone (ADH), which is transported to and released from the posterior pituitary gland. The principle action of ADH is to regulate the amount of water excreted by the kidneys. ADH (also known as vasopressin) causes direct water reabsorption from the kidney tubules, and salts and wastes are concentrated in what will eventually be excreted as urine. The hypothalamus controls the mechanisms of ADH secretion, either by regulating blood volume or the concentration of water in the blood. Dehydration or psychological stress can can cause an increase in osmolarity above 300 mOsm/L, which in turn, raises ADH secretion and causes water to be retained, causing an increase in blood pressure. ADH travels in the bloodstream to the kidneys. Once at the kidneys, ADH changes the kidneys to become more permeable to water by temporarily inserting water channels, aquaporins, into the kidney tubules. Water moves out of the kidney tubules through the aquaporins, reducing urine volume. The water is reabsorbed into the capillaries lowering blood osmolarity back toward normal. As blood osmolarity decreases, a negative feedback mechanism reduces osmoreceptor activity in the hypothalamus, and ADH secretion is reduced. ADH release can be reduced by certain substances, including alcohol, which can cause increased urine production and dehydration.
Chronic underproduction of ADH or a mutation in the ADH receptor results in diabetes insipidus. If the posterior pituitary does not release enough ADH, water cannot be retained by the kidneys and is lost as urine. This causes increased thirst, but water taken in is lost again and must be continually consumed. If the condition is not severe, dehydration may not occur, but severe cases can lead to electrolyte imbalances due to dehydration.
Another hormone responsible for maintaining electrolyte concentrations in extracellular fluids is aldosterone, a steroid hormone that is produced by the adrenal cortex. In contrast to ADH, which promotes the reabsorption of water to maintain proper water balance, aldosterone maintains proper water balance by enhancing Na+ reabsorption and K+ secretion from extracellular fluid of the cells in kidney tubules. Because it is produced in the cortex of the adrenal gland and affects the concentrations of minerals Na+ and K+, aldosterone is referred to as mineralocorticoid, a corticosteroid that affects ion and water balance. Aldosterone release is stimulated by a decrease in blood sodium levels, blood volume, or blood pressure, or an increase in blood potassium levels. It also prevents the loss of Na+ from sweat, saliva, and gastric juice. The reabsorption of Na+ also results in the osmotic reabsorption of water, which alters blood volume and blood pressure.
Aldosterone production can be stimulated by low blood pressure, which triggers a sequence of chemical release. When blood pressure drops, the renin-angiotensin-aldosterone system (RAAS) is activated. Cells in the juxtaglomerular apparatus, which regulates the functions of the nephrons of the kidney, detect this and release renin. Renin, an enzyme, circulates in the blood and reacts with a plasma protein produced by the liver called angiotensinogen. When angiotensinogen is cleaved by renin, it produces angiotensin I, which is then converted into angiotensin II in the lungs. Angiotensin II functions as a hormone and then causes the release of the hormone aldosterone by the adrenal cortex, resulting in increased Na+ reabsorption, water retention, and an increase in blood pressure. Angiotensin II in addition to being a potent vasoconstrictor also causes an increase in ADH and increased thirst, both of which help to raise blood pressure.
Hormonal regulation of the reproductive system is a process that requires the action of hormones from the pituitary gland, the adrenal cortex, and the gonads. During puberty in both males and females, the hypothalamus produces gonadotropin-releasing hormone (GnRH), which stimulates the production and release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. These hormones regulate the gonads (testes in males and ovaries in females) and are therefore called gonadotropins. In both males and females, FSH stimulates gamete production and LH stimulates production of hormones by the gonads. An increase in gonad hormone levels inhibits GnRH production through a negative feedback loop.
In the testes, FSH stimulates the maturation of sperm cells. FSH production is inhibited by the hormone inhibin, which is released by the testes. LH stimulates the production of the sex hormones (androgens) by the interstitial cells of the testes and therefore is also called interstitial cell-stimulating hormone.
The most commonly known androgen in males is testosterone, which promotes the production of sperm and growth and development of the testes and reproductive organs, skeletal and muscle growth, larynx growth, body hair growth, and sexual drive. Testosterone secretion is regulated by both the hypothalamus and the anterior pituitary gland. The hypothalamus sends releasing hormones that stimulate the release of gonadotropins from the anterior pituitary gland.
The ovarian reproductive system regulation occurs in the ovaries, as FSH stimulates development of egg cells, called ova, which develop in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production. LH also plays a role in the development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the ovaries (as well as testosterone production by the testes). Estradiol and progesterone are steroid hormones that serve several functions in the human body. Estradiol causes the egg to mature and release during the menstrual cycle, and thickens the uterine lining prior to egg implantation. Estradiol also helps with bone health, nitric oxide production, and brain function. During puberty, estradiol produces increased development of breast tissue, redistribution of fat toward hips, legs, breast, and the maturation of the uterus and vagina. Both estradiol and progesterone regulate the menstrual cycle.
In addition to producing FSH and LH, the anterior portion of the pituitary gland also produces the hormone prolactin (PRL). Prolactin stimulates the production of milk by the mammary glands following childbirth. Prolactin release inhibits the release of GnRH from the hypothalamus, resulting in a loss of FSH and LH release from the anterior pituitary. Prolactin levels are regulated by the hypothalamic hormones prolactin-releasing hormone (PRH) and prolactin-inhibiting hormone (PIH), which is now known to be dopamine. PRH stimulates the release of prolactin and PIH inhibits it.
The posterior pituitary releases the hormone oxytocin, which stimulates uterine contractions during childbirth. The uterine smooth muscles are not very sensitive to oxytocin until late in pregnancy when the number of oxytocin receptors in the uterus peaks. Stretching of tissues in the uterus and cervix stimulates oxytocin release during childbirth. Contractions increase in intensity as blood levels of oxytocin rise via a positive feedback mechanism until the birth is complete. Oxytocin also stimulates the contraction of myoepithelial cells around the milk-producing mammary glands. As these cells contract, milk is forced from the secretory alveoli into milk ducts and is ejected from the breasts in milk ejection (let-down) reflex. Oxytocin release is stimulated by the suckling of an infant, which triggers the synthesis of oxytocin in the hypothalamus and its release into circulation at the posterior pituitary.
Regulation of Metabolism by Hormones
Blood glucose levels vary through out the day as food is consumed and digested. Insulin and glucagon are the two hormones primarily responsible for maintaining homeostasis of blood glucose levels and additional regulation is accomplished with the thyroid hormones.
Managing nutrient intake involves storing excess intake and utilizing reserves when necessary and this is accomplished as the body uses hormones to moderate energy stores. Insulin is produced by the beta cells of the pancreas, which are stimulated to release insulin as blood glucose levels rise such as during food intake. Insulin lowers blood glucose levels by enhancing the rate of glucose uptake and utilization by target cells, which use glucose for ATP production. It also stimulates the liver to convert glucose to glycogen, which is then stored by cells for later use. Insulin also increases glucose transport into certain cells, such as muscle cells and the liver, resulting from an insulin-mediated increase in the number of glucose transporter proteins in the cell membranes, which remove glucose from circulation by facilitated diffusion. As insulin binds to its target cell via insulin receptors and signal transduction, it triggers the cell to incorporate glucose transport proteins into its membrane. This allows glucose to enter the cell, where it can be used as an energy source. However, this does not occur in all cells. Some cells, including those in the kidney and brain, can access glucose without the use of insulin. Insulin also stimulates the conversion of glucose to fat in adipocytes and the synthesis of proteins. These actions mediated by insulin cause blood glucose concentrations to fall, called a hypoglycemic low sugar effect, which inhibits further insulin release from beta cells through a negative feedback loop.
Damaged insulin function can lead to a condition called diabetes mellitus, with main symptoms of lethargy, stupor, excessive thirst and hunger, blurred vision, smell of acetone, weight loss, hyperventilation, nausea, vomiting, abdominal pain, frequent urination, glucose in urine. Diabetes mellitus can be caused by low levels of insulin production by the beta cells of the pancreas, or by reduced sensitivity of tissue cells to insulin. This prevents glucose from being absorbed by cells, causing high levels of blood glucose, or hyperglycemia (high sugar). High blood glucose levels make it difficult for the kidneys to recover all of the glucose from nascent urine, resulting in glucose being lost in urine. High glucose levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced and this can cause dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system. Oversecretion of insulin can cause hypoglycemia, low blood glucose levels. This causes insufficient glucose availability to cells, often leading to muscle weakness, and can sometimes cause unconsciousness or death if left untreated.
When blood glucose levels decline below normal levels (fasting or exercise), the hormone glucagon is released from the alpha cells of the pancreas. Glucagon raises blood glucose levels, eliciting what is called a hyperglycemic effect, by stimulating the breakdown of glycogen to glucose in skeletal muscle cells and liver cells in a process called glycogenolysis. Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells. Glucagon also stimulates absorption of amino acids from the blood by the liver, which then converts them to glucose. This process of glucose synthesis is called gluconeogenesis. Glucagon also stimulates adipose cells to release fatty acids into the blood. These actions mediated by glucagon result in an increase in blood glucose levels to normal homeostatic levels. Rising blood glucose levels inhibit further glucagon release by the pancreas via negative feedback mechanism. Therefore, insulin and glucagon work together to maintain homeostatic glucose levels.
Regulation of blood glucose levels by thyroid hormones
The basal metabolic rate, which is the amount of calories that required by the body at rest, is determined by two hormones produced by the thyroid gland: thyroxene (T4) and triiodothyronine (T3). These hormones affect many cells in the body and are transported across the plasma membrane of target cells and bind to receptors on the mitochondria resulting in increased ATP production. In the nucleus, T3 and T4 activate genes involved in energy production and glucose oxidation, resulting in increased rates of metabolism and body heat production, also known as the calorigenic effect.
T3 and T4 release from the thyroid gland is stimulated by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary. TSH binding at the receptors of the follicle of the thyroid triggers the production of T3 and T4 from a glycoprotein called thyroglobulin. Thyroglobulin is present in the follicles of the thyroid, and is converted to thyroid hormones with the addition of iodine. Iodine is formed from iodide ions that are actively transported into the thyroid follicle from the bloodstream.
The follicular cells of the thyroid require iodides (anions of iodine) in order to synthesize T3 and T4. Most diets in North America produce enough iodine from salts in foods. Diets with inadequate iodine intake that occur in developing countries are not able to produce enough T3 and T4, and the thyroid gland enlarges during a condition called goiter, when TSH is overproduced without the formation of the thyroid hormone.
Disorders in humans can result from underproduction and overproduction of thyroid hormones. Hypothyroidism is the underproduction of thyroid hormones and can cause a low metabolic rate leading to weight gain, sensitivity to cold, and reduced mental activity. Hyperthyroidism is the overproduction of thyroid hormones and can cause an increased metabolic rate, weight loss, excess heat production, sweating, and increased heart rate.
Hormonal control of blood calcium levels are important for generation of muscle contractions and nerve impulses, which are electrically stimulated. High calcium levels can cause a decrease of membrane permeability to sodium and make membranes less responsive. Low calcium levels can cause membrane permeability to sodium increases and cause convulsions or muscle spasms.
Blood calcium levels are regulated by the parathyroid hormone (PTH), which is produced by the parathyroid glands. PTH obtains calcium from bones and releases it into the bloodstream to increase calcium levels. PTH stimulates osteoclasts, causing bone to be reabsorbed and releasing calcium into the blood. PTH also inhibits osteoblasts, reducing calcium deposition into the bone.
PTH also obtains calcium from the kidneys and intestines to release into the bloodstream by reabsorption. Hyperparathyroidism results from the overproduction of the parathyroid hormone (PTH). Hypoparathyroidism is the underproduction of PTH and causes low levels of blood calcium.
The hormone calcitonin is produced by the parafollicular or C cells of the thyroid. Calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and stimulating calcium excretion by the kidneys, which causes calcium to be added to bones for structural integrity.
Hormonal regulation of growth is required for the growth and replication of most cells in the body. Growth hormone (GH) produced by the anterior portion of the pituitary gland, accelerates the rate of protein synthesis, particularly in skeletal muscle and bones. Growth hormone has direct and indirect mechanisms of action. The first direct action of GH is stimulation of triglyceride breakdown (lipolysis) and release into the blood by adipocytes. This results in a switch by most tissues from utilizing glucose as an energy source to utilizing fatty acids. This process is called a glucose-sparing effect.
In another direct mechanism of action, GH stimulates glycogen breakdown in the liver and the glycogen is then released into the blood as glucose. Blood glucose levels increase as most tissues are utilizing fatty acids instead of glucose for their energy needs. The GH mediated increase in blood glucose levels is called a diabetogenic effect because it is similar to the high blood glucose levels seen in diabetes mellitus.
The indirect mechanism of GH action is mediated by insulin-like growth factors (IGFs) or somatomedins, which are a family of growth-promoting proteins produced by the liver, which stimulates tissue growth. IGFs stimulate the uptake of amino acids from the blood, allowing the formation of new proteins, particularly in skeletal muscle cells, cartilage cells, and other target cells. This process is important after a meal, when glucose and amino acid concentration levels are high in the blood. GH levels are regulated by two hormones produced by the hypothalamus. GH release is stimulated by growth hormone-releasing hormone (GHRH) and is inhibited by growth hormone-inhibiting hormone (GHIH), also called somatostatin.
Growth hormone production balance is important for proper growth and development. Underproduction of GH in adults is not very important, however, in children, underproduction of GH can cause pituitary dwarfism, where growth is reduced and children grow to sizes much smaller than normal, but the body is still symmetrical. Oversecretion of growth hormone can cause gigantism in children, which leads to excessive growth. In adults, excessive GH can lead to acromegaly, which causes excessive growth in the bones of the face, hands, and feet.
Hormonal regulation of stress occurs as the body responds to stressful situations and releases hormones that will help the situation. The effects of the stress response include increased heart rate, dry mouth, and hair standing up. The sympathetic nervous system regulates the stress response through the hypothalamus. Stressful stimuli cause the hypothalamus to signal the adrenal medulla (which mediates short-term stress responses) via nerve impulses and the adrenal cortex, which mediates long-term stress responses, through the hormone adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary.
Short-term stress response occurs when the body responds by calling for the release of hormones that provide a burst of energy. The hormones epinephrine (or adrenaline) and norepinephrine (or noradrenaline) are released by the adrenal medulla. Epinephrine and norepinephrine provide a burst of energy by increasing blood glucose levels by stimulating the liver and skeletal muscles to break down glycogen and by stimulating glucose release by liver cells. Additionally, these hormones increase oxygen availability to cells by increasing the heart rate and dilating the bronchioles. The hormones also prioritize body function by increasing blood supply to essential organs such as the heart, brain, and skeletal muscles, while restricting blood flow to organs not in immediate need, such as the skin, digestive system, and kidneys. Epinephrine and norepinephrine are collectively called catecholamines.
Long-term stress response is different from short-term stress response because the body cannot sustain the bursts of energy from epinephrine and norepinephrine for long periods. Therefore, other hormones help out in the stress response. In a long-term stress response, the hypothalamus triggers the release of ACTH from the anterior pituitary gland. The adrenal cortex is stimulated by the ACTH to release steroid hormones called corticosteroids. Corticosteroids turn on transcription of certain genes in the nuclei of target cells. They change enzyme concentrations in the cytoplasm and affect cellular metabolism. There are two main corticosteroids: glucocorticoids such as cortisol, and mineralcorticoids such as aldosterone. These hormones target the breakdown of fat into fatty acids in the adipose tissue. The fatty acids are released into the bloodstream for other tissues to use for ATP production. The glucocorticoids primarily affect glucose metabolism by stimulating glucose synthesis. Glucocorticoids also have anti-inflammatory properties through inhibition of the immune system, however, it cannot be used long term because it suppresses the immune system.
Mineralcorticoids function to regulate ion and water balance of the body. The hormone aldosterone stimulates the reabsorption of water and sodium ions in the kidney, which results in increased blood pressure and volume.
Excessive production of glucocorticoids can cause Cushing's disease, which causes the shifting of fat storage areas of the body and can also cause the accumulation of adipose tissue in the face and neck, and excessive glucose in the blood. Hyposecretion, or excessive production of corticosteroids can cause Addison's disease, which can result in bronze skin, hypoglycemia, and low electrolyte levels in the blood.
The regulation of hormone production and release are primarily controlled by negative feedback systems, where a stimulus causes the release of a substance and once the substance reaches a certain level, it sends a signal that stops the further release of the substance. Therefore, the concentration of hormones in the blood can be regulated in this way.
Three mechanisms cause endocrine glands to be stimulated to synthesize and release hormones: humoral stimuli, hormonal stimuli, and neural stimuli.
Humoral stimuli refer to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. Hormonal stimuli refer to the release of hormones in response to another hormone, such as when endocrine glands release hormones when stimulated by hormones released by other endocrine glands. Neural stimuli occur when the nervous system directly stimulates endocrine glands to release hormones and the nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.
Endocrine glands in the endocrine system work together to maintain homeostasis. The endocrine system and the nervous system use chemical signals to communicate and regulate the body's physiology. The endocrine system releases hormones that act on target cells to regulate development, growth, energy metabolism, reproduction, and other behaviors. The nervous system releases neurotransmitters or neurohormones that regulate neurons, muscle cells, and endocrine cells. Since the neurons can regulate the release of hormones, the nervous system and endocrine systems work together to regulate the body physiology.
The hypothalamus in vertebrates integrates the endocrine and nervous systems. The hypothalamus is an endocrine organ located in the diencephalon of the brain and it receives input from the body and other brain areas and initiates endocrine responses to environmental changes. The hypothalamus synthesizes hormones and transports them along axons to the posterior pituitary gland. It synthesizes and secretes regulatory hormones that control the endocrine cells in the anterior pituitary gland. The hypothalamus contains autonomic centers that control endocrine cells in the adrenal medulla via neural control.
The pituitary gland is known as the hypophysis or master gland and is located at the base of the brain in the sella turcica, a groove of the sphenoid bone of the skull. The pituitary gland is attached to the hypothalamus via a stalk called the pituitary stalk (or infundibulum). The anterior portion of the pituitary gland is regulated by releasing or release-inhibiting hormones produced by the hypothalamus, and the posterior pituitary receives signals via neurosecretory cells to release hormones produced by the hypothalamus. The pituitary has two distinct regions: the anterior pituitary and posterior pituitary. Both of these together secrete nine different peptide or protein hormones. The posterior lobe of the pituitary gland contains axons of the hypothalamic neurons.
The anterior pituitary gland, or adenohypophysis, is surrounded by a capillary network that extends from the hypothalamus, down along the infundibulum, and to the anterior pituitary. This capillary network is part of the hypophyseal portal system that carries substances from the hypothalamus to the anterior pituitary and hormones from the anterior pituitary into the circulatory system. A portal system carries blood from one capillary network to another. Therefore, the hypophyseal portal system allows hormones produced by the hypothalamus to be carried directly to the anterior pituitary without first entering the circulatory system.
The anterior pituitary produces seven hormones: growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), melanin-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Anterior pituitary hormones are sometimes called tropic hormones because they control the functioning of other organs. While these hormones are produced by the anterior pituitary, their production is controlled by regulatory hormones produced by the hypothalamus. These hormones can be releasing or inhibiting hormones regulating the amount of hormones secreted. These hormones travel from the hypothalamus through the hypophyseal portal system to the anterior pituitary where they exert their effect. Negative feedback then regulates the amount of hormone secretion.
The posterior pituitary is significantly different in structure from the anterior pituitary. The posterior pituitary is part of the brain, extending down from the hypothalamus, and contains mostly nerve fibers and neuroglial cells, which support axons that extend from the hypothalamus to the posterior pituitary. The posterior pituitary and the infundibulum together are referred to as the neurohypophysis.
The hormones antidiuretic hormone (ADH), also known as vasopressin, and oxytocin are produced by neurons in the hypothalamus and transported with these axons along the infundibulum to the posterior pituitary. These hormones are released into the circulatory system via neural signaling from the hypothalamus.
The thyroid gland is located in the neck, just below the larynx and in front of the trachea. The thyroid gland is a butterfly-shaped gland with two lobes that are connected by the isthmus. The color is dark red due to its extensive vascular system. Sometimes the thyroid can swell during disfunction.
The thyroid is made up of many spherical thyroid follicles, which are lined with a simple cuboidal epithelium. These follicles contain a viscous fluid, called colloid, which stores the glycoprotein thyroglobulin, the precursor to thyroid hormones. The follicles produce hormones that can be stored in the colloid or released into the surrounding capillary network for transport to the rest of the body via the circulatory system.
Thyroid follicle cells synthesize the hormone thyroxine, (or T4, four atoms of iodine), and triiodothyronine (T3, three atoms of iodine). Follicle cells are stimulated to release stored T3 and T4 by thyroid stimulating hormone (TSH), which is produced by the anterior pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.
A third hormone, calcitonin, is produced by parafollicular cells of the thyroid either releasing hormones or inhibiting hormones. Calcitonin release is not controlled by TSH, but instead is released when calcium ion concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body fluids and acts in the bones to inhibit osteoclast activity and in the kidney to stimulate excretion of calcium, the combination of which lowers body fluid levels of calcium.
Parathyroid glands are located on the posterior surface of the thyroid gland and most people have between two to six of these glands. There is also usually a superior gland and inferior gland associated with the thyroid's two lobes. Each parathyroid gland is covered by connective tissue and contains many secretory cells that are associated with a capillary network. Parathyroid glands produce the parathyroid hormone (PTH). PTH increases blood calcium concentrations when calcium ion levels fall below normal. PTH enhances reabsorption of Ca2+ by the kidneys, stimulates osteoclast activity and inhibits osteoblast activity, and stimulates synthesis and secretion of calcitriol by the kidneys, which enhances Ca2+ absorption by the digestive system. PTH is produced by the chief cells of the parathyroid. PTH and calcitonin work in opposition to one another to maintain homeostatic calcium levels in the body fluids.
Adrenal glands are associated with the kidney, as one gland is located on top of each kidney. The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla and these regions secrete different hormones.
The adrenal cortex is made up of layers of epithelial cells and associated capillary networks. These layers form three distinct regions: an outer zona glomerulosa that produces mineralocorticoids, a middle zona fasciculata that produces glucocorticoids, and an inner zona reticularis that produces androgens.
The main mineralocorticoid is aldosterone, which regulates the concentration of Na+ ions in urine, sweat, pancreas, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.
The three main glucocorticoids are cortisol, corticosterone, and cortisone. The glucocorticoids stimulate the synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells and they promote the release of fatty acids from adipose tissue. These hormones increase blood glucose levels to maintain levels within a normal range between meals. These hormones are secreted in response to ACTH and levels are regulated by negative feedback.
The adrenal cortex also produces small amounts of testosterone precursor, although the role of this additional hormone production is not fully understood. Testosterone is a type of androgen that promotes a suite of characteristics such as the growth and development of the testes and penis, increased skeletal and muscular growth, enlargement of the larynx, increased growth and redistribution of body hair, and increased sexual drive. Testosterone secretion is regulated by both the hypothalamus and the anterior pituitary gland. The hypothalamus sends releasing hormones that stimulate the release of gonadotropins from the anterior pituitary gland. Testosterone produced in small amounts in the adrenal cortex may work with sex hormones released from the gonads.
The Adrenal medulla contains large, irregularly-shaped cells that are closely associated with blood vessels. These cells are innervated by preganglionic autonomic nerve fibers from the central nervous system. The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline) and another that produces norepinephrine (noadrenaline). Epinephrine is the primary adrenal medulla hormone accounting for 75 to 80 percent of its secretions. Epinephrine and norepinephrine increase heart rate, breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels. They also accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue.
The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous system. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic fibers innervating the adrenal medulla. These neural impulses originate from the hypothalamus in response to stress to prepare the body for the fight-or flight response.
The pancreas is an elongated organ that is located between the stomach and the proximal portion of the small intestine. It contains both exocrine cells that excrete digestive enzymes and endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both endocrine and exocrine functions.
The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans. The pancreatic islets contain two primary cell types: alpha cells, which produce the hormone glucagon, and beta cells, which produce the hormone insulin. These hormones regulate blood glucose levels. As blood glucose levels decline, alpha cells release glucagon to raise the blood glucose levels by increasing rates of glycogen breakdown and glucose release by the liver. When blood glucose levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by increasing the rate of glucose uptake in most body cells, and by increasing glycogen synthesis in skeletal muscles and the liver. Together, glucagon and insulin regulate blood glucose levels.
The pineal gland produces the hormone melatonin and the rate of melatonin production is affected by the photoperiod. Collaterals from the visual pathways innervate the pineal gland. During the day photoperiod, little melatonin is produced; however, melatonin production increases during the dark photoperiod (night). In some mammals, melatonin has an inhibitory effect on reproductive functions by decreasing production and maturation of sperm, oocytes, and reproductive organs. Melatonin is an effective antioxidant, protecting the CNS from free radicals such as nitric oxide and hydrogen peroxide. Lastly, melatonin is involved in biological rhythms, particularly circadian rhythms such as the sleep-wake cycle and eating habits.
The gonads include the testes and ovaries and produce steroid hormones. The testes produce androgens, testosterone being the most prominent, which allow for the growth and development of the testes and penis, increased skeletal and muscular growth, enlargement of the larynx, increased growth and redistribution of body hair, and the production of sperm cells. The ovaries produce estradiol and progesterone, which cause secondary sex characteristics and prepare the body for childbirth.
Endocrine glands and their associated hormones and effects include the following:
The hypothalamus endocrine gland and releasing and inhibiting hormones regulate hormone release from the pituitary gland. Also produces oxytocin, produces uterine contractions and milk secretion in breast tissue.
The hypothalamus endocrine gland and antidiuretic hormone (ADH) causes water reabsorption from the kidneys and vasoconstriction to increase blood pressure.
The pituitary (anterior) endocrine gland and the growth hormone (GH) promotes growth of body tissues, protein synthesis, and metabolic functions. The pituitary and prolactin (PRL) promote milk production. The pituitary and thyroid stimulating hormone (TSH) stimulates thyroid hormone release. The pituitary and adrenocorticotropic hormone (ACTH) stimulates hormone release by adrenal cortex and glucocorticoids. The pituitary and follicle-stimulating hormone (FSH) stimulates gamete production (both ova and sperm) and secretion of estradiol. The pituitary and luteinizing hormone (LH) stimulates androgen production by gonads, ovulation and secretion of progesterone. The pituitary and melanocyte-stimulating hormone (MSH) stimulates melanocytes of the skin increasing melanin pigment production.
The pituitary (posterior) endocrine gland and antidiuretic hormone (ADH) stimulates water reabsorption by the kidneys. The posterior pituitary and oxytocin hormone stimulates uterine contractions during childbirth and milk ejection, stimulates ductus deferens and prostate gland contraction during emission.
The thyroid endocrine gland and thyroxine, triiodothyronine hormones stimulate and maintain metabolism, growth, and development. The thyroid and calcitonin hormones reduce blood Ca2+ levels.
The parathyroid endocrine gland and parathyroid hormone (PTH) increases blood Ca2+ levels.
The adrenal cortex endocrine gland and aldosterone hormone increases blood Na+ levels and increases K+ secretion. The adrenal cortex and cortisol, corticosterone, and cortisone hormones increase blood glucose levels and have anti-inflammatory effects.
The adrenal medulla endocrine gland and epinephrine, norepinephrine hormones stimulate fight-or-flight responses, increase blood glucose levels and increase metabolic activities.
The pancreas endocrine gland and insulin hormone reduces blood glucose levels. The pancreas endocrine gland and glucagon hormone increases blood glucose levels.
The pineal endocrine gland and melatonin hormone regulates some biological rhythms and protects CNS from free radicals.
The testes endocrine gland and androgen hormones regulate, promote, increase, or maintain sperm production and include characteristics of growth and development of the testes and penis, increased skeletal and muscular growth, enlargement of the larynx, and increased growth and redistribution of body hair.
The ovaries endocrine gland and estrogen hormones promote uterine lining growth, increased development of breast tissue, redistribution of fat towards hips, legs, and breast and the maturation of the uterus and vagina. The ovaries endocrine gland and the progestins hormone promotes and maintains uterine lining growth.
Several organs in the body exist whose primary functions are not endocrine but also possess endocrine functions. These include the heart, kidneys, intestines, thymus, gonads, and adipose tissue.
The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These cells release the hormone atrial natriuretic peptide (ANP) in response to increased blood volume. High blood volume causes the cells to be stretched, resulting in hormone release. ANP also causes the kidneys to reduce the reabsorption of Na+, causing Na+ and water to be excreted in the urine. ANP also reduces the amount of renin released by the kidneys and aldosterone released by the adrenal cortex, which further prevents the retention of water. Therefore, ANP causes a reduction in blood volume and blood pressure, and reduces the concentration of Na+ in the blood.
The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located in the mucosa of the GI tract throughout the stomach and small intestine. Some of the hormones produced include gastrin, secretin, and cholecystokinin, which are secreted in the presence of food, and some of which act on other organs such as the pancreas, gallbladder, and liver. They trigger the release of gastric juices, which help to break down and digest food in the GI tract.
The kidneys have endocrine function in addition to the adrenal glands. Renin is released in response to decreased blood volume or pressure and is part of a system that leads to the release of aldosterone. Aldosterone then causes the retention of Na+ and water, raising blood volume. The kidneys also release calcitriol, which aids in the absorption of Ca2+ and phosphate ions.
Erythropoietin (EPO) is a protein hormone that triggers the formation of red blood cells in the bone marrow. EPO is released in response to low oxygen levels. Because red blood cells are oxygen carriers, increased production results in greater oxygen delivery throughout the body. High EPO levels can also cause health problems, however.
The thymus is found behind the sternum and it is most prominent in infants, becoming smaller in size through adulthood. The thymus produces hormones called thymosins, which contribute to the development of the immune response.
Adipose tissue is a connective tissue found throughout the body and it produces the hormone leptin in response to food intake. Leptin increases the activity of anorexigenic neurons, producing a feeling of satiety or fullness after eating, and therefore it reduces appetite. Leptin is also associated with improving reproduction fertility and it must be present for GnRH (gonadotropin-releasing hormone) and gonadotropin synthesis to occur. GnRH is the hormone produced by the hypothalamus in the brain that stimulates the pituitary gland to release two other hormones to regulate the gonads: FSH (follicle-stimulating hormone) and LH (luteinizing hormone).