Homeostasis

What you'll learn

Homeostasis is the maintenance of a constant internal environment within narrow limits, despite changes in external conditions. This topic covers how the human body regulates blood glucose concentration, body temperature, and water balance through negative feedback mechanisms. Understanding these regulatory systems is essential for explaining how organisms survive in changing environments.

Key terms and definitions

Homeostasis — the maintenance of a constant internal environment within narrow limits, keeping conditions optimal for enzyme function and cellular processes

Negative feedback — a control mechanism where a change in a factor triggers responses that counteract the change and restore the original level

Glucagon — a hormone produced by the pancreas that increases blood glucose concentration by stimulating glycogen breakdown in the liver

Insulin — a hormone produced by the pancreas that decreases blood glucose concentration by stimulating glucose uptake by cells and glycogen synthesis in the liver

Glycogen — a polysaccharide storage molecule formed from glucose, stored primarily in the liver and muscles

Vasodilation — the widening of blood vessels (arterioles) near the skin surface to increase heat loss from the body

Vasoconstriction — the narrowing of blood vessels (arterioles) near the skin surface to reduce heat loss from the body

ADH (antidiuretic hormone) — a hormone produced by the pituitary gland that increases water reabsorption in the kidney, producing more concentrated urine

Core concepts

Why homeostasis is important

The human body must maintain stable internal conditions for optimal functioning. Key factors requiring control include:

• Blood glucose concentration — must remain relatively constant to provide cells with a steady energy supply
• Body temperature — must stay around 37°C for enzymes to work at their optimum rate
• Water content — must be balanced to prevent cells from bursting (too much water) or shrinking (too little water)

If these conditions vary too far from normal:

• Enzymes may denature (at high temperatures) or work too slowly (at low temperatures)
• Cells may be damaged by osmotic stress
• Metabolic reactions cannot proceed efficiently

The central nervous system and hormonal system coordinate homeostatic responses through negative feedback mechanisms.

Control of blood glucose concentration

Blood glucose levels must remain stable between approximately 80-100 mg per 100 cm³ of blood. The pancreas detects changes in blood glucose and secretes hormones to correct deviations.

When blood glucose rises (after eating carbohydrate-rich food)

1. The pancreas detects the rise in blood glucose concentration
2. The pancreas secretes insulin into the bloodstream
3. Insulin travels to target organs, particularly the liver and muscles
4. Insulin causes:
   • Increased glucose uptake by cells for respiration
   • Conversion of glucose to glycogen for storage (glycogenesis)
   • Increased use of glucose in respiration
5. Blood glucose concentration returns to normal
6. The pancreas reduces insulin secretion (negative feedback)

When blood glucose falls (during exercise or between meals)

1. The pancreas detects the fall in blood glucose concentration
2. The pancreas secretes glucagon into the bloodstream
3. Glucagon travels to the liver
4. Glucagon causes:
   • Breakdown of glycogen into glucose (glycogenolysis)
   • Release of glucose into the blood
5. Blood glucose concentration returns to normal
6. The pancreas reduces glucagon secretion (negative feedback)

This is a classic example of negative feedback — the response counteracts the original change, maintaining blood glucose within narrow limits.

Type 1 diabetes

Type 1 diabetes is a condition where the pancreas produces insufficient or no insulin. This results in:

• Blood glucose concentration rising to dangerously high levels after eating
• Glucose appearing in urine (the kidneys cannot reabsorb all the excess glucose)
• Weight loss and fatigue (cells cannot take up enough glucose for respiration)

Treatment involves:

• Regular insulin injections (or insulin pump therapy)
• Careful monitoring of blood glucose levels
• Controlled carbohydrate intake matched to insulin doses
• Regular exercise to help regulate blood glucose

Type 1 diabetes usually develops in childhood or adolescence and requires lifelong management.

Control of body temperature

Humans are endotherms (warm-blooded), maintaining a constant body temperature around 37°C. The thermoregulatory centre in the hypothalamus of the brain monitors blood temperature and coordinates responses.

Temperature receptors are located in:

• The hypothalamus (detecting core body temperature)
• The skin (detecting external temperature changes)

When body temperature rises (hot environment or during exercise)

Vasodilation occurs:

• Arterioles supplying blood to skin capillaries dilate (widen)
• More blood flows through capillaries near the skin surface
• More heat is lost by radiation from the skin

Increased sweating:

• Sweat glands secrete more sweat onto the skin surface
• Water in sweat evaporates, removing heat energy from the body (latent heat of vaporisation)
• This cools the skin and blood flowing near the surface

Additional responses:

• Reduction in metabolic rate
• Hairs lie flat (less significant in humans than in other mammals)

When body temperature falls (cold environment)

Vasoconstriction occurs:

• Arterioles supplying blood to skin capillaries constrict (narrow)
• Less blood flows through capillaries near the skin surface
• Less heat is lost by radiation

Reduced sweating:

• Sweat glands produce less or no sweat
• Less heat is lost through evaporation

Shivering:

• Rapid, involuntary muscle contractions
• Respiration in muscle cells releases heat energy
• Increases heat production

Additional responses:

• Hairs stand upright due to contraction of erector muscles (trapping an insulating layer of air — more effective in furry mammals)
• Increased metabolic rate, particularly in the liver

Both responses demonstrate negative feedback — the corrective mechanisms work to restore the normal temperature.

Control of water balance in the body

Water enters the body through:

• Drinking fluids
• Food (many foods contain water)
• Metabolic reactions (e.g., respiration produces water)

Water is lost from the body through:

• Urine produced by the kidneys
• Sweat from the skin
• Water vapour in exhaled air
• Faeces

The kidneys regulate water balance by controlling the volume and concentration of urine produced.

Structure and function of the kidney

The kidneys filter blood and produce urine through three main processes:

Ultrafiltration (in the glomerulus and Bowman's capsule):

• Blood enters the glomerulus under high pressure
• Small molecules (water, glucose, urea, salts) are filtered out of the blood
• Large molecules (proteins, blood cells) remain in the blood
• The filtrate passes into the Bowman's capsule

Selective reabsorption (in the nephron tubules):

• All glucose is actively reabsorbed back into the blood (in healthy individuals)
• Most water is reabsorbed by osmosis
• Useful mineral ions are reabsorbed
• All reabsorption occurs into the network of capillaries surrounding the tubules

Formation of urine:

• Urea and excess water and salts continue along the tubule
• This forms urine, which flows to the bladder

Role of ADH

The concentration of urine is controlled by antidiuretic hormone (ADH), produced by the pituitary gland in the brain.

When the body is dehydrated (low water content in blood):

1. Receptors in the hypothalamus detect increased blood concentration
2. The pituitary gland releases more ADH into the bloodstream
3. ADH travels to the kidneys
4. ADH makes the collecting ducts more permeable to water
5. More water is reabsorbed from the tubules back into the blood
6. A smaller volume of more concentrated urine is produced
7. Water content of blood returns to normal (negative feedback)

When excess water is consumed (high water content in blood):

1. Receptors detect decreased blood concentration
2. The pituitary gland releases less ADH
3. Collecting ducts become less permeable to water
4. Less water is reabsorbed
5. A larger volume of more dilute urine is produced
6. Water content of blood returns to normal (negative feedback)

This is another example of negative feedback maintaining homeostasis.

Removal of waste products

While controlling water balance, the kidneys also remove metabolic waste products:

Urea:

• Produced in the liver from excess amino acids (deamination)
• Toxic if allowed to accumulate
• Filtered out of blood by the kidneys
• Excreted in urine

The kidneys maintain the correct balance of water and salts while removing urea, demonstrating their dual role in homeostasis and excretion.

Worked examples

Example 1: Explaining negative feedback in blood glucose control

Question: Describe and explain what happens when blood glucose concentration becomes too high after eating a meal rich in carbohydrates. (4 marks)

Mark scheme answer:

• The pancreas detects the rise in blood glucose (1 mark)
• The pancreas secretes insulin into the blood (1 mark)
• Insulin causes the liver to convert glucose to glycogen / causes cells to take up more glucose (1 mark)
• Blood glucose concentration decreases back to normal (1 mark)

Examiner note: Each process step earns one mark. Be specific about the hormone (insulin, not just "a hormone") and the storage molecule (glycogen). Mention the pancreas as both detector and effector.

Example 2: Temperature regulation

Question: Explain how the body responds to prevent overheating during vigorous exercise on a hot day. (5 marks)

Mark scheme answer:

• Temperature receptors detect the rise in body temperature / hypothalamus detects rise (1 mark)
• Vasodilation occurs / arterioles supplying skin capillaries widen (1 mark)
• More blood flows near the skin surface / more heat is lost by radiation (1 mark)
• Sweat glands produce more sweat (1 mark)
• Evaporation of sweat removes heat from the skin / cooling effect (1 mark)

Examiner note: Explain both mechanisms (vasodilation and sweating). Use correct terminology — "vasodilation" not "blood vessels move closer to the skin." Explain the cooling mechanism of evaporation.

Example 3: Kidney function and ADH

Question: A person exercises intensely for one hour in hot conditions without drinking water. Explain how the body responds to maintain water balance. (5 marks)

Mark scheme answer:

• Water is lost through sweating / water content of blood decreases (1 mark)
• Detected by receptors in the hypothalamus / brain (1 mark)
• Pituitary gland releases more ADH (1 mark)
• ADH makes kidney tubules / collecting ducts more permeable to water (1 mark)
• More water is reabsorbed / smaller volume of more concentrated urine is produced (1 mark)

Examiner note: Link the stimulus (water loss) to the detection mechanism, then explain the hormone's action and effect. Mention both the change in volume and concentration of urine.

Common mistakes and how to avoid them

• Confusing insulin and glucagon — Remember: insulin lowers blood glucose (think "in-sulin puts glucose in to storage"), glucagon raises it. Learn which does what and stick to one explanation rather than mixing them up.

• Saying "blood vessels move to the skin surface" — Blood vessels don't move. Instead, arterioles dilate (vasodilation) allowing more blood to flow through capillaries already near the surface, or constrict (vasoconstriction) reducing blood flow to these capillaries.

• Forgetting that sweating alone doesn't cool you — It's the evaporation of sweat that causes cooling (requires energy). Simply producing sweat without evaporation has minimal cooling effect.

• Mixing up Type 1 and Type 2 diabetes — At IGCSE level, focus on Type 1 diabetes (pancreas doesn't produce enough insulin, treated with insulin injections). Type 2 is not required for this specification.

• Thinking ADH produces urine — ADH doesn't make urine; it controls how much water is reabsorbed from the filtrate. More ADH means more reabsorption and therefore more concentrated urine.

• Not explaining negative feedback — Always complete the loop: state what is detected, what response occurs, what the effect is, and that this returns the factor to normal, which then reduces the corrective response.

Exam technique for "Homeostasis"

• Command words matter: "Describe" means state what happens (observations, steps in a process). "Explain" means give reasons why it happens (use "because," "so that," "this causes"). Explanation questions require cause-and-effect links and usually carry more marks.

• Use correct terminology consistently: Learn the precise terms (vasodilation, not "widening of blood vessels near the skin"; glycogen, not "animal starch"). Examiners expect and reward accurate biological vocabulary.

• Follow the mark scheme structure: For a 4-mark question on negative feedback, you need four distinct points. Break responses into: detection → hormone/response → effect on target organ → return to normal. Check you've made enough separate points.

• Draw clear sequences: For processes like blood glucose control, use numbered steps or flow diagrams in your revision. In exams, structure your answer sequentially to avoid missing steps and losing marks.

Quick revision summary

Homeostasis maintains constant internal conditions through negative feedback mechanisms. Blood glucose is controlled by insulin (lowers glucose by promoting glycogen storage) and glucagon (raises glucose by promoting glycogen breakdown), both secreted by the pancreas. Body temperature is regulated through vasodilation, sweating, vasoconstriction, and shivering, coordinated by the hypothalamus. Water balance is controlled by ADH from the pituitary gland, which regulates water reabsorption in the kidneys. All homeostatic systems involve detection of changes, coordinated responses, and restoration of normal conditions.