Homeostasis and feedbackEdexcel International A Level Biology: Revision notes

Homeostasis is the maintenance of a stable internal environment within narrow limits, despite changes in the external environment or in activity such as exercise. Factors controlled include temperature, blood glucose, blood pH and water potential.

Homeostasis is a state of dynamic equilibrium: the factor is never fixed, but fluctuates around a set point, and every deviation is corrected.

It is important because enzymes and cells work best only within narrow ranges. Too high a temperature denatures enzymes; too low slows reactions.

Negative feedback reverses a change so the factor returns to its set point. The sequence is:

1. A receptor detects a change away from the set point.
2. A coordinator (such as the hypothalamus) processes the information.
3. Effectors carry out a response that reverses the change.
4. The receptor detects the return to the set point and the response is reduced.

Because it works in both directions (e.g. heating and cooling), negative feedback keeps the factor within narrow limits. There is a time lag, so small fluctuations always occur.

Positive feedback amplifies a change, moving the factor further from its starting point. It does not maintain a set point.

Example: in childbirth, stretching of the cervix by the baby's head causes oxytocin release, which makes the uterus contract more strongly, causing more stretching and more oxytocin. The cycle ends when the baby is born.

Positive feedback is useful for rapid, completed events, but it would be unstable for controlling temperature or glucose.

The hypothalamus contains thermoreceptors that monitor the temperature of the blood. It also receives impulses from thermoreceptors in the skin.

Too hot (heat loss centre):

• vasodilation of skin arterioles: more heat lost by radiation
• more sweating: evaporation cools the skin
• erector pili muscles relax, hairs lie flat

Too cold (heat gain centre):

• vasoconstriction of skin arterioles: less heat lost
• shivering: muscle respiration releases heat
• less sweating; erector pili contract, hairs raised; metabolic rate increased by hormones such as adrenaline and thyroxine

Exercising muscles release heat from respiration, so core temperature tends to rise. Negative feedback increases sweating and skin blood flow to match heat loss to heat production. Temperature rises slightly, then stabilises at a slightly higher level.

Homeostasis also keeps blood glucose and pH within narrow limits so respiration continues. Without it, enzymes would denature and muscles stop working.