The extremes of environmental conditions upset the internal environment of cells and organisms, which can severely damage their system and performance or even life. There is only a very narrow range of environmental conditions under which organisms and cells can maintain their life activities. A stable internal environment must be maintained, even in the face of sudden changes in external conditions. For example, when you feel "cold," that perception sets in motion a sequence of events that involve three different hormones, and three different types of tissues: nerve cells, muscle cells, and endocrine cells. The result is the production of body heat, to maintain a constant body temperature. Maintaining a constant internal environment in spite of changes in the external environment is known as nerve cells) and effectors (which initiates a response, such as shivering muscle cells).

There are two factors, which playa role in maintaing the internal environment. They are regulation of water and regulation of temperature.

Regulation of Water Balance

Water is a very important element of life. There is a constant and steady loss of water from the body in different ways. For example, transpiration and excretion can reduce the water content of the body substantially and therefoR should be replaced. Water is the medium for biochemical reactions and is needed in sufficient quantities for the easy diffusion and flow of dissolved and suspended materials within the body. A disturbance in the water balance in the body can disturb entire metabolic activities.

In animals, the site of homeostasis is the kidney. Kidneys are concerned with the isolation and excretion of metabolic waste products. Kidneys filter a large volume of water, solutes, and wastes everyday. But major amounts of water and solutes are reabsorbed. Only a small amount of water, solutes, and all wastes are excreted as urine. This is enough to create a shift in the water balance of the body. But we take in a lot of water along with food and otherwise. In the case of fresh water fishes, since the solute concentration in the cells is higher than the surrounding water excess water gets into their cells. The excess water is removed by excreting very dilute urine. Whereas, fish living in salt water have a different mechanism for water regulation. They take in a lot of salt water and the excess salt is pumped out through their gills. This is an active mechanism and needs lot of energy. In addition to this they produ::e highly concentrated urine in their specialized kidneys. Thus, they can remove a lot of salt by conserving water. This is the mechanism of homeostasis in marine organisms.

Regulation of Temperature

Regulation of temperature is known as thermoregulation. It is almost as important as osmoregulation. Biochemical reactions of the cells are taking place under an optimal temperature. Therefore, the stability of the optimal body temperature is very essential for the normal functioning of all biological reactions. The variations in the body temperature can affect the functioning of the organisms in a very lethal way. Depending on the influence of the environmental temperature on the body temperature the animals can be of two types: ectotherms and endotherms organisms.

Ectotherms use the external environment and behavioral mechanisms to maintain a thermal balance as much as possible. But their metabolic machinery must be generalized. The enzymes and metabolic reactions have to be functional in a wide temperature ranges. Their enzymes have the ability to work in a wide range of temperatures. For example, insects tend to be maxitherms when given the choice (fish, to digest, move down to cooler temperatures to digest food, while feeding in warm surface waters). Many ectotherms show behavioral and structural adaptations, which help them to adjust to temperature changes. For example, amphibians undergo hibernation during the hot summer. Another example, marine iguanas, feed in cold ocean waters, where they lose heat quickly. They try to gain heat by orienting their black-colored body parts directly to the radiant heat of sun. They also try to get heat by pressing their belly against the rocks that have been warmed by the sun.

Endotherms are organisms that have a constant body temperature and use a variety of physiological mechanisms to maintain a constant internal temperature. The basic mechanism is to have a constantly active metabolic machine. Endotherms fiuch as mammals and birds have very effective temperature control methods. They have a very high respiratory rate, which generates metabolic heat. They also conserve heat by minimizing the heat loss. The heat loss can be minimized by decreasing conductivity or by increasing insulation, or by vasoconstriction or vasodilatation, or by changing the color of insulation. For example, feathers. Air trapped in fur or feathers acts as an insulator, which prevents the loss of heat.

Panting by birds and marrunals and sweating in mammals are effective methods for cooling the system. In certain organisms minimized respiration prevents the loss of excess heat from the body. In mammals there is the mechanism of thermogenesis by shivering, a type of active thermoregulation controlled by the hypothalamus of the brain through a negative feedback thermostat.

There are some leaves with a shiny leaf surface to cut down the radiant heat by reflection. Certain desert plants are provided with folded ridges on the trunks, so that sunlight falls only at shallow angles resulting in reduced absorption of radiant heat.

All living organisms are provided with different structural and physiological modifications and adaptations to maintain homeostasis.

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