Osmotic Regulation and Excretion BIO 41 by Owen Borville August 13, 2025
Osmosis is the diffusion of water across a membrane in response to osmotic pressure cause by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved into water. A non-electrolyte doesn't dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance.
The body's fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body, (such as pleural, serous, and cell membranes), are semi-permeable membranes. Semi permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. A cell placed in water tends to swell because of the water and low-salt environment (hypotonic). A cell placed in a higher salt concentration solution will tend to shrivel up due to water loss in the "high salt" environment (hypertonic). Isotonic cells have an equal concentration of solutes inside and outside the cell and osmotic pressure is equalized on both sides of the semi-permeable cell membrane.
In the body, there is constant input of water and electrolytes into the system. After osmoregulation, and excess electrolytes and wastes are transported to the kidneys and excreted to maintain osmotic balance.
Osmoregulation is needed because as food, water, and nutrients are consumed by humans and animals, and excreted in the form of sweat, urine, and feces, there is a need to regulate osmotic pressure and prevent the consumption of toxic waste and water.
Systems in mammals can regulate not only osmotic pressure across membranes, but also specific concentrations of important electrolytes in major body fluids: blood plasma, extracellular fluid, and intracellular fluid. The movement of water across membranes affects osmotic pressure, including blood pressure from blood plasma.
Electrolytes such as sodium chloride ionize in water and dissociate into ions. Electrolytes are lost from the body during urination and perspiration (sweating). Therefore, these electrolytes should be replenished with fluids with electrolytes.
Osmotic pressure is influenced by the concentration of solutes in a solution and is directly proportional to the number of solute atoms or molecules, and not dependent on the size of the solute molecules. Since electrolytes dissociate into ions, more solute particles are added to the solution and this has a greater affect on osmotic pressure than compounds that do not dissociate in water.
Water can pass through membranes by passive diffusion, but electrolyte ions cannot and require a special mechanism to cross the semi-permeable membranes in the body. This special mechanism is through facilitated diffusion and active transport. Facilitated diffusion requires protein-based channels for moving the solute. Active transport requires energy from ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.
Osmotic pressure is calculated by units of the mole, molarity, and molality. The units for measuring solutes are the mole. One mole is the gram molecular weight of the solute. Molarity is the number of moles of solute per liter of solution. The molality is the number of moles of solute per kilogram of solvent. With a water solvent, one kilogram of water is equal to one liter of water.
Electrolyte concentrations are calculated in milliequivalents per liter (mEq/L) equal to the ion concentration in millimoles multiplied by the number of electrical charges on the ion.
Osmoregulatory mechanisms help organisms such as fish survive aquatic environments of variable salinity from freshwater to saltwater. For example, when fish are live in freshwater, they drink less water, actively take up ions through their gills, absorb water through their skin, and excretes dilute urine. In a saltwater environment, however, the fish drink much water, lose water through its skin, excrete ions through its gills, and excrete concentrated urine.
Most marine invertebrates are isotonic in seawater, or osmoconformers, because their body concentrations and osmotic pressures conform to changes in seawater concentration to match the seawater. Cartilaginous marine fishes like sharks use organic compounds like urea and TMAO (trymethylamine oxide) to achieve isotonicity with the sea. Urea helps with osmotic balance but destabilizes proteins while TMAO in turn, stabilizes proteins.
The kidneys are the primary organ for osmoregulation, but the skin and lungs also play a role. Water and electrolytes are lost through sweat glands in the skin, and the lungs also release a small amount of water.
The kidneys are a pair of bean-shaped structures that are located just below and posterior to the liver in the peritoneal cavity of the body. The adrenal glands sit on top of each kidney and are also called the suprarenal glands. Kidneys filter blood and purify it many times per day. Kidneys use 25 percent of the oxygen absorbed through the lungs to perform this process by creating energy in the form of ATP through aerobic respiration. The filtrate coming out of the kidneys is called urine.
The kidneys are surrounded by three layers. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule, which helps anchor the kidneys in place. The third and most inner layer is the renal capsule.
Inside the kidney, there are three regions: the outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney and it is also the point of exit for the ureters. The renal cortex is granular due to the presence of nephrons, which are the functional unit of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. Each kidney has about eight renal pyramids. The renal pyramids along with the adjoining cortical region are called the lobes of the kidney. The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood supply starts with the branching of the aorta into the renal arteries and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery splits further into several interlobar arteries and enters the renal columns, which supply the renal lobes. The interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate "bow shaped" arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries radiate out from the arcuate arteries. The cortical radiate arteries branch into numerous afferent arterioles, and then enter the capillaries of the supplying the nephrons. Veins trace the path of the arteries and have similar names, except there are no segmental veins.
The nephron is the functional unit of the kidney. Each kidney is made up of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned sagittally. There are two types of nephrons: The cortical nephrons are 85 percent of nephrons and are located deep in the renal cortex. Juxtamedullary nephrons are 15 percent of nephrons and are located in the renal cortex close to the renal medulla. A nephron consists of three parts: a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries.
The renal corpuscle, located in the renal cortex, is made of a network of capillaries known as the glomerulus and the capsule, a cup-shaped chamber that surrounds it, is called the glomerular or Bowman's capsule.
The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function: The proximal convoluted tubule (PCT) is near the glomerulus and stays in the renal cortex. The Loop of Henle, or nephritic loop, forms a loop with ascending and descending limbs that goes through the renal medulla. The third part is the distal convoluted tubule (DCT) which is also located in the renal cortex. The DCT is the last part of the nephron and connects and empties its contents into collecting ducts that line the medullary pyramids. The collecting ducts collect contents from multiple nephrons and fuse together as they enter the papillae of the renal medulla.
The capillary network that originates from the renal arteries supplies the nephron with blood that needs to be filtered. The branch that enters the glomerulus is called the afferent arteriole. The branch that exits the glomerulus is called the afferent arteriole. Within the glomerulus, the network of capillaries is called the glomerular capillary bed. Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network, which surrounds and interacts with the parts of the renal tubule. In cortical nephrons, the peritubular capillary network surrounds the PCT and DCT. In juxtamedullary nephrons, the peritubular capillary network forms a network around the loop of Henle and is called the vasa recta.
Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus and most solutes except for proteins are filtered out into the glomerulus by a process called glomerular filtration. Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network. Water is also reabsorbed during this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is basically the opposite of tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis.
Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole, but proteins are not filtered out. Glomerular filtration rate is the volume of glomerular filtrate formed per minute by the kidneys and is regulated by multiple mechanisms.
Tubular reabsorption occurs in the PCT part of the renal tubule. Most nutrients are reabsorbed by passive or active transport. Reabsorption of water and electrolytes are regulated and can be influenced by hormones. Sodium Na+ is the most abundant ion. Water is also independently reabsorbed into the peritubular capillaries by pressure differences. Every solute has a transport maximum and the excess is not reabsorbed.
In the loop of Henle, the permeability of the membrane changes in each limb. The descending limb is permeable to water but not solutes. The opposite is true for the ascending limb. Because each limb performs the opposite function, it acts as a countercurrent multiplier and the vasa recta around it acts as the countercurrent exchanger.
When the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. Additional water can be reabsorbed. Waste products and urea are excreted by tubular secretion in the DCT. Balance of pH is maintained in the kidney by secreting excess H+ ions.
Chemical messengers between different cell types are exchanged in the renal tubules, both proximal and distal. For example, the macula densa is a mass of cells in the loop of Henle that are in contact with the mass of juxtaglomerular cells.
Some animals don't have kidneys, like microorganisms, worms, and insects, but have excretory systems that include vacuoles, flame cells, and Malpighian tubules.
Microorganisms like bacteria, protozoa, and fungi use contractile vacuoles to remove waste. These microorganisms have cells that are bound by cell membranes. Some cells are able to ingest food by endocytosis when vesicles are formed by involution of the cell membrane within the cell. The food vesicle fuses with a lysosome, which digests the food. Waste is excreted by exocytosis.
Metabolites can also be exchanged with the intracellular environment. Contractile vacuoles like those in the amoeba merge with the cell membrane and remove waste to the environment during exocytosis. Contractile vacuoles should not be confused with vacuoles, which store food and water in the cell.
Flatworms called Planaria live in freshwater and have an excretory system with two tubules connected to a highly branched duct system. The cells in the tubules are called flame cells (or protonephridia) because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope. The cilia push waste matter down the tubules and out of the body through excretory pores that open on the body surface. Cilia also pull water from the interstitial fluid and filtration is performed. Any valuable metabolites are recovered by reabsorption. Flame cells are found in flatworms, tapeworms, and planaria. Flame cells also maintain the organisms osmotic balance.
Earthworms (annelids) have excretory structures called nephridia and a pair of nephridia is located on each segment of the earthworm. These are similar to flame cells in that they have a tubule with cilia. Excretion occurs through a pore called the nephridiopore. These have a system for tubular reabsorption by a capillary network before excretion.
Malpighian tubules are the excretory system of insects and these tubules are found lining the gut of some arthropods like the bee. These tubes are usually in pairs and the number of tubules varies with each insect. The tubes are convoluted (twisted or folded), which increases their surface area, and they are lined with microvilli for reabsorption and maintenance of the osmotic balance. The tubes work with specialized glands in the wall of the rectum. Body fluids are not filtered and urine is produced by tubular secretion mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph (a mixture of blood and interstitial fluid in insects). Metabolic wastes like uric acid freely diffuse in the tubules. Exchange pumps line the tubules, and ions are exchanged as H+ ions are transferred in and K+ and Na+ ions out of the cell. Water forms urine. The secretion of ions alters the osmotic pressure, which brings water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low water environments, and uric acid is excreted as thick paste or powder. Not dissolving wastes in water helps these organisms to conserve water, which is very important in dry environments.
Nitrogen is present in proteins and nucleic acid macromolecules. When nitrogen-containing macromolecules break down, carbon, hydrogen, and oxygen are extracted and stored in the form of carbohydrates and fats. Excess nitrogen is excreted from the body.
Nitrogenous wastes tend to form toxic ammonia, which raises the pH of body fluids. The formation of ammonia itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system. Animals that live in aquatic environments tend to release ammonia into the water. Animals that excrete ammonia are called ammonotelic.
Terrestrial organisms have other mechanisms to excrete nitrogenous wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric acid. Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce uric acid. Animals that secrete urea as the primary nitrogenous waste material are called ureotelic animals.
The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The chemical reaction conversion of ammonia to urea:
2NH3 (ammonia) + CO2 +3ATP+H2O => H2N-CO-NH2(urea)+2ADP+4Pi+AMP
The urea cycle has five intermediate steps, catalyzed by five different enzymes to convert ammonia to urea. The amino acid L-ornithine is converted into different intermediates before being regenerated at the end of the urea cycle. The urea cycle is also called the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to the accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function.
Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or guanine (guano) instead of urea. Mammals also form some uric acid during breakdown of nucleic acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder and is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea. Many invertebrates and aquatic species excrete ammonia. Mammals, adult amphibians, and some marine species excrete urea. Insects, land snails, birds, and many reptiles excrete uric acid.
Mammals use uric acid crystals as an antioxidant. However, too much uric acid can lead to form kidney stones or gout, where uric acid crystals accumulate in the joints.
Hormones in the body help control osmoregulatory functions in partnership with the kidneys. Hormones are small molecules that act as messengers within the body. Hormones are usually secreted from one cell and travel in the bloodstream to affect a target cell in another location in the body. Different regions of the nephron bear specialized cells that have receptors to respond to chemical messengers and hormones.
Hormones that affect osmoregulation include:
Epinephrine and norepinephrine are hormones produced in the adrenal medulla when the body is under stress and can decrease kidney function temporarily by vasoconstriction of the smooth muscles of the blood vessels. Blood flow to the nephrons stops. Then the renin-angiotensin-aldosterone hormones are triggered.
The renin-angiotensin-aldosterone hormone system produces angiotensin II, which stabilizes blood pressure and volume.
Renin is a hormone produced in the kidney nephrons that increases blood pressure by acting on angiotensinogen. Renin is secreted by a part of the juxtaglomerular complex and is produced by the granular cells of the afferent and efferent arterioles to control blood pressure and volume. Renin converts angiotensinogen made in the liver to convert it to angiotensin I.
Angiotensin is a hormone produced in the liver that affects multiple processes and increases blood pressure. The ACE (angiotensin converting enzyme) converts angiotensin I to angiotensin II. Angiotensin II raises blood pressure by constricting blood vessels and also triggers the release of mineralocorticoid aldosterone from the adrenal cortex, which in turn stimulates the renal tubules to reabsorb more sodium. Angiotensin II also causes the release of anti-diuretic hormone (ADH) from the hypothalamus, leading to more water retention in the kidneys. It acts directly on the nephrons and decreases glomerular filtration rate.
Aldosterone is a hormone produced in the adrenal cortex that prevents loss of sodium and water.
Mineralocorticoids are hormones made by the adrenal cortex that affect osmotic balance. Aldosterone is a mineralocorticoid that regulates sodium levels in the blood. In addition, aldosterone also manages water levels in body fluids. Aldosterone also stimulates potassium secretion along with sodium reabsorption.
Anti-diuretic hormone ADH (vasopressin) is a hormone produced in the hypothalamus and stored and released in the posterior pituitary that prevents water loss. ADH helps the body conserve water when body fluid volume, especially the blood, is low. ADH acts by inserting aquaporins in the collecting ducts and promotes the reabsorption of water. ADH also acts as a vasoconstrictor and increases blood pressure during hemorrhaging.
Atrial natriuretic peptide (ANP) is produced in the heart atrium and decreases blood pressure by acting as a vasodilator and increasing glomerular filtration rate. ANP also decreases sodium reabsorption in the kidneys. ANP is released by cells in the atrium of the heart in response to high blood pressure and in patients with sleep apnea. ANP affects salt release, and because water passively follows salt to maintain osmotic balance, it also has a diuretic effect. ANP also prevents sodium reabsorption by the renal tubules, decreasing water reabsorption and acting as a diuretic, thereby lowering blood pressure. ANP stops the actions of aldosterone, ADH, and renin.
Osmosis is the diffusion of water across a membrane in response to osmotic pressure cause by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved into water. A non-electrolyte doesn't dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance.
The body's fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body, (such as pleural, serous, and cell membranes), are semi-permeable membranes. Semi permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. A cell placed in water tends to swell because of the water and low-salt environment (hypotonic). A cell placed in a higher salt concentration solution will tend to shrivel up due to water loss in the "high salt" environment (hypertonic). Isotonic cells have an equal concentration of solutes inside and outside the cell and osmotic pressure is equalized on both sides of the semi-permeable cell membrane.
In the body, there is constant input of water and electrolytes into the system. After osmoregulation, and excess electrolytes and wastes are transported to the kidneys and excreted to maintain osmotic balance.
Osmoregulation is needed because as food, water, and nutrients are consumed by humans and animals, and excreted in the form of sweat, urine, and feces, there is a need to regulate osmotic pressure and prevent the consumption of toxic waste and water.
Systems in mammals can regulate not only osmotic pressure across membranes, but also specific concentrations of important electrolytes in major body fluids: blood plasma, extracellular fluid, and intracellular fluid. The movement of water across membranes affects osmotic pressure, including blood pressure from blood plasma.
Electrolytes such as sodium chloride ionize in water and dissociate into ions. Electrolytes are lost from the body during urination and perspiration (sweating). Therefore, these electrolytes should be replenished with fluids with electrolytes.
Osmotic pressure is influenced by the concentration of solutes in a solution and is directly proportional to the number of solute atoms or molecules, and not dependent on the size of the solute molecules. Since electrolytes dissociate into ions, more solute particles are added to the solution and this has a greater affect on osmotic pressure than compounds that do not dissociate in water.
Water can pass through membranes by passive diffusion, but electrolyte ions cannot and require a special mechanism to cross the semi-permeable membranes in the body. This special mechanism is through facilitated diffusion and active transport. Facilitated diffusion requires protein-based channels for moving the solute. Active transport requires energy from ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.
Osmotic pressure is calculated by units of the mole, molarity, and molality. The units for measuring solutes are the mole. One mole is the gram molecular weight of the solute. Molarity is the number of moles of solute per liter of solution. The molality is the number of moles of solute per kilogram of solvent. With a water solvent, one kilogram of water is equal to one liter of water.
Electrolyte concentrations are calculated in milliequivalents per liter (mEq/L) equal to the ion concentration in millimoles multiplied by the number of electrical charges on the ion.
Osmoregulatory mechanisms help organisms such as fish survive aquatic environments of variable salinity from freshwater to saltwater. For example, when fish are live in freshwater, they drink less water, actively take up ions through their gills, absorb water through their skin, and excretes dilute urine. In a saltwater environment, however, the fish drink much water, lose water through its skin, excrete ions through its gills, and excrete concentrated urine.
Most marine invertebrates are isotonic in seawater, or osmoconformers, because their body concentrations and osmotic pressures conform to changes in seawater concentration to match the seawater. Cartilaginous marine fishes like sharks use organic compounds like urea and TMAO (trymethylamine oxide) to achieve isotonicity with the sea. Urea helps with osmotic balance but destabilizes proteins while TMAO in turn, stabilizes proteins.
The kidneys are the primary organ for osmoregulation, but the skin and lungs also play a role. Water and electrolytes are lost through sweat glands in the skin, and the lungs also release a small amount of water.
The kidneys are a pair of bean-shaped structures that are located just below and posterior to the liver in the peritoneal cavity of the body. The adrenal glands sit on top of each kidney and are also called the suprarenal glands. Kidneys filter blood and purify it many times per day. Kidneys use 25 percent of the oxygen absorbed through the lungs to perform this process by creating energy in the form of ATP through aerobic respiration. The filtrate coming out of the kidneys is called urine.
The kidneys are surrounded by three layers. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule, which helps anchor the kidneys in place. The third and most inner layer is the renal capsule.
Inside the kidney, there are three regions: the outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney and it is also the point of exit for the ureters. The renal cortex is granular due to the presence of nephrons, which are the functional unit of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. Each kidney has about eight renal pyramids. The renal pyramids along with the adjoining cortical region are called the lobes of the kidney. The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood supply starts with the branching of the aorta into the renal arteries and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery splits further into several interlobar arteries and enters the renal columns, which supply the renal lobes. The interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate "bow shaped" arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries radiate out from the arcuate arteries. The cortical radiate arteries branch into numerous afferent arterioles, and then enter the capillaries of the supplying the nephrons. Veins trace the path of the arteries and have similar names, except there are no segmental veins.
The nephron is the functional unit of the kidney. Each kidney is made up of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned sagittally. There are two types of nephrons: The cortical nephrons are 85 percent of nephrons and are located deep in the renal cortex. Juxtamedullary nephrons are 15 percent of nephrons and are located in the renal cortex close to the renal medulla. A nephron consists of three parts: a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries.
The renal corpuscle, located in the renal cortex, is made of a network of capillaries known as the glomerulus and the capsule, a cup-shaped chamber that surrounds it, is called the glomerular or Bowman's capsule.
The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function: The proximal convoluted tubule (PCT) is near the glomerulus and stays in the renal cortex. The Loop of Henle, or nephritic loop, forms a loop with ascending and descending limbs that goes through the renal medulla. The third part is the distal convoluted tubule (DCT) which is also located in the renal cortex. The DCT is the last part of the nephron and connects and empties its contents into collecting ducts that line the medullary pyramids. The collecting ducts collect contents from multiple nephrons and fuse together as they enter the papillae of the renal medulla.
The capillary network that originates from the renal arteries supplies the nephron with blood that needs to be filtered. The branch that enters the glomerulus is called the afferent arteriole. The branch that exits the glomerulus is called the afferent arteriole. Within the glomerulus, the network of capillaries is called the glomerular capillary bed. Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network, which surrounds and interacts with the parts of the renal tubule. In cortical nephrons, the peritubular capillary network surrounds the PCT and DCT. In juxtamedullary nephrons, the peritubular capillary network forms a network around the loop of Henle and is called the vasa recta.
Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus and most solutes except for proteins are filtered out into the glomerulus by a process called glomerular filtration. Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network. Water is also reabsorbed during this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is basically the opposite of tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis.
Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole, but proteins are not filtered out. Glomerular filtration rate is the volume of glomerular filtrate formed per minute by the kidneys and is regulated by multiple mechanisms.
Tubular reabsorption occurs in the PCT part of the renal tubule. Most nutrients are reabsorbed by passive or active transport. Reabsorption of water and electrolytes are regulated and can be influenced by hormones. Sodium Na+ is the most abundant ion. Water is also independently reabsorbed into the peritubular capillaries by pressure differences. Every solute has a transport maximum and the excess is not reabsorbed.
In the loop of Henle, the permeability of the membrane changes in each limb. The descending limb is permeable to water but not solutes. The opposite is true for the ascending limb. Because each limb performs the opposite function, it acts as a countercurrent multiplier and the vasa recta around it acts as the countercurrent exchanger.
When the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. Additional water can be reabsorbed. Waste products and urea are excreted by tubular secretion in the DCT. Balance of pH is maintained in the kidney by secreting excess H+ ions.
Chemical messengers between different cell types are exchanged in the renal tubules, both proximal and distal. For example, the macula densa is a mass of cells in the loop of Henle that are in contact with the mass of juxtaglomerular cells.
Some animals don't have kidneys, like microorganisms, worms, and insects, but have excretory systems that include vacuoles, flame cells, and Malpighian tubules.
Microorganisms like bacteria, protozoa, and fungi use contractile vacuoles to remove waste. These microorganisms have cells that are bound by cell membranes. Some cells are able to ingest food by endocytosis when vesicles are formed by involution of the cell membrane within the cell. The food vesicle fuses with a lysosome, which digests the food. Waste is excreted by exocytosis.
Metabolites can also be exchanged with the intracellular environment. Contractile vacuoles like those in the amoeba merge with the cell membrane and remove waste to the environment during exocytosis. Contractile vacuoles should not be confused with vacuoles, which store food and water in the cell.
Flatworms called Planaria live in freshwater and have an excretory system with two tubules connected to a highly branched duct system. The cells in the tubules are called flame cells (or protonephridia) because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope. The cilia push waste matter down the tubules and out of the body through excretory pores that open on the body surface. Cilia also pull water from the interstitial fluid and filtration is performed. Any valuable metabolites are recovered by reabsorption. Flame cells are found in flatworms, tapeworms, and planaria. Flame cells also maintain the organisms osmotic balance.
Earthworms (annelids) have excretory structures called nephridia and a pair of nephridia is located on each segment of the earthworm. These are similar to flame cells in that they have a tubule with cilia. Excretion occurs through a pore called the nephridiopore. These have a system for tubular reabsorption by a capillary network before excretion.
Malpighian tubules are the excretory system of insects and these tubules are found lining the gut of some arthropods like the bee. These tubes are usually in pairs and the number of tubules varies with each insect. The tubes are convoluted (twisted or folded), which increases their surface area, and they are lined with microvilli for reabsorption and maintenance of the osmotic balance. The tubes work with specialized glands in the wall of the rectum. Body fluids are not filtered and urine is produced by tubular secretion mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph (a mixture of blood and interstitial fluid in insects). Metabolic wastes like uric acid freely diffuse in the tubules. Exchange pumps line the tubules, and ions are exchanged as H+ ions are transferred in and K+ and Na+ ions out of the cell. Water forms urine. The secretion of ions alters the osmotic pressure, which brings water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low water environments, and uric acid is excreted as thick paste or powder. Not dissolving wastes in water helps these organisms to conserve water, which is very important in dry environments.
Nitrogen is present in proteins and nucleic acid macromolecules. When nitrogen-containing macromolecules break down, carbon, hydrogen, and oxygen are extracted and stored in the form of carbohydrates and fats. Excess nitrogen is excreted from the body.
Nitrogenous wastes tend to form toxic ammonia, which raises the pH of body fluids. The formation of ammonia itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system. Animals that live in aquatic environments tend to release ammonia into the water. Animals that excrete ammonia are called ammonotelic.
Terrestrial organisms have other mechanisms to excrete nitrogenous wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric acid. Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce uric acid. Animals that secrete urea as the primary nitrogenous waste material are called ureotelic animals.
The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The chemical reaction conversion of ammonia to urea:
2NH3 (ammonia) + CO2 +3ATP+H2O => H2N-CO-NH2(urea)+2ADP+4Pi+AMP
The urea cycle has five intermediate steps, catalyzed by five different enzymes to convert ammonia to urea. The amino acid L-ornithine is converted into different intermediates before being regenerated at the end of the urea cycle. The urea cycle is also called the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to the accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function.
Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or guanine (guano) instead of urea. Mammals also form some uric acid during breakdown of nucleic acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder and is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea. Many invertebrates and aquatic species excrete ammonia. Mammals, adult amphibians, and some marine species excrete urea. Insects, land snails, birds, and many reptiles excrete uric acid.
Mammals use uric acid crystals as an antioxidant. However, too much uric acid can lead to form kidney stones or gout, where uric acid crystals accumulate in the joints.
Hormones in the body help control osmoregulatory functions in partnership with the kidneys. Hormones are small molecules that act as messengers within the body. Hormones are usually secreted from one cell and travel in the bloodstream to affect a target cell in another location in the body. Different regions of the nephron bear specialized cells that have receptors to respond to chemical messengers and hormones.
Hormones that affect osmoregulation include:
Epinephrine and norepinephrine are hormones produced in the adrenal medulla when the body is under stress and can decrease kidney function temporarily by vasoconstriction of the smooth muscles of the blood vessels. Blood flow to the nephrons stops. Then the renin-angiotensin-aldosterone hormones are triggered.
The renin-angiotensin-aldosterone hormone system produces angiotensin II, which stabilizes blood pressure and volume.
Renin is a hormone produced in the kidney nephrons that increases blood pressure by acting on angiotensinogen. Renin is secreted by a part of the juxtaglomerular complex and is produced by the granular cells of the afferent and efferent arterioles to control blood pressure and volume. Renin converts angiotensinogen made in the liver to convert it to angiotensin I.
Angiotensin is a hormone produced in the liver that affects multiple processes and increases blood pressure. The ACE (angiotensin converting enzyme) converts angiotensin I to angiotensin II. Angiotensin II raises blood pressure by constricting blood vessels and also triggers the release of mineralocorticoid aldosterone from the adrenal cortex, which in turn stimulates the renal tubules to reabsorb more sodium. Angiotensin II also causes the release of anti-diuretic hormone (ADH) from the hypothalamus, leading to more water retention in the kidneys. It acts directly on the nephrons and decreases glomerular filtration rate.
Aldosterone is a hormone produced in the adrenal cortex that prevents loss of sodium and water.
Mineralocorticoids are hormones made by the adrenal cortex that affect osmotic balance. Aldosterone is a mineralocorticoid that regulates sodium levels in the blood. In addition, aldosterone also manages water levels in body fluids. Aldosterone also stimulates potassium secretion along with sodium reabsorption.
Anti-diuretic hormone ADH (vasopressin) is a hormone produced in the hypothalamus and stored and released in the posterior pituitary that prevents water loss. ADH helps the body conserve water when body fluid volume, especially the blood, is low. ADH acts by inserting aquaporins in the collecting ducts and promotes the reabsorption of water. ADH also acts as a vasoconstrictor and increases blood pressure during hemorrhaging.
Atrial natriuretic peptide (ANP) is produced in the heart atrium and decreases blood pressure by acting as a vasodilator and increasing glomerular filtration rate. ANP also decreases sodium reabsorption in the kidneys. ANP is released by cells in the atrium of the heart in response to high blood pressure and in patients with sleep apnea. ANP affects salt release, and because water passively follows salt to maintain osmotic balance, it also has a diuretic effect. ANP also prevents sodium reabsorption by the renal tubules, decreasing water reabsorption and acting as a diuretic, thereby lowering blood pressure. ANP stops the actions of aldosterone, ADH, and renin.