1The first homeostatic mechanism that we will examine is thermoregulation, the process by which animals maintain an internal temperature within a tolerable range. Each species of which animals maintain an internal temperature within a tolerable range. Each species of animal has an optimal temperature range. Thermoregulation helps keep body temperature within that range, enabling cells to function optimally even when the external temperature fluctuates. Internal metabolism and the external environment provide the sources of heat for thermoregulation. Most birds and mammals are ENDOTHERMS, meaning that they are warmed mostly by heat generated by their own metabolism. In contrast, most amphibians, lizards, many fishes, and most invertebrates are ECTOTHERMS, meaning that they gain most of their heat from external sources. Keep in mind, though, that endothermic and ectothermic modes of thermoregulation are not mutually exclusive. 2An animal can exchange heat with the environment by four physical processes. CONDUCTION is the transfer of thermal motion (heat) between molecules of objects that CONDUCTION is the transfer of thermal motion (heat) between molecules of objects that are in direct contact, as when an animal is physically touching an object in its environment. Heat is always transferred from an object of higher temperature to one of lower temperature. CONVECTION is the transfer of heat by the movement of air or liquid past a surface. RADIATION is the emission of electromagnetic waves. Radiation can transfer heat between objects that are not in direct contact, as when an animal absorbs heat radiating from the sun.EVAPORATION is the loss of heat from the surface of a liquid that is losing some of its molecules as a gas. A lizard loses heat as moisture evaporates from its nostrils.3METABOLIC HEAT PRODUCTION: In cold weather,hormonal changes tend to boost the metabolic rate of birds and In cold weather,hormonal changes tend to boost the metabolic rate of birds and mammals, increasing their heat production. Simply moving around more or shivering also produces heat as a metabolic by-product of the contraction of skeletal muscles. INSULATION: Adaption in mammals and birds is insulation- hair (often called fur), feathers, or fat layers. Most land mammals and birds react to cold by raising their fur or feathers, which traps a thicker layer of air next to the warm skin, improving insulation. Aquatic mammals and aquatic birds are insulated by a thick layer of fat called BLUBBER.CIRCULATORY ADAPTIONS:In COUNTERCURRENT HEAT EXCHANGE, warm and cold blood flow in opposite (countercurrent) directions in two adjacent blood vessels. EVAPORATIVE COOLING: Some animals have adaptions that greatly increase evaporative cooling, such as panting, sweating, or even spreading saliva on body surfaces.BEHAVIORAL COOLING:Both endotherms and ectotherms control body temperature through behavioral responses. Some birds and butterflies migrate, other animals such as dessert lizards, bask in the sun when it is cold and find cool, damp areas or burrows when it is hot. 4OSMOREGULATION: Involves balancing the uptake and loss of water and solutes, such as salt (NaCl) and other ions. Osmoregulation is based largely on regulating solutes because salt (NaCl) and other ions. Osmoregulation is based largely on regulating solutes because water follows the movement of solutes by osmosis. (Recall that osmosis is a net movement of water across a selectively permeable membrane from a solution with lower solute concentration to a solution with higher solute concentration)Some sea-dwelling animals such as most marine invertebrates have body fluids with a solute concentration equal to that of seawater. Called OSMOCONFORMERS, such animals do not undergo a net gain or loss of water. OSMOREGULATORS, such as freshwater animals, land animals, and marine vertebrates have body fluids whose solute concentration differs from that of their environment. Therefore, they must regulate water loss or gain.A freshwater fish maintains water and solute balance by food that helps supply ions, and its gills actively take up salt. The fish’s kidneys produce large amounts of URINE, the waste material produced by its urinary system. By excreting dilute urine, the fish disposes of excess water and conserves solutes. 5Metabolism produces a number of toxic by-products, such as the nitrogenous (nitrogen-containing) wastes that result from the breakdown of proteins and nucleic acids. An animal containing) wastes that result from the breakdown of proteins and nucleic acids. An animal deposes these metabolic wastes through the process of excretion, the third and final homeostatic control mechanism. Most aquatic animals dispose of their nitrogenous wastes as AMMONIA. Among the most toxic of all metabolic by-products, ammonia (NH3) is formed when amino groups (-NH2) are removed from amino acids and nucleic acids. Ammonia is to toxic to store in the body, but it is highly soluble in water and diffuses rapidly across cell membranes. If an animal is surrounded by water, ammonia readily diffuses out of its cell and body. Ammonia excretion does not work well for land animals. Land animals convert ammonia into less toxic compounds, either urea or uric acid, that can be safely transported and stored in the body and released periodically by the urinary system. The disadvantage of excreting urea or uric acid is that the animal must use energy to produce these compounds from ammonia. Mammals, most amphibians, sharks, and some bony fishes excrete UREA. Urea is produced in the vertebrate liver by a metabolic cycle that combines ammonia with carbon dioxide. Urea is highly soluble in water and it is also 100,000 times less toxic than ammonia, so it can be held in a concentrated solution in the body and disposed of with relatively little water loss. Land animals that excrete URIC ACID (birds and many other reptiles, insects, land snails, and few amphibians living in deserts) avoid the water loss problems almost completely. 6The URINARY SYSTEM plays a central role in homeostasis, forming and excreting urine while regulating the amount of water and ions in the body fluids. In humans, the main processing regulating the amount of water and ions in the body fluids. In humans, the main processing centers of the urinary system are the two kidneys. From this enormous traffic of blood, our kidneys extract a fluid called FILTRATE, consisting of water, urea, and a number of