Homeostasis

What you'll learn

This revision guide covers homeostasis as specified in the WJEC GCSE Biology curriculum. You'll learn how the body maintains stable internal conditions despite external changes, focusing on temperature regulation, blood glucose control, and water balance. This topic is essential for understanding how organisms survive in varying environments and forms a significant portion of your GCSE exam.

Key terms and definitions

Homeostasis — The maintenance of a stable internal environment within an organism, keeping conditions constant despite external changes.

Negative feedback — A control mechanism where a change in a factor triggers a response that counteracts the initial change, returning the factor to its normal level.

Receptors — Cells or organs that detect changes (stimuli) in the internal or external environment.

Effectors — Muscles or glands that bring about responses to restore normal conditions.

Thermoregulatory centre — The region in the hypothalamus of the brain that monitors and controls body temperature.

Insulin — A hormone produced by the pancreas that reduces blood glucose concentration by stimulating glucose uptake by cells and conversion to glycogen.

Glucagon — A hormone produced by the pancreas that increases blood glucose concentration by stimulating the conversion of glycogen to glucose in the liver.

Vasodilation — The widening of blood vessels near the skin surface to increase heat loss from the body.

Core concepts

Principles of homeostasis

Homeostasis involves automatic control systems that maintain optimal conditions for enzyme action and cellular function. The human body must regulate several key factors:

• Body temperature (normally around 37°C)
• Blood glucose concentration
• Water content
• Ion content

All homeostatic control systems contain three essential components:

1. Receptors detect changes in the environment (stimuli)
2. Coordination centres (such as the brain, spinal cord, or pancreas) receive and process information, then organise a response
3. Effectors (muscles or glands) carry out the response

These components work together through negative feedback loops. When a factor deviates from its normal level, the body detects this change and responds to return the factor to its optimum level. Once the normal level is restored, the corrective mechanisms are switched off.

Temperature control in humans

Body temperature must be maintained close to 37°C for enzymes to function optimally. The thermoregulatory centre in the hypothalamus constantly monitors blood temperature and receives information from temperature receptors in the skin.

When body temperature is too high:

• Vasodilation occurs — blood vessels near the skin surface dilate (widen), allowing more blood to flow near the surface, increasing heat loss by radiation
• Sweat glands produce more sweat, which evaporates from the skin surface, removing heat energy and cooling the body
• Hairs lie flat against the skin (minimal effect in humans)
• Metabolic rate may decrease slightly

When body temperature is too low:

• Vasoconstriction occurs — blood vessels near the skin surface constrict (narrow), reducing blood flow to the surface and minimising heat loss
• Sweat production stops
• Skeletal muscles contract rapidly (shivering), and the energy released from increased respiration warms the body
• Hairs stand upright to trap an insulating layer of air (minimal effect in humans due to lack of body hair)
• Metabolic rate increases, generating more heat from respiration

These responses demonstrate negative feedback: high temperature triggers cooling mechanisms; low temperature triggers warming mechanisms. Once normal temperature is restored, the corrective responses stop.

Control of blood glucose concentration

Blood glucose concentration must be regulated because cells require a constant supply of glucose for respiration, but levels must not become too high or too low. The pancreas monitors blood glucose levels and secretes hormones to control them.

When blood glucose concentration is too high (e.g., after eating):

1. The pancreas detects the rise in blood glucose
2. Beta cells in the pancreas secrete insulin into the bloodstream
3. Insulin travels to target organs, particularly the liver and muscle cells
4. Insulin causes cells to take up more glucose from the blood
5. Insulin stimulates the liver to convert excess glucose into glycogen for storage (glycogenesis)
6. Blood glucose concentration returns to normal

When blood glucose concentration is too low (e.g., during exercise or between meals):

1. The pancreas detects the fall in blood glucose
2. Alpha cells in the pancreas secrete glucagon into the bloodstream
3. Glucagon travels to the liver
4. Glucagon stimulates the liver to convert stored glycogen back into glucose (glycogenolysis)
5. Glucose is released into the bloodstream
6. Blood glucose concentration returns to normal

This represents another negative feedback system. Insulin and glucagon have antagonistic effects (opposite actions), working together to maintain blood glucose within narrow limits.

Type 1 and Type 2 diabetes

Type 1 diabetes occurs when the pancreas fails to produce sufficient insulin. This usually develops in childhood and results from the immune system destroying beta cells in the pancreas.

Characteristics of Type 1 diabetes:

• Blood glucose concentration can rise to fatal levels after eating
• The kidneys cannot reabsorb all the glucose, so some appears in urine
• Uncontrolled, can lead to death

Treatment involves:

• Regular insulin injections, typically before meals
• Careful monitoring of diet, particularly carbohydrate intake
• Regular blood glucose testing
• Matching insulin dose to food intake and activity levels

Type 2 diabetes occurs when body cells no longer respond properly to insulin produced by the pancreas (insulin resistance). It typically develops in adults and is strongly linked to obesity and lack of exercise.

Risk factors for Type 2 diabetes:

• Obesity (particularly excess abdominal fat)
• Poor diet high in simple sugars and low in fibre
• Lack of physical exercise
• Family history
• Increasing age
• Ethnicity (higher risk in people of African-Caribbean, South Asian descent)

Treatment involves:

• Dietary modifications (reducing simple carbohydrates, increasing fibre)
• Regular exercise to use glucose and improve insulin sensitivity
• Weight loss
• Medication to help control blood glucose (in some cases)
• Insulin injections (if the condition progresses)

Type 2 diabetes is increasingly common in younger people due to rising obesity rates. Unlike Type 1, it can often be prevented or reversed through lifestyle changes.

Water and nitrogen balance

The body must balance water intake and water loss to maintain proper hydration and cellular function. Water is gained through drinking, eating, and as a product of respiration. Water is lost through:

• Urine (variable amount, controlled by kidneys)
• Sweat (variable, depending on temperature and exercise)
• Exhaled air (fixed loss)
• Faeces (small, relatively fixed amount)

The kidneys regulate water content by adjusting the concentration and volume of urine produced. On a hot day or during exercise, less urine is produced, and it is more concentrated. When water intake is high, more dilute urine is produced.

The kidneys also regulate ion content by controlling the reabsorption of mineral ions. Excess ions are removed in urine.

Removal of waste products:

The kidneys remove urea, a toxic waste product formed in the liver from the breakdown of excess amino acids. This process is called deamination. Amino acids cannot be stored in the body, so excess amino acids from protein digestion are broken down:

1. Amino acids are transported to the liver
2. The amino group is removed (deamination)
3. The amino group forms ammonia, which is toxic
4. Ammonia is immediately converted to urea, which is less toxic
5. Urea dissolves in blood plasma and is filtered out by the kidneys
6. Urea is excreted in urine

The nervous system and coordination

The nervous system allows rapid, coordinated responses to stimuli. It consists of:

• Central nervous system (CNS) — brain and spinal cord
• Peripheral nervous system — nerves connecting the CNS to receptors and effectors

The nervous system uses electrical impulses travelling along nerve cells (neurones) for rapid communication. A typical pathway is:

Stimulus → Receptor → Sensory neurone → CNS → Motor neurone → Effector → Response

This pathway is called a reflex arc when the response is automatic and rapid (a reflex action). Examples include:

• Withdrawing hand from a hot object
• Blinking when an object approaches the eye
• Knee-jerk reflex

Synapses are junctions between neurones where the electrical impulse triggers the release of chemical neurotransmitters. These chemicals diffuse across the tiny gap and trigger a new electrical impulse in the next neurone. Synapses ensure impulses travel in one direction only.

Reflex actions are important because they:

• Are rapid and automatic
• Protect the body from harm
• Do not require conscious thought
• Allow the brain to coordinate more complex behaviours

Worked examples

Question 1: Explain how the body responds when core body temperature increases above normal. (4 marks)

Mark scheme answer:

• The thermoregulatory centre in the brain/hypothalamus detects the temperature increase (1 mark)
• Blood vessels near the skin surface dilate/vasodilation occurs (1 mark)
• More blood flows near the surface, increasing heat loss by radiation (1 mark)
• Sweat glands produce more sweat, which evaporates and cools the skin (1 mark)

Examiner tip: This question uses the command word "explain," so you must give reasons and mechanisms, not just list responses. Link each response to its effect on temperature.

---

Question 2: A student measures their blood glucose concentration at different times during one day. The results are shown in the table below.

Time	Blood glucose (mg/100 cm³)
8:00 (before breakfast)	85
9:00 (after breakfast)	140
10:00	95

(a) Explain what causes the change in blood glucose between 8:00 and 9:00. (2 marks)

(b) Describe how the body reduces blood glucose concentration between 9:00 and 10:00. (3 marks)

Mark scheme answers:

(a)

• Food/breakfast contains carbohydrates/glucose (1 mark)
• Carbohydrates are digested and glucose is absorbed into the blood (1 mark)

(b)

• The pancreas detects high blood glucose (1 mark)
• The pancreas releases insulin into the blood (1 mark)
• Insulin causes the liver to convert glucose to glycogen/causes cells to take up more glucose (1 mark)

Examiner tip: Notice that part (a) uses "explain" (give reasons), while part (b) uses "describe" (say what happens). Both require detail but "explain" needs causation.

---

Question 3: Type 2 diabetes is becoming more common in the UK. Suggest why. (3 marks)

Mark scheme answer:

• Increased obesity/more people are overweight (1 mark)
• Due to poor diet/eating more sugar/processed foods/high-energy foods (1 mark)
• Less exercise/more sedentary lifestyle (1 mark)

Examiner tip: The command word "suggest" indicates there isn't a single correct answer, but your suggestions must be scientifically valid and linked to the context (Type 2 diabetes in the UK).

Common mistakes and how to avoid them

• Confusing Type 1 and Type 2 diabetes. Remember: Type 1 = no insulin produced; Type 2 = insulin produced but cells don't respond. Type 1 develops in childhood; Type 2 typically in adults and is linked to lifestyle.

• Stating that blood vessels move closer to or further from the skin surface. Blood vessels don't move. They dilate (widen) or constrict (narrow) to control blood flow near the surface. Use the correct terms: vasodilation and vasoconstriction.

• Saying glucose is "turned into" glycogen rather than "converted to" glycogen. Use precise biological terminology. Also specify that this happens in the liver (and muscles).

• Writing about the "brain" controlling homeostasis without being specific. Always state the thermoregulatory centre (for temperature) or identify the pancreas as the coordination centre (for blood glucose). The GCSE mark scheme rewards precision.

• Forgetting that homeostasis involves negative feedback. Don't just describe what happens when a factor increases; explain that the response reduces the factor back to normal, which then switches off the corrective mechanism.

• Confusing glucagon and glycogen. Glucagon is a hormone; glycogen is a storage carbohydrate. Pay attention to spelling in exams — marks can be lost for incorrect terminology.

Exam technique for "Homeostasis"

• Command words matter. "Describe" requires you to state what happens; "Explain" requires reasons and mechanisms. For "Explain" questions about homeostasis, always link the response to its effect on the factor being controlled.

• Use the mark allocation as a guide. A 4-mark question typically requires four distinct points. If you've only written two sentences for a 4-mark question, you've probably missed marks. Break down processes into clear steps.

• Include specific organ names and hormones. Don't write "organs release chemicals." Write "the pancreas secretes insulin." The more precise your biological terminology, the more likely you are to access full marks.

• For 6-mark questions on homeostasis, structure your answer logically: Start with the stimulus/change → detection by receptors → coordination centre processes information → effectors respond → effect of response → return to normal (negative feedback). This systematic approach ensures you cover all assessment criteria.

Quick revision summary

Homeostasis maintains stable internal conditions through negative feedback mechanisms involving receptors, coordination centres, and effectors. Body temperature is regulated by the thermoregulatory centre through vasodilation, sweating, vasoconstriction, and shivering. Blood glucose is controlled by the pancreas secreting insulin (reduces glucose by converting it to glycogen) and glucagon (increases glucose by converting glycogen to glucose). Type 1 diabetes results from insufficient insulin production; Type 2 from insulin resistance linked to obesity. The kidneys regulate water, ion content, and remove urea produced from amino acid breakdown.