Homeostasis

Homeostasis is defined by AQA as 'the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal AND external changes'.

In simpler terms: your body works hard to keep things like temperature, blood sugar and water content roughly the same, no matter what's happening outside (cold day, big meal, intense exercise) or inside (digesting food, fighting an infection).

Why is it essential?

• Enzymes (which control every chemical reaction in cells) only work in narrow temperature and pH ranges. Outside these ranges they slow down or even denature.
• Cells lose water by osmosis if blood is too concentrated; they burst if blood is too dilute.
• The brain and other vital organs need a constant supply of glucose for respiration.

The three big things AQA wants you to know are regulated in the human body:

1. Body temperature — kept close to 37 °C.
2. Blood glucose concentration — kept within a narrow band (more on this in 4.5.3).
3. Water and ion levels in the body (covered in 4.5.4).

Every homeostatic control system in the body — temperature, glucose, water — has the same three components. Learn these as a unit: receptors → coordination centre → effectors.

1. Receptors. Specialised cells (often in the sense organs or in glands) that detect a stimulus — a change in the internal or external environment. Examples:

• Thermoreceptors in the skin and hypothalamus (detect temperature change).
• Receptors in the pancreas (detect blood glucose change).
• Osmoreceptors in the hypothalamus (detect water concentration of blood).

2. Coordination centres. Where information from the receptors is processed and a response is decided. The main coordination centres are:

• Brain (especially the hypothalamus for temperature/water).
• Spinal cord (for reflex responses).
• Pancreas (for blood glucose).

3. Effectors. The parts of the body that carry out the response — they bring the level back towards the set point. Effectors are always muscles (which contract) or glands (which release hormones or substances). Examples:

• Sweat glands (release sweat to cool you down).
• Muscles in skin arrector pili (raise hairs to trap warm air).
• Pancreas (releases insulin / glucagon to adjust blood glucose).

The chain of communication uses either:

• Electrical impulses along nerves (fast, short-lasting).
• Hormones released into the blood (slower, longer-lasting).

Negative feedback means the body responds to a deviation from the set point by triggering the OPPOSITE change. This pushes the level back towards the set point.

Worked example — body temperature.

• Set point: 37 °C.
• If body temperature rises above 37 °C:
  • Sweat glands (effectors) release more sweat → evaporation cools the skin.
  • Blood vessels (effectors) supplying the skin dilate (vasodilation) → more heat is lost from the surface.
  • Hairs lie flat (no insulation needed).
• If body temperature falls below 37 °C:
  • Sweat production reduces.
  • Blood vessels constrict (vasoconstriction) → less heat lost.
  • Muscles shiver → respiration generates heat.
  • Hairs are raised (in mammals — traps insulating air layer).

In both cases the change is reversed, so temperature returns close to 37 °C.

Worked example — blood glucose.

• Set point: a normal blood glucose concentration (~5 mmol/L).
• After a meal, blood glucose rises → pancreas (coordination centre) detects it → releases insulin → cells take up glucose → blood glucose falls back.
• Between meals or during exercise, blood glucose falls → pancreas releases glucagon → liver converts glycogen back to glucose → blood glucose rises back.

(The detail of insulin/glucagon is in spec 4.5.3.)

What does 'negative' mean? 'Negative' = opposite. The body's response is OPPOSITE to the original change. Negative feedback is therefore a stabilising mechanism — it resists change and keeps the level close to the set point.

Information in homeostatic control systems is carried in two ways:

Nervous communication.

• Carried by electrical impulses along neurones (nerve cells).
• FAST (impulses travel many metres per second).
• Short-lived effect.
• Precise target — one neurone to one effector.
• Used for: pulling your hand off a hot plate (reflex), running away from danger.

Chemical (hormonal) communication.

• Carried by hormones — chemicals released by endocrine glands into the bloodstream.
• SLOWER (depends on blood flow).
• Longer-lasting effects.
• Widespread — a hormone in the blood reaches almost every cell, but only acts on cells with the right receptor.
• Used for: adjusting blood glucose, controlling growth, puberty, regulating water balance.

Feature	Nervous	Hormonal
Signal type	Electrical impulse	Chemical (hormone)
Speed	Very fast (ms)	Slower (s to minutes)
Duration	Brief	Longer
Target	Specific (one effector)	Widespread (any cell with the receptor)
Example	Reflex action	Insulin response

Both systems are essential; the body uses whichever is best suited to the situation. AQA Paper 1 questions often ask you to compare them.

Thyroxine is a hormone released by the thyroid gland, located in the front of the neck. It contains iodine.

What thyroxine does:

• Controls basal metabolic rate (BMR) — the rate at which energy is released from food in resting cells.
• A higher thyroxine level = a faster metabolic rate = cells respire faster and release more heat.
• Thyroxine is also essential for growth and development, especially in children.

What goes wrong if thyroxine is abnormal:

• Too much (hyperthyroidism) — increased BMR, weight loss, racing heart, sweating, restlessness, bulging eyes (Graves' disease).
• Too little (hypothyroidism) — slower BMR, weight gain, feeling cold, tiredness, slow growth in children.

Iodine is needed to make thyroxine. Iodine deficiency causes the thyroid to swell — a goitre.

Thyroxine is the classic AQA example of negative feedback in hormone control. The system has three players:

• Hypothalamus (in the brain) — monitors thyroxine in blood. Releases TRH.
• Pituitary gland (just below the hypothalamus) — releases TSH (thyroid-stimulating hormone).
• Thyroid gland (in the neck) — releases thyroxine when stimulated by TSH.

The loop, step by step:

1. Thyroxine level drops (below the set point). 2. The hypothalamus and pituitary detect the low thyroxine. 3. The pituitary releases more TSH (thyroid-stimulating hormone) into the blood. 4. TSH stimulates the thyroid to release more thyroxine. 5. Thyroxine level rises back to normal. 6. The high thyroxine then INHIBITS the release of TSH — so thyroxine stops rising. 7. Now the level is back at the set point.

This is negative feedback because the response (more thyroxine) opposes the original change (low thyroxine). The same principle works in reverse if thyroxine gets too high — TSH release is inhibited, thyroid output falls, thyroxine returns to normal.

Adrenaline is produced by the adrenal glands, which sit on top of the kidneys (the name = 'ad-renal' = 'on/above the kidney').

When is adrenaline released?

• In situations of fear, stress, danger or excitement — anything that triggers 'fight-or-flight'.
• The brain (hypothalamus) sends nerve impulses to the adrenal glands telling them to release adrenaline.

What does adrenaline do? It prepares the body for vigorous physical action (running away or fighting back). Effects include:

• Heart rate increases — pumps more blood per minute.
• Breathing rate and depth increase — more oxygen taken in.
• Blood vessels to muscles dilate — more oxygen and glucose delivered to muscles.
• Blood vessels to digestive system / skin constrict — blood diverted away from non-essential organs.
• Liver releases glucose (from glycogen breakdown) into the blood — more fuel for respiration.
• Pupils dilate — better vision.

Why is this useful? In our evolutionary past, fight-or-flight situations (predators, fighting rivals) needed sudden bursts of energy. The body had to be ready in seconds. Today the same response kicks in for exams, public speaking, near-misses while driving — sometimes useful, sometimes not.

Is adrenaline under negative feedback? No — adrenaline is released directly in response to a stimulus (the brain sensing danger). When the danger passes, no more adrenaline is released and the existing adrenaline is broken down. There is no TSH-like 'inhibition loop'.