Homeostasis and Response — AQA Combined Science: Trilogy

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes. This unit covers the nervous and hormonal control systems that achieve this.

Principles of homeostasis

The body keeps conditions such as blood glucose concentration, body temperature and water levels stable. All control systems include:

Receptors — detect a change (stimulus).
Coordination centres — such as the brain, spinal cord and pancreas, which process information.
Effectors — muscles or glands that bring about a response to restore conditions.

Many systems use negative feedback: when a level rises too high or falls too low, the system acts to bring it back to normal.

The nervous system

The nervous system lets humans react to their surroundings and coordinate behaviour. Information from receptors travels as electrical impulses along neurones.

The pathway is: stimulus → receptor → sensory neurone → coordinator (CNS) → motor neurone → effector → response

The central nervous system (CNS) is the brain and spinal cord.

Reflex actions

Reflexes are automatic and rapid; they do not involve the conscious part of the brain, which makes them protective (e.g. pulling your hand off a hot object).

In a reflex arc, the impulse passes from a sensory neurone, across a synapse to a relay neurone in the spinal cord, across another synapse to a motor neurone, and then to the effector.

At a synapse, the electrical impulse triggers the release of chemicals (neurotransmitters) that diffuse across the gap and start a new impulse in the next neurone.

Required practical: investigating reaction time (e.g. the ruler-drop test) and how it is affected by factors such as caffeine or practice.

The endocrine system

The endocrine system is made of glands that secrete hormones — chemical messengers carried in the bloodstream to target organs. Compared with nervous responses, hormonal responses are usually slower but longer lasting and more widespread.

Key glands:

Pituitary gland — the "master gland" in the brain; releases hormones that control other glands.
Thyroid — controls metabolic rate.
Pancreas — controls blood glucose.
Adrenal glands — release adrenaline.
Ovaries and testes — release sex hormones.

Control of blood glucose

The pancreas monitors and controls blood glucose concentration.

If blood glucose is too high, the pancreas releases insulin, which causes cells (especially liver and muscle) to take up glucose and store excess as glycogen.
If blood glucose is too low, the pancreas releases glucagon, which causes glycogen to be converted back to glucose and released into the blood.

This is a clear example of negative feedback.

Diabetes

Type 1 diabetes — the pancreas produces little or no insulin, so blood glucose can rise dangerously. Treated with insulin injections, plus careful diet and exercise.
Type 2 diabetes — body cells stop responding to insulin. Linked to obesity as a risk factor. Treated with a carbohydrate-controlled diet, exercise and sometimes drugs.

You should be able to interpret data on diabetes, BMI and waist:hip ratios.

Hormones in human reproduction

At puberty, sex hormones cause secondary sexual characteristics to develop.

Testosterone (from the testes) stimulates sperm production.
Oestrogen (from the ovaries) is the main female hormone.

The menstrual cycle

Four hormones control the cycle:

FSH — causes an egg to mature in the ovary.
Oestrogen — causes the uterus lining to build up; stops FSH and triggers LH.
LH — stimulates the release of an egg (ovulation) at about day 14.
Progesterone — maintains the uterus lining; falling levels lead to menstruation.

Contraception

Hormonal methods: the pill (contains oestrogen and/or progesterone to stop FSH so eggs don't mature), implants, injections and patches. Very effective but can have side effects.
Non-hormonal methods: condoms and diaphragms (barrier methods, also protect against STIs), IUDs, spermicides, abstinence and surgical sterilisation.

You should be able to evaluate the different methods, considering effectiveness, side effects and ease of use.

Exam tips

Learn the control-system structure: receptor → coordination centre → effector, and apply it to any example.
For reflex arcs, get the order of neurones right and mention the synapse and neurotransmitters.
Insulin lowers blood glucose; glucagon raises it — don't mix them up.
Be able to describe negative feedback in your own words for blood glucose and temperature.
Practise evaluating contraception methods and interpreting menstrual-cycle graphs.

Biology: Homeostasis and Response