Homeostasis and response

Homeostasis and response is the GCSE Biology topic about how your body keeps its internal conditions steady. Homeostasis is the regulation of the internal conditions of a cell or organism to keep them at the best level for working, whatever changes happen inside or outside the body. It relies on two control systems: the nervous system, which gives fast responses, and the hormonal (endocrine) system, which is slower but lasts longer.

What is homeostasis?

Cells can only survive within narrow limits. They need a steady temperature and pH, and a constant supply of dissolved food and water. Homeostasis keeps conditions right for enzymes and every other cell function. In the human body, the AQA specification names three things that are controlled:

• blood glucose concentration
• body temperature
• water levels

These control systems are automatic. They can use nervous responses, chemical (hormone) responses, or both.

How a control system works

Every control system has the same three parts:

• Receptors: cells that detect stimuli, which are changes in the environment.
• Coordination centres: such as the brain, spinal cord and pancreas, which receive and process information from receptors.
• Effectors: muscles or glands, which bring about responses that restore the best levels.

Most of these systems work by negative feedback: when a level moves away from normal, the response pushes it back the other way. If it then overshoots, the opposite response kicks in.

The nervous system

The nervous system lets you react to your surroundings and coordinate your behaviour. Information from receptors travels along nerve cells (neurones) as electrical impulses to the central nervous system (CNS), which is the brain and spinal cord. The CNS coordinates the response of effectors: muscles contracting or glands secreting hormones.

Reflex actions are automatic and rapid: you do not have to think about them, because the conscious part of the brain is not involved. They protect you, for example by pulling your hand away from something hot. See the reflex arc diagram guide for the full pathway. Required practical 7 investigates a factor that affects human reaction time.

The brain (Biology only)

The brain is made of billions of interconnected neurones, with different regions doing different jobs. You need to identify three on a diagram:

The eye (Biology only)

The eye is a sense organ with receptors sensitive to light intensity and colour. Accommodation is changing the shape of the lens to focus on near or distant objects. In myopia (short sight) and hyperopia (long sight), light rays do not focus on the retina; spectacle lenses refract the rays so that they do. The eye diagram guide labels every part.

Controlling body temperature (Biology only)

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. It contains receptors sensitive to the temperature of the blood, and temperature receptors in the skin send impulses to it.

The endocrine system

The endocrine system is made of glands that secrete hormones directly into the bloodstream. The blood carries each hormone to a target organ, where it has an effect. The pituitary gland in the brain is called the master gland, because several of its hormones act on other glands and make them release their own hormones.

Controlling blood glucose

Blood glucose concentration is monitored and controlled by the pancreas.

• Too high (after a meal): the pancreas produces insulin, which makes glucose move from the blood into cells. In liver and muscle cells, extra glucose is turned into glycogen to store it.
• Too low (Higher tier): the pancreas produces glucagon, which makes glycogen turn back into glucose and be released into the blood.

Insulin and glucagon work against each other in a negative feedback cycle, which keeps blood glucose within a narrow range. Practise reading a glucose level and choosing the right hormone in this question from the Human Physiology course.

Type 1 and Type 2 diabetes

Water and nitrogen balance (Biology only)

If body cells gain or lose too much water by osmosis, they do not work efficiently. Water leaves the body from the lungs when you breathe out, and water, ions and urea are lost in sweat; there is no control over these losses. The kidneys remove extra water, ions and urea in urine. They filter the blood, then selectively reabsorb useful substances such as glucose, some ions and water.

• ADH (Higher tier): when the blood is too concentrated, the pituitary gland releases ADH, so the kidney tubules let more water pass back into the blood. This is controlled by negative feedback. The osmoregulation guide goes deeper.
• Urea (Higher tier): excess amino acids from digested protein are deaminated in the liver to form ammonia, which is toxic, so it is immediately turned into urea for safe excretion.
• Kidney failure can be treated with a transplant or with dialysis.

Hormones in human reproduction

At puberty, reproductive hormones cause secondary sexual characteristics to develop. Oestrogen, from the ovaries, is the main female reproductive hormone; eggs start to mature and one is released about every 28 days (ovulation). Testosterone, from the testes, is the main male hormone and stimulates sperm production. In the menstrual cycle:

• FSH makes an egg mature in the ovary.
• LH stimulates the release of the egg.
• Oestrogen and progesterone keep the lining of the uterus in place.

Knowing how these hormones work led to hormonal contraceptives, such as pills that stop FSH being produced so no eggs mature, and to fertility treatment. In IVF (Higher tier), FSH and LH make several eggs mature; the eggs are collected, fertilised in the laboratory, and one or two tiny embryos are put into the uterus.

Adrenaline and thyroxine (Higher tier)

• Adrenaline: when you are frightened or stressed, the adrenal glands release adrenaline. It speeds up the heart, so more oxygen and glucose reach the brain and muscles, ready for fight or flight.
• Thyroxine from the thyroid gland speeds up the basal metabolic rate and matters for growth and development. Its level is controlled by negative feedback.

Plant hormones (Biology only)

Plants also respond to their surroundings using hormones. Auxin controls growth towards light (phototropism) and in response to gravity (gravitropism, or geotropism): an uneven spread of auxin makes one side of a shoot or root grow faster than the other. At Higher tier you also need gibberellins, which start seed germination, and ethene, which controls cell division and fruit ripening. People use auxins as weed killers and rooting powders, and ethene to control ripening while fruit is stored and transported.

Exam questions with answers

A runner gets hot during a race. Explain how her body temperature is brought back down.

1. Temperature receptors detect the rise and the thermoregulatory centre in the brain is sent impulses.
2. Blood vessels supplying the skin capillaries dilate, so more warm blood flows near the skin surface.
3. Sweat glands release more sweat, and evaporating sweat transfers energy away from the skin.
4. The skin loses more energy to the air, so body temperature falls back towards normal.

Why does blood glucose rise after a meal and then fall again?

Carbohydrates in the meal are digested into glucose, which is absorbed into the blood. The pancreas detects the rise and releases insulin. Insulin makes cells take in glucose, and liver and muscle cells store the excess as glycogen, so the concentration falls back to normal.

Common mistakes

• Saying blood vessels move. They dilate or constrict; they do not move nearer to or further from the skin.
• Mixing up glucagon and glycogen. Glucagon is the hormone; glycogen is the stored carbohydrate.
• Saying insulin turns glucose into glycogen. Insulin makes cells take in glucose; liver and muscle cells then convert it.
• Saying Type 2 diabetics make no insulin. In Type 2 the cells stop responding to insulin.
• Calling hormones nerve signals. Hormones travel in the blood; nerve impulses travel along neurones.
• Leaving out the coordination centre. Name the brain, spinal cord or pancreas in every control pathway.

Practise with a short timed paper

Homeostasis and response is in AQA GCSE Biology and Combined Science (Paper 2 in Biology). The daily GCSE Biology Higher and GCSE Combined Science Higher papers are short timed papers of 8 questions, with a new one every day.

Read next

Biology · Nerves and homeostasisOsmoregulation: ADH, the kidneys and negative feedbackOsmoregulation for GCSE and A-level Biology: how the kidneys and ADH control the water potential of the blood by negative feedback, plus the nephron.