What is Homeostasis?

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Homeostasis is involved in keeping the body’s internal environment constant (like the thermostat of a central heating system). Homeostasis keeps the body’s temperature at a certain level (36.5oC) and it keeps the pH of the body at a certain level so that enzymes don’t denature. Blood glucose is kept constant, CO2 levels and O2 levels are monitored to ensure that enough oxygen and not too much carbon dioxide are in the blood. The overall concentration and volume of blood is also monitored homeostatically. The term Homeostasis was first used by Cannon in the late 1920s. Homeostasis is very important to animals because it allows them to rely on the external environment. A constant internal environment allows a considerable degree of independence and allows animals to live in areas from the arctic to the tropics.

Many of the mechanisms involved rely on negative feedback. A movement from the set level (e.g. a rise or fall in body temperature) is detected by receptors. These receptors then send information to the control centre in the brain which reacts by returning to the original value.



For example, the temperature control mechanism. Humans maintain body temperature within 1
oC of 36.5. If the temperature rises too high, the resulting increase in blood temperature is detected by receptors in the hypothalamus in the brain. The heat loss centre also in the hypothalamus sends impulses to arteries and sweat glands which eventually results in a fall in temperature. Cold conditions are detected by receptors in the skin.


      The nervous system plays an important role in homeostatic functions. Hunger is an important homeostatic function as it tells us when to eat and when to stop eating. The role of the mouth in hunger was studied by Spiegel (1973). He asked participants to swallow a tube that delivered food directly to the stomach. The intake was established so that it maintained body weight but the meals weren’t satisfying, the participants wanted to taste and chew the food. This implied that the taste and sensation of the food in the mouth are important in the regulation of meal size but not essential features. Antin et al. (1975) supported this by sham feeding rats. Everything that the rat ate was passed out of the oesophagus through a tube before it reached the stomach. The rat consequently ate far more than it would usually have done. This study showed that the presence of food in the mouth is not enough alone to regulate food intake.

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      The taste of food is sensed by taste receptors on the tongue and the information is then passed on to the brain. Brala and Hagan (1983) found that sweet flavours made people feel hungrier and made them eat more. Rats were found to press bars to receive saccharin which is sweet but non-nutritious and over eat sugary foods if they are available to them (Morgan, 1965). Rolls et al. (1981) showed that people would eat a larger meal if more than one type of sandwich filling were available rather than only one.

      The stomach was thought to ...

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