Hormones and homeostasis

Homeostasis is keeping the conditions inside the body steady, such as blood glucose, body temperature and water levels, so that cells and enzymes work well. It uses the nervous system and hormones. Hormones are chemicals made by glands of the endocrine system and carried in the blood to their target organs.

Key facts

  • Control systems have receptors that detect a change, a coordination centre (the brain, spinal cord or pancreas) and effectors that respond. Many work by negative feedback: a change is reversed.
  • Endocrine glands: the pituitary (the “master gland”: ADH, FSH, LH, TSH, growth hormone), thyroid (thyroxine), adrenal glands (adrenaline), pancreas (insulin, glucagon), ovaries (oestrogen, progesterone) and testes (testosterone). Hormones act more slowly than nerves but last longer.
  • When blood glucose rises, the pancreas releases insulin: cells take in glucose and the liver and muscles store it as glycogen. When it falls, the pancreas releases glucagon: the liver turns glycogen back into glucose.
  • Type 1 diabetes: the pancreas makes too little insulin; it is treated with insulin injections. Type 2 diabetes: cells no longer respond to insulin; it is treated with diet and exercise, and obesity is a risk factor.
  • The thermoregulatory centre in the brain monitors body temperature. Too hot: sweating and vasodilation. Too cold: shivering and vasoconstriction.
  • Menstrual cycle: FSH makes an egg mature; oestrogen builds up the uterus lining; LH triggers ovulation (about day 14 of a 28-day cycle); progesterone keeps the lining thick. Contraceptive pills use oestrogen and progesterone to stop FSH and LH.

Worked examples

Example 1

The graph shows blood glucose after a meal. Explain the shape of the curve.

  1. After the meal, glucose is absorbed from the gut, so blood glucose rises.
  2. The pancreas detects the rise and releases insulin.
  3. Insulin makes cells take in glucose and the liver store it as glycogen.
  4. Blood glucose falls back to normal: negative feedback.

Answer: glucose rises as food is digested and absorbed; the pancreas releases insulin, cells take in glucose and the liver stores glycogen, so it falls back to normal

Example 2

A woman's cycle lasts 30 days. Ovulation happens about 14 days before her next period. On which day does she ovulate?

  1. The next period starts after day 30.
  2. Count back 14 days. 30 − 14 = 16
  3. So ovulation is about day 16.

Answer: about day 16

Example 3

Describe how the body responds when it gets too cold.

  1. The thermoregulatory centre detects the fall in temperature.
  2. Muscles contract quickly (shivering): respiration releases heat.
  3. Blood vessels supplying the skin narrow (vasoconstriction), so less heat is lost.
  4. Sweating stops and hairs stand up to trap air.

Answer: shivering releases heat by respiration; vasoconstriction keeps warm blood away from the skin; hairs stand up

Example 4

Compare Type 1 and Type 2 diabetes.

  1. In Type 1 diabetes the pancreas does not make enough insulin, so glucose stays high.
  2. It is treated with insulin injections, usually several times a day.
  3. In Type 2 diabetes the body cells no longer respond to insulin.
  4. It is controlled with a carbohydrate-controlled diet and exercise; obesity is a risk factor.

Answer: Type 1: the pancreas makes too little insulin, treated with insulin injections; Type 2: cells stop responding to insulin, controlled by diet and exercise