Animal Body Form and Function Design by Owen Borville January 2, 2025 BIO 33
Animal body plans are patterns related to symmetry and include asymmetrical, radial, and bilateral forms.
Asymmetrical animals have no pattern of symmetry in the body, such as in a sponge marine animal. Radial symmetry occurs when the animal produces two equal halves when cut in half equally across any longitudinal line. Many radial animals are marine. Bilateral symmetry occurs when an animal can be cut into two equal halves at one location across a longitudinal bisecting line. Bilateral animals are found on land and in the oceans and these animals are highly mobile.
Terms describing animal form include anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach).
Fusiform shape is a body form among animals with bilateral symmetry and living in water characterized by a tubular shaped body that is tapered at both ends. Fusiform shape design allows the animal to move in the water or swim at high speeds by reducing drag forces on the body.
Because of drag forces higher in water than air, aquatic speed is slowed down in animals, but this force is less present in air with land mammals. However, land mammals must overcome the force of gravity to travel at a certain speed.
Most animals have an exoskeleton, a hard covering or shell that protects an animal's body from predators or water loss, and helps provide attachment of muscles in insects and invertebrates.
The exoskeleton is commonly composed of chitin and calcium carbonate, fused to the animal's epidermis. Apodemes are ingrowths of the exoskeleton that function as attachment sites for muscles, like tendons.
A new exoskeleton grows under the old exoskeleton, and as the old exoskeleton is shed, and the new exoskeleton appears and allows for growth. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. Therefore, most animals with an exoskeleton are small (insects, invertebrates).
Endoskeletons are similar to exoskeletons, but the muscles are attached on the outside, and increased mass is easier to accommodate. Animals with endoskeletons can grow as long as the tissues and muscles for movement can be supported. As the body size increases, both bone and muscle mass increase. The speed of the endoskeleton animal is determined by the amount of bone and muscle that support the movement of the entire body size and mass.
Diffusion is the process where the exchange of nutrients and wastes between a cell and its watery environment occurs. Diffusion limits the size that a cell in the body can become in single cell and multicellular animals. The efficiency of diffusion depends on the surface to volume ratio. As the cell becomes larger, its surface to volume ratio decreases, and diffusion becomes less efficient.
Multicellular organisms can become larger than single cell organisms because in multicellular organisms, individual cells become specialized on specific tasks, which takes pressure off of the rest of the cells in the body. So multiple cells are more efficient at doing fewer tasks. Cells in circulatory cells specialize on different tasks than respiratory cells and cells of other organ systems in the body. The surface to volume ratio applies to other areas of animal development, such as between muscle mass and cross-sectional surface area in supporting skeletons, in addition to muscle mass and heat circulation.
Animals obtain energy from food ingestion or absorption, which is converted to ATP for storage and use, glycogen, or triglycerides. Animal metabolism produces waste energy in the form of heat. Endothermic (warm-blooded) animals can conserve this heat and maintain relatively constant body temperature. Fur, fat, and feathers help animals conserve body heat and insulate. Ectothermic animals have an absence of insulation and are more dependent on the environment for body heat.
Metabolic rate is the amount of energy expended by an animal over a specific time (measured in joules or calories). Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Endothermic animals require a large amount of energy to maintain constant body temperature (up to 1,800 kcal/day) for humans and (60 kcal/day) for alligators (an ectotherm).
Smaller endothermic animals have a greater surface area for their mass compared to larger endothermic animals. These smaller endothermic animals lose heat at a faster rate than larger animals and require more energy to maintain constant internal temperature (higher BMR). The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR or SMR.
Torpor is a process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions of temperature or food shortage. Torpor can be used by animals for long periods, such as during a state of hibernation in the winter months, enabling them to maintain a reduced body temperature.
Estivation is torpor that occurs during the summer months with high temperature and little water (desert animals). Torpor can be used on a daily basis in bats and hummingbirds. Torpor daily allows some smaller animals to be endotherms.
In body form of animals, a standing vertebrate can be divided by several planes geometrically. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (coronal plane) separates the front from the back. A transverse plane (horizontal plane) divides the animal into upper and lower portions (also called cross-section). If the transverse cut is at an angle, it is an oblique plane.
Vertebrate animals have several body cavities, and two of these have several smaller cavities inside them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which contains the abdominal and pelvic cavities.
Animal Primary Tissues
Tissues are a group of similar cells that carry out related functions. Tissues combine to form organs with specific, specialized functions. Organs are organized into organ systems that perform particular functions. Organ systems combine to form one organism.
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. Epithelia composed of a single layer of cells is called simple epithelia. Epithelial tissue composed of multiple layers is called stratified epithelia. Different types of epithelial tissue include:
Squamous epithelial tissue has flat, irregular and round shape, and central nucleus. Cells fit together to form a covering. Simple squamous epithelial tissue are located in the lung alveoli and capillaries; facilitate diffusion and exchange gas, nutrients, and waste. Stratified squamous epithelial tissue is found in the skin, mouth, and vagina.
Cuboidal epithelial tissue is cubed shaped with a central nucleus and is located in the glands and renal tubules (kidney), liver. Usually these are a single layer of simple epithelia that make and secrete glandular material.
Columnar epithelial tissue is tall, narrow columnar stack with the nucleus toward the base and tall, narrow nucleus along the cell. Commonly found as a simple epithelial tissue and found in the digestive tract. Pseudostratified columnar epithelial tissue is found in the respiratory tract appear to be stratified but are actually one layer epithelial tissue as each cell is attached to the base membrane of the tissue. These cells absorb material of the lumen of the digestive tract and prepare it for entry into the body through the circular and lymphatic systems. This cellular covering has cilia at the apical or free surface of the cells. The cilia enhance the movement of mucus and trapped particles out of the respiratory tract, which helps protect the system from invasive microorganisms and harmful material that enters the body by respiration. Goblet cells are found in some tissues such as the lining of the trachea and these goblet cells contain mucus that traps irritants and prevents them from entering the lungs.
Transitional epithelial tissue is round and simple but appears stratified and is located only in the urinary system, primarily the urinary bladder. These cells appear to pile up on top of each other in a relaxed, empty bladder. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine entering. As the bladder expands, its lining becomes thinner.
Connective tissues are made up of a matrix consisting of living cells and a nonliving substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principle cell of the connective tissues is called the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and sometimes leukocytes can be found in some of the tissues. Some tissues have specialized cells. The matrix in the connective tissues gives the tissues its density. When a tissue has a high concentration of cells or fibers, it has proportionately a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin. This fiber can stretch to one and a half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected.
Loose connective tissue (areolar) has all of the components of connective tissue. It has fibroblasts and macrophages. Collagen fibers are relatively wide and light pink, while elastic fibers are thin and dark blue or black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps keep the vessel in place. The tissue is also found around and between most body organs. This tissue is tough but flexible and comprises membranes.
Fibrous connective tissues contain large amounts of collagen fibers and a few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage is a connective tissue with a large amount of matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae. A cartilage with few collagen and elastic fibers is hyaline cartilage. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Animals can have a cartilaginous skeleton or partially cartilaginous skeleton.
Elastic cartilage has a large amount of elastic fibers, which gives it great flexibility. Fibrocartilage contains a large amount of collagen fibers, which gives it great strength.
Bone (osseous tissue) is a connective tissue that has a large amount of matrix material, including organic and inorganic matrix. Organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers, and providing strength and flexibility to the tissue. The inorganic matrix consists of mineral salts, mostly calcium salts, that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks. Without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone has two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi, which connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae that serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Adipose tissue (fat tissue) is a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of material for viewing.
Blood is a connective tissue because it has a matrix. The living cell types are red blood cells (erythrocytes), and white blood cells (leukocytes). The fluid portion of the whole blood, the matrix, is commonly called plasma. The cell found in the greatest abundance in the blood is the erythrocyte, which are counted in the millions in a blood sample. Erythrocytes are usually the same size in a species but the size varies between species. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. However, other animals retain their nuclei and mitochondria during the cell's life. The main job of the erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood and number in the thousands in blood measurements. Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies.
Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that have entered the body.
Another leucocyte found in the peripheral blood is the monocyte, which gives rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Other leucocytes in the blood are eosinophils and basophils, both of which help facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow, called the platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. The primary function of blood is to transport material through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. These differ by the presence or absence of striations or bands, the number and location of nuclei, and whether they are voluntarily or involuntarily controlled, and their location in the body.
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus. Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle is also called non-striated because it lacks the banded appearance of skeletal and cardiac muscle. Walls of blood vessels, tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle. Smooth muscle is located in visceral organs.
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones.
Cardiac muscle is found only in the heart and has cross striations in its cells along with a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line that extends along the end of the cell as it borders the next cardiac cell in the row. This line is called an intercalated disc and it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron. The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also seen in the structure. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Homeostasis is the phenomenon where animal organs and organ systems constantly adjust to internal and external changes, such as temperature or level of glucose or calcium in the blood in order to maintain dynamic equilibrium in the body. The animal body is constantly adjusting to changes in the body's systems through homeostasis to reach an equilibrium state within specific ranges.
The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. The body systems can fluctuate from the set point, but eventually stay at or near the set point. A change in the internal or external environment is called a stimulus, which is detected by a receptor. The system responds to the stimulus by adjusting and moving toward the set point for such things as temperature and glucose level.
Homeostasis allows for adjustments when there are changes in the environment of the animal. The receptor senses the changes in the environment, and then a signal is sent to the control center (brain), which generates a response that is signaled to an effector. The effector is a muscle that contracts or relaxes or a gland that secretes. Homeostasis is maintained by negative feedback loops, as positive feedback loops cause the organism to move further from homeostasis. Homeostasis is controlled by the nervous and endocrine systems of mammals.
A negative feedback loop is the most prominent homeostatic mechanism that changes the direction of the stimulus, either by increasing or decreasing the stimulus. A high stimulus will be changed to a decreasing stimulus and a low stimulus will be changed to an increasing stimulus, such as with blood glucose levels. The endocrine system releases a specific hormone to lower (insulin) or raise (glucagon) glucose levels. A similar phenomenon occurs with the regulation of blood calcium levels in homeostasis.
A positive feedback loop maintains the direction of the stimulus, and can also accelerate the stimulus. An example of a positive feedback loop in animals is the chemical reactions that lead up to blood clotting, where these reactions occur until a clot is formed. Another example is the uterine contractions during childbirth made possible by the hormone oxytocin from the endocrine system stimulates the contraction of the uterus in females.
The set point of an animal system can be adjusted, as the feedback loop works to maintain the new setting, such as with blood pressure. Blood pressure can increase so much or for so long that the body recognizes this increase as normal and does not work to lower it. Blood pressure can then be lowered with medication, in a process called alteration.
Changes can be made in a group of body organ systems in order to maintain a set point in another system, called acclimatization. An example is when an animal moves to a higher altitude location than what it is accustomed. The higher altitude causes oxygen levels to decrease and the animal body must adjust by increasing the number of red blood cells in the blood circulation for distribution throughout the body. Another example is when animals grow a heavier coat in the winter cold season for increased heat protection and grow a lighter coat in the summer warm season for cooling.
Thermoregulation in homeostasis occurs as animal body temperature rises, enzyme activity rises as well in general. When the temperature becomes too high, enzymes and proteins begin to lose their function (about 50 degrees Celsius in mammals). Enzyme activity decreases in animals as temperature lowers.
Homeostasis in Endotherms and Ectotherms
Some animals can maintain a constant body temperature while other animals have a body temperature that varies with the environment. Ectotherms are animals that rely on external temperatures to set their body temperature. Ectotherms are commonly called cold-blooded, but some desert animals have a very high body temperature. Poikilotherms are animals with constantly varying internal temperatures. Homeotherms are animals that maintain a constant body temperature despite the environmental changes. Endotherms are animals that rely on internal sources for maintenance of relatively constant body temperature in varying environmental temperatures. Endotherms can maintain metabolic activity in cooler temperatures that an ectotherm cannot do. However, since ectotherms and poikilotherms usually live in a constant temperature environment, their body temperature can stay constant.
Heat can be exchanged between an animal and its environment using four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic heat waves from the sun and radiates from dry skin. Heat can be removed with liquid from a surface during evaporation, for example, when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.
Animals can conserve or dissipate heat in several ways. Some endothermic animals have insulation in the form of fur, fat layers, feathers, or a combination of these. Polar bears, seals, arctic foxes are examples of animals that can conserve heat and maintain body temperature. Arrector pili muscles (goose bumps) cause small hairs to stand up when a mammal is cold, increasing body temperature.
Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals can transfer heat from arteries to veins in the circulatory system, which warms the blood returning to the heart, called countercurrent heat exchange, which prevents the cold venous blood from cooling the heart and other organs. This same system can reverse this process in order to prevent the body from overheating.
Some ectothermic animals use changes in their behavior to help regulate body temperature, such as staying in cooler locations during warm temperature and staying in warmer locations in cooler weather. Some animals use water for cooling or stay in groups for warmth.
Many animals use metabolic waste heat as a heat source. When muscles are contracted, such as shivering, this wasted energy is converted into heat. Some animals have a type of tissue called brown fat that generates heat for the body.
The nervous system of animals helps control thermoregulation and the processes of homeostasis and temperature control are located in the hypothalamus of the animal brain. The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold.
The hypothalamus also responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the body temperature. This process causes body temperature increase during a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal's enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Heat itself can kill pathogens in the body and is a normal defense mechanism.
Animal body plans are patterns related to symmetry and include asymmetrical, radial, and bilateral forms.
Asymmetrical animals have no pattern of symmetry in the body, such as in a sponge marine animal. Radial symmetry occurs when the animal produces two equal halves when cut in half equally across any longitudinal line. Many radial animals are marine. Bilateral symmetry occurs when an animal can be cut into two equal halves at one location across a longitudinal bisecting line. Bilateral animals are found on land and in the oceans and these animals are highly mobile.
Terms describing animal form include anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach).
Fusiform shape is a body form among animals with bilateral symmetry and living in water characterized by a tubular shaped body that is tapered at both ends. Fusiform shape design allows the animal to move in the water or swim at high speeds by reducing drag forces on the body.
Because of drag forces higher in water than air, aquatic speed is slowed down in animals, but this force is less present in air with land mammals. However, land mammals must overcome the force of gravity to travel at a certain speed.
Most animals have an exoskeleton, a hard covering or shell that protects an animal's body from predators or water loss, and helps provide attachment of muscles in insects and invertebrates.
The exoskeleton is commonly composed of chitin and calcium carbonate, fused to the animal's epidermis. Apodemes are ingrowths of the exoskeleton that function as attachment sites for muscles, like tendons.
A new exoskeleton grows under the old exoskeleton, and as the old exoskeleton is shed, and the new exoskeleton appears and allows for growth. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. Therefore, most animals with an exoskeleton are small (insects, invertebrates).
Endoskeletons are similar to exoskeletons, but the muscles are attached on the outside, and increased mass is easier to accommodate. Animals with endoskeletons can grow as long as the tissues and muscles for movement can be supported. As the body size increases, both bone and muscle mass increase. The speed of the endoskeleton animal is determined by the amount of bone and muscle that support the movement of the entire body size and mass.
Diffusion is the process where the exchange of nutrients and wastes between a cell and its watery environment occurs. Diffusion limits the size that a cell in the body can become in single cell and multicellular animals. The efficiency of diffusion depends on the surface to volume ratio. As the cell becomes larger, its surface to volume ratio decreases, and diffusion becomes less efficient.
Multicellular organisms can become larger than single cell organisms because in multicellular organisms, individual cells become specialized on specific tasks, which takes pressure off of the rest of the cells in the body. So multiple cells are more efficient at doing fewer tasks. Cells in circulatory cells specialize on different tasks than respiratory cells and cells of other organ systems in the body. The surface to volume ratio applies to other areas of animal development, such as between muscle mass and cross-sectional surface area in supporting skeletons, in addition to muscle mass and heat circulation.
Animals obtain energy from food ingestion or absorption, which is converted to ATP for storage and use, glycogen, or triglycerides. Animal metabolism produces waste energy in the form of heat. Endothermic (warm-blooded) animals can conserve this heat and maintain relatively constant body temperature. Fur, fat, and feathers help animals conserve body heat and insulate. Ectothermic animals have an absence of insulation and are more dependent on the environment for body heat.
Metabolic rate is the amount of energy expended by an animal over a specific time (measured in joules or calories). Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Endothermic animals require a large amount of energy to maintain constant body temperature (up to 1,800 kcal/day) for humans and (60 kcal/day) for alligators (an ectotherm).
Smaller endothermic animals have a greater surface area for their mass compared to larger endothermic animals. These smaller endothermic animals lose heat at a faster rate than larger animals and require more energy to maintain constant internal temperature (higher BMR). The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR or SMR.
Torpor is a process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions of temperature or food shortage. Torpor can be used by animals for long periods, such as during a state of hibernation in the winter months, enabling them to maintain a reduced body temperature.
Estivation is torpor that occurs during the summer months with high temperature and little water (desert animals). Torpor can be used on a daily basis in bats and hummingbirds. Torpor daily allows some smaller animals to be endotherms.
In body form of animals, a standing vertebrate can be divided by several planes geometrically. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (coronal plane) separates the front from the back. A transverse plane (horizontal plane) divides the animal into upper and lower portions (also called cross-section). If the transverse cut is at an angle, it is an oblique plane.
Vertebrate animals have several body cavities, and two of these have several smaller cavities inside them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which contains the abdominal and pelvic cavities.
Animal Primary Tissues
Tissues are a group of similar cells that carry out related functions. Tissues combine to form organs with specific, specialized functions. Organs are organized into organ systems that perform particular functions. Organ systems combine to form one organism.
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. Epithelia composed of a single layer of cells is called simple epithelia. Epithelial tissue composed of multiple layers is called stratified epithelia. Different types of epithelial tissue include:
Squamous epithelial tissue has flat, irregular and round shape, and central nucleus. Cells fit together to form a covering. Simple squamous epithelial tissue are located in the lung alveoli and capillaries; facilitate diffusion and exchange gas, nutrients, and waste. Stratified squamous epithelial tissue is found in the skin, mouth, and vagina.
Cuboidal epithelial tissue is cubed shaped with a central nucleus and is located in the glands and renal tubules (kidney), liver. Usually these are a single layer of simple epithelia that make and secrete glandular material.
Columnar epithelial tissue is tall, narrow columnar stack with the nucleus toward the base and tall, narrow nucleus along the cell. Commonly found as a simple epithelial tissue and found in the digestive tract. Pseudostratified columnar epithelial tissue is found in the respiratory tract appear to be stratified but are actually one layer epithelial tissue as each cell is attached to the base membrane of the tissue. These cells absorb material of the lumen of the digestive tract and prepare it for entry into the body through the circular and lymphatic systems. This cellular covering has cilia at the apical or free surface of the cells. The cilia enhance the movement of mucus and trapped particles out of the respiratory tract, which helps protect the system from invasive microorganisms and harmful material that enters the body by respiration. Goblet cells are found in some tissues such as the lining of the trachea and these goblet cells contain mucus that traps irritants and prevents them from entering the lungs.
Transitional epithelial tissue is round and simple but appears stratified and is located only in the urinary system, primarily the urinary bladder. These cells appear to pile up on top of each other in a relaxed, empty bladder. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine entering. As the bladder expands, its lining becomes thinner.
Connective tissues are made up of a matrix consisting of living cells and a nonliving substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principle cell of the connective tissues is called the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and sometimes leukocytes can be found in some of the tissues. Some tissues have specialized cells. The matrix in the connective tissues gives the tissues its density. When a tissue has a high concentration of cells or fibers, it has proportionately a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin. This fiber can stretch to one and a half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected.
Loose connective tissue (areolar) has all of the components of connective tissue. It has fibroblasts and macrophages. Collagen fibers are relatively wide and light pink, while elastic fibers are thin and dark blue or black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps keep the vessel in place. The tissue is also found around and between most body organs. This tissue is tough but flexible and comprises membranes.
Fibrous connective tissues contain large amounts of collagen fibers and a few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones).
Cartilage is a connective tissue with a large amount of matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae. A cartilage with few collagen and elastic fibers is hyaline cartilage. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Animals can have a cartilaginous skeleton or partially cartilaginous skeleton.
Elastic cartilage has a large amount of elastic fibers, which gives it great flexibility. Fibrocartilage contains a large amount of collagen fibers, which gives it great strength.
Bone (osseous tissue) is a connective tissue that has a large amount of matrix material, including organic and inorganic matrix. Organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers, and providing strength and flexibility to the tissue. The inorganic matrix consists of mineral salts, mostly calcium salts, that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks. Without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone has two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi, which connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae that serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons.
Adipose tissue (fat tissue) is a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of material for viewing.
Blood is a connective tissue because it has a matrix. The living cell types are red blood cells (erythrocytes), and white blood cells (leukocytes). The fluid portion of the whole blood, the matrix, is commonly called plasma. The cell found in the greatest abundance in the blood is the erythrocyte, which are counted in the millions in a blood sample. Erythrocytes are usually the same size in a species but the size varies between species. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. However, other animals retain their nuclei and mitochondria during the cell's life. The main job of the erythrocyte is to carry and deliver oxygen to the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood and number in the thousands in blood measurements. Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies.
Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that have entered the body.
Another leucocyte found in the peripheral blood is the monocyte, which gives rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Other leucocytes in the blood are eosinophils and basophils, both of which help facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow, called the platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. The primary function of blood is to transport material through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. These differ by the presence or absence of striations or bands, the number and location of nuclei, and whether they are voluntarily or involuntarily controlled, and their location in the body.
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus. Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle is also called non-striated because it lacks the banded appearance of skeletal and cardiac muscle. Walls of blood vessels, tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle. Smooth muscle is located in visceral organs.
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones.
Cardiac muscle is found only in the heart and has cross striations in its cells along with a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line that extends along the end of the cell as it borders the next cardiac cell in the row. This line is called an intercalated disc and it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells.
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron. The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also seen in the structure. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Homeostasis is the phenomenon where animal organs and organ systems constantly adjust to internal and external changes, such as temperature or level of glucose or calcium in the blood in order to maintain dynamic equilibrium in the body. The animal body is constantly adjusting to changes in the body's systems through homeostasis to reach an equilibrium state within specific ranges.
The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. The body systems can fluctuate from the set point, but eventually stay at or near the set point. A change in the internal or external environment is called a stimulus, which is detected by a receptor. The system responds to the stimulus by adjusting and moving toward the set point for such things as temperature and glucose level.
Homeostasis allows for adjustments when there are changes in the environment of the animal. The receptor senses the changes in the environment, and then a signal is sent to the control center (brain), which generates a response that is signaled to an effector. The effector is a muscle that contracts or relaxes or a gland that secretes. Homeostasis is maintained by negative feedback loops, as positive feedback loops cause the organism to move further from homeostasis. Homeostasis is controlled by the nervous and endocrine systems of mammals.
A negative feedback loop is the most prominent homeostatic mechanism that changes the direction of the stimulus, either by increasing or decreasing the stimulus. A high stimulus will be changed to a decreasing stimulus and a low stimulus will be changed to an increasing stimulus, such as with blood glucose levels. The endocrine system releases a specific hormone to lower (insulin) or raise (glucagon) glucose levels. A similar phenomenon occurs with the regulation of blood calcium levels in homeostasis.
A positive feedback loop maintains the direction of the stimulus, and can also accelerate the stimulus. An example of a positive feedback loop in animals is the chemical reactions that lead up to blood clotting, where these reactions occur until a clot is formed. Another example is the uterine contractions during childbirth made possible by the hormone oxytocin from the endocrine system stimulates the contraction of the uterus in females.
The set point of an animal system can be adjusted, as the feedback loop works to maintain the new setting, such as with blood pressure. Blood pressure can increase so much or for so long that the body recognizes this increase as normal and does not work to lower it. Blood pressure can then be lowered with medication, in a process called alteration.
Changes can be made in a group of body organ systems in order to maintain a set point in another system, called acclimatization. An example is when an animal moves to a higher altitude location than what it is accustomed. The higher altitude causes oxygen levels to decrease and the animal body must adjust by increasing the number of red blood cells in the blood circulation for distribution throughout the body. Another example is when animals grow a heavier coat in the winter cold season for increased heat protection and grow a lighter coat in the summer warm season for cooling.
Thermoregulation in homeostasis occurs as animal body temperature rises, enzyme activity rises as well in general. When the temperature becomes too high, enzymes and proteins begin to lose their function (about 50 degrees Celsius in mammals). Enzyme activity decreases in animals as temperature lowers.
Homeostasis in Endotherms and Ectotherms
Some animals can maintain a constant body temperature while other animals have a body temperature that varies with the environment. Ectotherms are animals that rely on external temperatures to set their body temperature. Ectotherms are commonly called cold-blooded, but some desert animals have a very high body temperature. Poikilotherms are animals with constantly varying internal temperatures. Homeotherms are animals that maintain a constant body temperature despite the environmental changes. Endotherms are animals that rely on internal sources for maintenance of relatively constant body temperature in varying environmental temperatures. Endotherms can maintain metabolic activity in cooler temperatures that an ectotherm cannot do. However, since ectotherms and poikilotherms usually live in a constant temperature environment, their body temperature can stay constant.
Heat can be exchanged between an animal and its environment using four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic heat waves from the sun and radiates from dry skin. Heat can be removed with liquid from a surface during evaporation, for example, when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.
Animals can conserve or dissipate heat in several ways. Some endothermic animals have insulation in the form of fur, fat layers, feathers, or a combination of these. Polar bears, seals, arctic foxes are examples of animals that can conserve heat and maintain body temperature. Arrector pili muscles (goose bumps) cause small hairs to stand up when a mammal is cold, increasing body temperature.
Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals can transfer heat from arteries to veins in the circulatory system, which warms the blood returning to the heart, called countercurrent heat exchange, which prevents the cold venous blood from cooling the heart and other organs. This same system can reverse this process in order to prevent the body from overheating.
Some ectothermic animals use changes in their behavior to help regulate body temperature, such as staying in cooler locations during warm temperature and staying in warmer locations in cooler weather. Some animals use water for cooling or stay in groups for warmth.
Many animals use metabolic waste heat as a heat source. When muscles are contracted, such as shivering, this wasted energy is converted into heat. Some animals have a type of tissue called brown fat that generates heat for the body.
The nervous system of animals helps control thermoregulation and the processes of homeostasis and temperature control are located in the hypothalamus of the animal brain. The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold.
The hypothalamus also responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the body temperature. This process causes body temperature increase during a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal's enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Heat itself can kill pathogens in the body and is a normal defense mechanism.