Homeostasis and Coordination
What you must understand
Homeostasis is the maintenance of a constant internal environment. Know two examples in detail: temperature control and blood glucose control. Body temperature is controlled by the hypothalamus. When too hot, sweating and vasodilation occur; when too cold, shivering, vasoconstriction and hair erection occur. Blood glucose is controlled by insulin, which lowers it, and glucagon, which raises it. Insulin converts glucose to glycogen in the liver; a failure to control glucose leads to diabetes.
The nervous system has a central part (brain and spinal cord) and peripheral nerves. A reflex arc runs: receptor → sensory neurone → relay neurone in the spinal cord → motor neurone → effector. Reflexes are rapid and automatic and protect the body. The eye, with its iris reflex, is a useful example.
Traps examiners set
- Saying sweating "cools the blood". Sweating cools the body by evaporation of sweat from the skin, taking latent heat.
- Confusing vasodilation with vasoconstriction; vasodilation widens vessels and increases heat loss.
- Mixing up insulin and glucagon effects.
- Forgetting that reflexes bypass conscious thought in the brain, which is why they are fast.
Worked example
A person's blood glucose rises after a meal. Explain the response. The pancreas detects the rise and secretes insulin; insulin causes liver and muscle cells to take up glucose and store it as glycogen; blood glucose falls back to normal.
Rule of thumb: for every homeostasis question, identify the stimulus, the receptor, the coordinating centre, the response and the negative feedback that restores normal.
Negative feedback
All homeostasis works by negative feedback: a change is detected and a response reverses it, keeping conditions within narrow limits.