What homeostasis means

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. It keeps conditions ideal for enzyme action and all cell functions.

In the human body the conditions controlled are:

• blood glucose concentration
• body temperature
• water levels

These automatic control systems may use nervous responses or chemical (hormonal) responses. Every control system has the same three parts:

• Receptors — cells that detect stimuli (changes in the environment).
• Coordination centres — e.g. the brain, spinal cord and pancreas — receive and process information from receptors.
• Effectors — muscles or glands — bring about responses that restore optimum levels.

Watch out: a receptor only detects; an effector only acts. Don't muddle them — the coordination centre sits in between and decides the response.

Receptor or effector?

The human nervous system

The nervous system lets humans react to their surroundings and coordinate behaviour. Information from receptors passes along cells called neurones as electrical impulses to the central nervous system (CNS) — the brain and spinal cord.

The CNS coordinates the response of effectors, which may be muscles contracting or glands secreting hormones. The pathway is always:

Where two neurones meet there is a tiny gap called a synapse. The impulse cannot jump the gap directly: a chemical (neurotransmitter) diffuses across, then triggers a new impulse in the next neurone.

The reflex arc

Reflex actions are automatic and rapid; they do not involve the conscious part of the brain. That speed protects you — e.g. pulling your hand off a hot plate. Trace the impulse through the reflex arc:

Misconception: reflexes are not "you reacting fast" — the conscious brain is bypassed entirely. The relay neurone in the CNS sends the response straight back to the effector, which is why it is so quick.

Order the reflex arc

Tap the parts in the order the impulse travels — first to last.

How can you move before you think?

Required practical: reaction time

You must be able to plan and carry out an investigation into the effect of a factor on human reaction time.

The classic method is the ruler-drop test:

• One person holds a ruler between a partner's open thumb and finger, at the 0 cm mark.
• They drop it without warning; the partner catches it as fast as they can.
• The distance the ruler falls is read off — a shorter drop means a faster reaction time.
• Repeat and find a mean; change one factor (e.g. caffeine, practice, distraction) and keep the rest the same.

Skills: you should be able to extract and interpret data about the nervous system, and translate reaction-time information between numerical and graphical forms.

Why reflexes are fast

The brain Triple

The brain controls complex behaviour. It is made of billions of interconnected neurones and has regions with different jobs. You must identify three on a diagram:

(HT only) Higher Neuroscientists map brain regions by studying brain damage, electrically stimulating regions and MRI scanning. The brain's complexity and delicacy make damage and disease very difficult to investigate and treat.

The eye & accommodation Triple

The eye is a sense organ with receptors sensitive to light intensity and colour. Know these structures: retina, optic nerve, sclera, cornea, iris, ciliary muscles, suspensory ligaments. The iris adapts to dim light by changing pupil size.

Accommodation is changing the shape of the lens to focus on near or distant objects:

Two common defects: myopia (short sight) and hyperopia (long sight) — light does not focus on the retina. Treatments: spectacle lenses, plus hard/soft contact lenses, laser surgery to reshape the cornea, or a replacement lens.

Focusing on a near object

What keeps your blood glucose steady?

Controlling body temperature Triple

Body temperature is monitored by the thermoregulatory centre in the brain, which has receptors sensitive to the temperature of the blood. The skin also has temperature receptors that send impulses to this centre.

• Too hot: blood vessels dilate (vasodilation) and sweat is produced — both transfer energy from skin to the environment.
• Too cold: blood vessels constrict (vasoconstriction), sweating stops, and skeletal muscles contract (shiver).

(HT only) Higher You should be able to explain how these mechanisms lower or raise body temperature in a given context — e.g. shivering releases energy by muscle respiration, warming the body.

The endocrine system

The endocrine system is made of glands that secrete chemicals called hormones directly into the bloodstream. The blood carries each hormone to a target organ where it has its effect. Compared with the nervous system the effects are slower but last longer.

The pituitary gland in the brain is the "master gland": it secretes several hormones in response to body conditions, and these act on other glands to release further hormones.

Gland → hormone

Tap a gland on the left, then its hormone on the right.

Control of blood glucose

Blood glucose concentration is monitored and controlled by the pancreas.

• Glucose too high: the pancreas releases insulin, which causes glucose to move from the blood into cells. In liver and muscle cells, excess glucose is converted to glycogen for storage.
• (HT only) Higher Glucose too low: the pancreas releases glucagon, which makes glycogen convert back to glucose, released into the blood.

Misconception: insulin lowers blood glucose (by storing it as glycogen); it does not raise it. Negative feedback opposes the change — high triggers insulin, low triggers glucagon.

Type 1 and Type 2 diabetes

• Type 1: the pancreas fails to produce enough insulin. Blood glucose runs uncontrollably high. Normally treated with insulin injections.
• Type 2: the body cells no longer respond to insulin. Treated with a carbohydrate-controlled diet and an exercise regime. Obesity is a risk factor.

Skills: you should be able to interpret graphs of insulin's effect on blood glucose in people with and without diabetes, and evaluate the link between obesity and Type 2 diabetes.

Blood-glucose response

What keeps your blood glucose steady?

Water balance & the kidney Triple

If body cells lose or gain too much water by osmosis they do not function efficiently. Water and substances leave the body in several ways:

• Water is lost via the lungs in exhalation, and via the skin in sweat (with ions and urea) — no control over these losses.
• Excess water, ions and urea are removed by the kidneys in urine.
• The kidneys make urine by filtration of the blood and selective reabsorption of useful substances (glucose, some ions and water).

(HT only) Higher Excess amino acids from protein digestion are deaminated in the liver to form ammonia, which is toxic and is immediately converted to urea for safe excretion.

ADH & dialysis Triple

(HT only) Higher Water level is controlled by the hormone ADH, which acts on the kidney tubules. When the blood is too concentrated, the pituitary gland releases ADH, which makes the tubules more permeable so more water is reabsorbed back into the blood. This is controlled by negative feedback.

Kidney failure can be treated by organ transplant or kidney dialysis. You should know the basic principles of dialysis and be able to evaluate dialysis (a machine) versus a transplant.

How does the body keep time without a clock?

Hormones in reproduction

At puberty, reproductive hormones cause secondary sex characteristics to develop. Oestrogen is the main female hormone (from the ovary); testosterone is the main male hormone (from the testes, stimulating sperm production). An egg is released roughly every 28 days — ovulation.

Four hormones run the menstrual cycle:

(HT only) Higher You must explain the interactions: FSH stimulates the ovary and oestrogen production; rising oestrogen inhibits FSH and stimulates LH; the LH surge triggers ovulation; progesterone maintains the lining and inhibits FSH and LH.

Which hormone releases the egg?

How does the body keep time without a clock?

Contraception & treating infertility

Fertility can be controlled by hormonal and non-hormonal contraception:

• Hormonal: oral contraceptives that inhibit FSH so no eggs mature; injection/implant/patch of slow-release progesterone to inhibit egg maturation and release.
• Non-hormonal: barrier methods (condoms, diaphragms); intrauterine devices; spermicides; abstaining when an egg may be present; sterilisation.

Treating infertility (HT only) Higher:

• A "fertility drug" of FSH and LH can be given so a woman may become pregnant normally.
• IVF: the mother is given FSH and LH to mature several eggs; eggs are collected and fertilised by sperm in the laboratory; the fertilised eggs become embryos, and one or two tiny balls of cells are inserted into the uterus.
• IVF is emotionally and physically stressful, success rates are low, and it can cause multiple births (a risk to mother and babies).

Adrenaline & thyroxine Higher

Adrenaline is produced by the adrenal glands in times of fear or stress. It increases heart rate and boosts delivery of oxygen and glucose to the brain and muscles — preparing the body for "fight or flight".

Thyroxine from the thyroid gland stimulates the basal metabolic rate and plays an important role in growth and development. Thyroxine levels are controlled by negative feedback:

How does a seedling bend towards the light?

Plant hormones & tropisms Triple

Plants make hormones to coordinate growth and responses to light (phototropism) and gravity (gravitropism / geotropism). An unequal distribution of auxin causes unequal growth rates in shoots and roots.

Misconception: auxin doesn't "pull" the shoot to the light. It collects on the shaded side and makes those cells grow longer, so the faster-growing dark side pushes the tip over towards the light.

(HT only) Higher Gibberellins initiate seed germination; ethene controls cell division and fruit ripening (mechanisms not required).

How does a seedling bend towards the light?

Practical & uses of plant hormones Triple

Required practical 8: investigate the effect of light or gravity on the growth of newly germinated seedlings. Record results as length measurements and as careful, labelled biological drawings.

(HT only) Higher Uses of plant hormones in agriculture and horticulture:

• Auxins: as weedkillers, as rooting powders, and for promoting growth in tissue culture.
• Ethene: controls ripening of fruit during storage and transport.
• Gibberellins: end seed dormancy, promote flowering, and increase fruit size.

A shoot in one-sided light

The big ideas to know

You've covered the key ideas in AQA 4.5 — homeostasis, the nervous system, the brain & eye, thermoregulation, the endocrine system, blood glucose, reproduction, feedback, plant hormones and the kidney. Press Finish to see your score.