Homeostasis

Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Homeostasis is keeping the internal environment of the body within narrow limits. Examples are body temperature, blood pH, blood glucose concentration and blood osmotic concentration.

It works by negative feedback. A change away from the set point is detected, and the response reverses the change. This works whether the variable rises too high or falls too low. Positive feedback would make the change larger, so it cannot keep a variable steady.

Blood glucose is controlled by two hormones from the pancreas, insulin and glucagon, which have opposite effects. Body temperature is controlled by the hypothalamus, which receives information from thermoreceptors and sends signals to the skin, muscles and other effectors.

Rules to remember

Blood glucose too high: beta cells secrete insulin. Liver and muscle cells take up glucose and store it as glycogen.
Blood glucose too low: alpha cells secrete glucagon. The liver breaks down glycogen and releases glucose.
Type 1 diabetes: beta cells are destroyed by the immune system, so little or no insulin is made. Treated with insulin.
Type 2 diabetes: target cells respond less to insulin. Risk factors include obesity, diet and lack of exercise.
Too cold: vasoconstriction, shivering, hair erection, uncoupled respiration in brown adipose tissue, more thyroxin (via the pituitary and thyroid).
Too hot: vasodilation of skin arterioles and sweating. Evaporation of sweat takes heat from the skin.

Common mistake

Students confuse glucagon with glycogen. Glucagon is the hormone that raises blood glucose. Glycogen is the storage carbohydrate in liver and muscle cells.

Worked example

A runner's body temperature rises above the set point during a race on a hot day. Explain how negative feedback returns it to normal.

Thermoreceptors detect the rise and the hypothalamus receives this information.
The hypothalamus signals the skin: arterioles dilate, so more blood flows near the surface and more heat is lost.
Sweat glands secrete sweat, and its evaporation takes heat from the skin.
Body temperature falls, so the hypothalamus reduces these responses.

Answer: The response (vasodilation and sweating) reverses the rise in temperature, which is negative feedback.

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D3.3 Homeostasis

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