7.2 Homeostasis

Why this matters

Your internal environment — blood glucose, core body temperature, water content, ion concentration, pH — is kept inside a narrow range despite huge changes in the world around you. You can eat a sugary meal, walk into a heatwave, run a 5k or fast for a day, and your blood glucose stays close to a set point, your core temperature stays close to 37 °C, and your blood water content barely moves. That is homeostasis, and Edexcel 1BI0 7.2 expects you to be able to define it and to apply it to three named systems. It matters because every chemical reaction in your body is catalysed by enzymes, and enzymes work at their optimum only inside a narrow range of temperature, pH and substrate concentration. If your internal conditions wandered too far, enzymes would slow down or denature, cells would dehydrate or burst by osmosis, and respiration — which cells need to release energy from glucose — would stop. Every homeostatic loop has the same architecture: a RECEPTOR detects a change, a COORDINATION CENTRE processes the information, and an EFFECTOR (a muscle or a gland) brings about a response that reverses the change. Edexcel names three concrete examples you must master: (1) blood glucose, controlled by the pancreas using insulin and glucagon acting on the liver and muscle cells; (2) body temperature, controlled by thermoreceptors in the hypothalamus and skin, with effectors that include sweat glands, the arterioles of the skin (vasodilation and vasoconstriction) and skeletal muscles (shivering); (3) water balance, controlled by the pituitary releasing ADH and acting on the kidneys. Type 1 diabetes (no insulin produced, treated by injection) and Type 2 diabetes (cells stop responding to insulin, managed by diet + exercise) are explicit content too. Restoring normal conditions allows effectors to function correctly and keeps the whole organism alive.

How to learn this topic

Build on what you already know

• Topic 1: enzymes work at an optimum temperature and pH — they denature outside that range.
• Topic 1: osmosis — water moves from dilute to more concentrated solutions across a partially-permeable membrane.
• Topic 2: cells respire to release energy from glucose — cells need glucose for respiration.
• Topic 7.1: hormones travel in the blood from glands to target organs (the endocrine system).

1. Define homeostasis precisely — 'maintains a stable internal environment' — and link it to enzymes working at their optimum.
2. List the three Edexcel-named conditions: blood glucose, body temperature, water balance.
3. Introduce the receptor → coordination centre → effector framework and apply it to the three examples.
4. Drill blood glucose regulation: insulin causes liver/muscle cells to take up glucose; glucose converted to glycogen; glucagon reverses this when glucose falls.
5. Drill thermoregulation in BOTH directions: too hot (sweat, vasodilation) and too cold (vasoconstriction, shivering, hairs erect).
6. Contrast Type 1 (no insulin → inject insulin) and Type 2 (cells stop responding → diet + exercise) diabetes.
7. Memorise the verbatim Edexcel marking phrases for each scenario.
8. Pre-empt common errors: 'blood vessels move', 'sweat cools by being cold', 'shivering when too hot'.

Key terms

homeostasis

The maintenance of a stable internal environment despite external changes, allowing cells and enzymes to function at their optimum. (Edexcel wording: 'homeostasis maintains a stable internal environment'. Don't just say 'keeping the body balanced'.)

stable internal environment

The set of internal conditions — chiefly blood glucose, body temperature and water/ion balance — kept inside a narrow range so cells can function. (Use 'stable internal environment' verbatim — it is the Edexcel marking phrase.)

set point

The 'normal' target value of a controlled variable — for example 37 °C body temperature or the normal blood-glucose range. Responses bring the variable back to the set point.

negative feedback

A control mechanism in which a change in a variable triggers a response that reverses the change, returning the variable to its set point.

receptor

A cell (or group of cells) that detects a stimulus — for example glucose-sensitive cells in the pancreas, or thermoreceptors in the skin and hypothalamus. (Receptors DETECT — they do not 'respond'. Effectors respond.)

thermoreceptor

A receptor cell that detects changes in temperature. Thermoreceptors in the hypothalamus monitor blood temperature; thermoreceptors in the skin monitor environmental temperature. (Edexcel wording for a too-hot answer starts with 'thermoreceptors detect the increase'.)

coordination centre

A structure that receives and processes information from receptors and signals to effectors. For blood glucose it is the pancreas; for temperature and water it is the hypothalamus.

effector

A muscle or gland that brings about a response. Examples: liver cells take up glucose; sweat glands secrete sweat; skin arterioles vasodilate/vasoconstrict; skeletal muscles shiver.

insulin

A hormone secreted by the pancreas when blood glucose is high. Insulin causes liver and muscle cells to take up glucose and convert it to glycogen for storage. (Always pair insulin with liver/muscle cells and with glycogen — these are the marking phrases.)

glucagon

A hormone secreted by the pancreas when blood glucose is low. Glucagon causes the liver to break glycogen down into glucose, which enters the blood.

glycogen

The storage form of glucose in liver and muscle cells. Many glucose molecules are joined together. Insulin causes glucose to be converted to glycogen; glucagon causes glycogen to be broken down into glucose. (Don't confuse glycogen (animal storage) with glucagon (the hormone) or glucose (the sugar).)

Type 1 diabetes

A condition in which the pancreas fails to produce insulin. Without insulin, blood glucose stays too high after meals. Treated by insulin injection.

Type 2 diabetes

A condition in which body cells stop responding to insulin. The pancreas still makes insulin but cells ignore it. Often managed through diet and exercise.

vasodilation

The widening of blood vessels (specifically the arterioles supplying the skin's surface capillaries). It increases blood flow near the skin surface so more heat is lost by radiation — used when the body is too hot. (Edexcel marking phrase: 'blood vessels near skin vasodilate / widen'.)

vasoconstriction

The narrowing of blood vessels (specifically the arterioles supplying the skin's surface capillaries). It decreases blood flow near the skin surface so less heat is lost — used when the body is too cold. (The blood vessels themselves narrow — they do NOT move deeper into the body.)

sweat gland

A gland in the skin that secretes sweat onto the skin's surface. When sweat evaporates it removes heat from the body, cooling the blood beneath.

Notes

What homeostasis is

Homeostasis is the maintenance of a stable internal environment despite changes in the external environment. It allows cells and enzymes to function at their optimum.

For Edexcel 1BI0 7.2 the wording matters. Examiners expect you to say:

• Homeostasis maintains a stable internal environment.
• Enzymes work at their optimum at normal conditions.
• Cells need glucose for respiration (so glucose must be kept available).
• Restoring normal conditions allows effectors to function correctly.

The three internal conditions Edexcel names for humans are:

• Blood glucose concentration — controlled by insulin and glucagon from the pancreas.
• Body temperature — controlled by the hypothalamus, with sweat glands, skin arterioles (vasodilation / vasoconstriction) and skeletal muscles (shivering) as effectors.
• Water balance — controlled by ADH from the pituitary, acting on the kidneys.

Why homeostasis matters

Every reaction in your body is catalysed by an enzyme, and enzymes work at their optimum at normal conditions. Move too far from the optimum temperature or pH and the enzyme denatures — its active site changes shape and the reaction stops. Cells also need a stable supply of glucose because cells need glucose for respiration to release the energy that keeps them alive. Homeostasis maintains a stable internal environment so that enzymes can keep catalysing reactions, cells can keep respiring, and restoring normal conditions allows effectors to function correctly.

The three parts of every control system

Every homeostatic loop has the same architecture:

• Receptors detect a stimulus (a change in the environment, internal or external). Examples: glucose-sensitive cells in the pancreas; thermoreceptors in the skin and in the hypothalamus.
• Coordination centres receive and process information from receptors. Examples: the pancreas (for blood glucose); the hypothalamus (for temperature and water).
• Effectors are muscles or glands that bring about a response — a muscle contracts (e.g. shivering) or a gland secretes (e.g. sweat glands secrete sweat, the pancreas secretes insulin).

Blood glucose regulation

### After a meal (glucose rises)

When you eat carbohydrates, carbohydrates broken down to glucose in the digestive system and glucose is absorbed into the blood. Blood glucose rises. The pancreas secretes insulin into the blood. Insulin causes liver/muscle cells to take up glucose. Inside the liver the glucose is converted to glycogen for storage. Blood glucose returns to normal / set point.

### Between meals or during exercise (glucose falls)

When blood glucose drops below the set point, the pancreas secretes glucagon. Glucagon acts on the liver: glycogen is broken down into glucose, and glucose enters the blood. This raises blood glucose back to normal — this maintains a stable internal environment.

Diabetes

Type 1 diabetes: the pancreas fails to produce insulin. After a meal, blood glucose rises but the body cannot bring it down. Treatment is insulin injection before meals — once injected, insulin causes liver/muscle cells to take up more glucose and insulin causes glucose to be converted to glycogen, so blood glucose returns to a safe level. Without insulin, blood glucose stays too high.

Type 2 diabetes: body cells stop responding to insulin (insulin resistance). The pancreas still makes insulin but cells ignore it, so blood glucose stays high. Type 2 is most commonly managed by diet and exercise — eating fewer carbohydrates and increasing physical activity so that muscle cells take up more glucose.

Body temperature regulation (thermoregulation)

Core body temperature must be held close to 37 °C because that is the optimum for human enzymes. Thermoreceptors in the hypothalamus monitor the temperature of the blood; thermoreceptors in the skin monitor the temperature of the environment. The hypothalamus is the coordination centre. The effectors are sweat glands, skin arterioles (which can undergo vasodilation or vasoconstriction) and skeletal muscles (which can shiver). Edexcel marks both directions.

### When the body is TOO HOT

1. Thermoreceptors detect the increase in temperature.
2. Sweat glands secrete more sweat onto the surface of the skin.
3. Sweat evaporates from skin, removing heat — evaporation takes thermal energy from the body, cooling the blood beneath.
4. Blood vessels near skin vasodilate / widen. This is vasodilation: the arterioles supplying the skin's surface capillaries get wider, so more warm blood flows close to the surface and more heat is lost by radiation.
5. Shivering does NOT happen when too hot — that would generate more heat.

Result: heat is lost faster than it is produced, and core temperature returns to 37 °C.

### When the body is TOO COLD

1. Thermoreceptors in the hypothalamus and skin detect the decrease in temperature.
2. Sweat glands stop secreting sweat — there is no evaporation to cool the body.
3. Skin arterioles undergo vasoconstriction — they narrow, so less warm blood flows near the surface and less heat is lost by radiation. (Blood is diverted to deeper vessels.)
4. Skeletal muscles shiver — rapid involuntary contractions release heat from respiration, warming the blood.
5. Hairs stand on end (erector muscles in the skin contract), trapping a layer of insulating air close to the skin.

Result: heat is conserved and generated, and core temperature returns to 37 °C.

Important wording note: the blood vessels (arterioles) widen and narrow — the blood vessels do NOT move closer to or further from the skin. Edexcel marks this carefully. Use the words vasodilation and vasoconstriction explicitly.

Water balance

Water content of the blood is monitored by the hypothalamus. If blood is too concentrated, the pituitary gland releases more ADH, which acts on the kidneys; the kidneys reabsorb more water from the urine into the blood. If blood is too dilute, less ADH is released and the kidneys reabsorb less water — producing more dilute urine. Detail is built up in topic 7.5 (the kidney).

Putting it together

For every Edexcel homeostasis question, build your answer from the same scaffold:

1. What is the stimulus? (e.g. blood glucose rises, body too hot)
2. What detects it? (receptor — e.g. pancreas, thermoreceptors)
3. What processes it? (coordination centre — e.g. pancreas, hypothalamus)
4. What is the effector? (muscle or gland — e.g. liver, sweat gland, skin arteriole)
5. What is the response? (e.g. insulin → glycogen, sweat evaporates, vasodilation)
6. End with: this maintains a stable internal environment.

Exam tips

• On any homeostasis definition question, write 'homeostasis maintains a stable internal environment' verbatim — it is the Edexcel mark.
• For 'why does homeostasis matter' questions, chain three ideas: enzymes work at their optimum at normal conditions; cells need glucose for respiration; restoring normal conditions allows effectors to function correctly.
• For blood-glucose-rising answers: pancreas secretes insulin → insulin causes liver/muscle cells to take up glucose → glucose converted to glycogen in liver → blood glucose returns to normal / set point. Four marks, four phrases.
• For blood-glucose-falling answers: pancreas secretes glucagon → glycogen is broken down into glucose → glucose enters the blood → this maintains a stable internal environment.
• For too-hot answers, get all four phrases in: thermoreceptors detect the increase; sweat glands secrete more sweat; sweat evaporates from skin, removing heat; blood vessels near skin vasodilate / widen.
• For too-cold answers, name vasoconstriction explicitly, then shivering (muscles release heat from respiration), then hairs standing on end (trapping insulating air).
• When writing about diabetes, name the type: Type 1 = no insulin produced → insulin injection; Type 2 = cells stop responding → diet + exercise.
• Use the words 'vasodilation' and 'vasoconstriction' explicitly — saying 'blood vessels get bigger/smaller' may not score.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in Edexcel GCSE Biology (1BI0)

• 7.1 Hormonal coordination