HomeostasisOxford AQA IGCSE Biology: Revision notes
Section 1
What is homeostasis and why is it essential?
Homeostasis is the maintenance of a constant internal environment despite external changes. This is essential because cells require stable conditions (temperature, glucose concentration, water potential) to function optimally and carry out metabolic reactions efficiently.
The body constantly monitors internal conditions through receptors and makes adjustments via effectors (muscles and glands) to keep variables within narrow ranges. This is a form of negative feedback, where any deviation from the set point triggers responses that counteract the change and restore equilibrium.
Key variables maintained by homeostasis include:
- Body temperature (around 37°C)
- Blood glucose concentration
- Water content and blood osmotic potential
- Blood pH and ion concentration
Examiners want you to explain why homeostasis is important—always mention that cells need stable conditions for enzymes to work and metabolic reactions to occur at optimal rates.
Think of homeostasis like a thermostat in a house: when temperature drops below the set point, the heating turns on; when it rises above, heating turns off. The body does the same with temperature, glucose, and water.
Section 2
How does the body regulate temperature through thermoregulation?
The body maintains core temperature at approximately 37°C through a series of coordinated responses. The hypothalamus acts as the body's thermostat, detecting temperature changes via thermoreceptors and triggering appropriate effector responses.
When body temperature rises (too hot):
- Sweating increases: sweat glands secrete sweat onto the skin surface, which evaporates and removes latent heat
- Vasodilation occurs: blood vessels in the skin dilate, allowing more blood to flow close to the skin surface where heat can be lost to the environment
- Metabolic rate decreases
When body temperature falls (too cold):
- Shivering increases: involuntary muscle contractions generate heat through respiration
- Vasoconstriction occurs: blood vessels in the skin constrict, reducing blood flow to the surface and conserving heat in the body core
- Metabolic rate increases
- Hair erector muscles contract (less significant in humans but raises body hair)
The hypothalamus contains two regions: the anterior hypothalamus (heat loss centre) and posterior hypothalamus (heat gain centre). These coordinate nervous and hormonal responses to maintain thermal equilibrium.
When describing thermoregulation, always explain the mechanism of how each response works. For sweating, say 'evaporation of sweat removes latent heat'; for vasodilation, say 'more blood flows to the skin surface where heat is lost to the environment'.
Students often confuse vasodilation and vasoconstriction or forget to explain that these changes affect blood flow to the skin. Remember: dilation = larger diameter = more blood to surface = more heat loss.
A person exercises and body temperature rises to 38°C. The hypothalamus detects this via thermoreceptors. The anterior hypothalamus triggers vasodilation (more blood to skin surface) and increased sweating (evaporation cools the body). These responses continue until temperature returns to 37°C, then stop—this is negative feedback.
Section 3
How is blood glucose regulated by the pancreas, insulin and glucagon?
Blood glucose concentration is tightly regulated to around 80–120 mg/100 cm³ (or 4.5–7 mmol/L) to ensure cells have a constant supply of glucose for respiration whilst preventing osmotic damage.
The pancreas contains islets of Langerhans with two types of cells:
- Beta (β) cells: secrete insulin when blood glucose is HIGH
- Alpha (α) cells: secrete glucagon when blood glucose is LOW
When blood glucose is HIGH (after a meal):
- Beta cells in the pancreas detect high glucose
- Insulin is secreted into the bloodstream
- Insulin binds to receptors on liver cells and muscle cells
- These cells increase glucose uptake and convert glucose to glycogen for storage (liver) or use glucose for respiration and protein synthesis (muscle)
- Blood glucose concentration decreases back to normal
When blood glucose is LOW (between meals or during exercise):
- Alpha cells in the pancreas detect low glucose
- Glucagon is secreted into the bloodstream
- Glucagon binds to receptors on liver cells
- Liver cells break down glycogen to glucose (glycogenolysis) and convert non-carbohydrates to glucose (gluconeogenesis)
- Glucose is released into the bloodstream
- Blood glucose concentration increases back to normal
Both hormones work via negative feedback: deviation from normal glucose triggers a response that restores normal levels.
Examiners expect you to describe the complete pathway: which cells detect the change, which hormone is released, where it acts, and what cellular response occurs. Always link this back to negative feedback.
Students often say insulin 'removes glucose from the blood' without explaining how—it doesn't remove it; it increases the rate at which cells take up and use or store glucose. Be precise with your language.
A student eats a large meal rich in carbohydrates. Blood glucose rises to 150 mg/100 cm³. Beta cells detect this rise and secrete insulin. Insulin causes liver and muscle cells to absorb glucose and convert it to glycogen. Over time, blood glucose falls back to 80 mg/100 cm³, beta cells reduce insulin secretion, and equilibrium is restored.
Section 4
What is the structure and function of the kidney in regulating water and urine formation?
The kidney is a bean-shaped organ that filters blood to form urine. Understanding its structure is essential to understanding how it regulates water and waste removal.
Kidney structure (from outside to inside):
- Renal artery: brings oxygenated blood and glucose, amino acids, urea, and excess ions to the kidney
- Cortex: outer region where ultrafiltration occurs; contains Bowman's capsules and convoluted tubules
- Medulla: middle region containing the loop of Henle and collecting ducts; involved in selective reabsorption
- Ureter: carries urine from the kidney to the bladder
- Renal vein: carries filtered blood away from the kidney
Ultrafiltration (in the cortex): Ultrafiltration is the process by which small molecules are forced out of the capillary in the Bowman's capsule into the Bowman's capsule lumen.
- Blood enters the glomerulus (a network of capillaries) under high pressure
- Small molecules (glucose, amino acids, ions, urea, water) are forced through the capillary wall into Bowman's capsule
- Large molecules (proteins and red blood cells) remain in the capillaries because they are too large to pass through the selectively permeable capillary wall
- The result is filtrate in the Bowman's capsule
Selective reabsorption (in the cortex and medulla): Selective reabsorption is the process by which useful molecules are reabsorbed from the filtrate back into the blood.
- In the proximal convoluted tubule (PCT): all glucose, amino acids, and useful ions are reabsorbed by active transport (requires energy)
- In the loop of Henle: water and ions are reabsorbed, particularly in the descending limb (water) and ascending limb (ions)
- In the collecting duct: water reabsorption is controlled by ADH (antidiuretic hormone); more ADH = more water reabsorbed
- Urine is what remains: urea, excess ions, excess water
Role of ADH (antidiuretic hormone):
- ADH is produced by the hypothalamus and released by the posterior pituitary gland
- High ADH levels (when blood is too concentrated): more water is reabsorbed in the collecting duct, urine is dilute (dark), blood osmotic potential returns to normal
- Low ADH levels (when blood is too dilute): less water is reabsorbed, urine is concentrated and copious (pale), blood osmotic potential returns to normal
- ADH works via negative feedback: deviation from normal water/osmotic potential triggers ADH release or suppression to restore equilibrium
For HT students: examiners test understanding of why ultrafiltration and selective reabsorption occur. Ultrafiltration uses pressure to force small molecules through; selective reabsorption uses active transport and concentration gradients to recover useful substances.
Students often confuse filtration (non-selective, pressure-driven) with selective reabsorption (selective, active transport). Remember: filtration removes small molecules; reabsorption recovers the useful ones back into blood.
Blood enters the glomerulus with glucose, urea, and water. Pressure forces all three into Bowman's capsule (ultrafiltration). In the PCT, glucose and ions are actively transported back into the blood (selective reabsorption). Urea remains in the filtrate and eventually becomes part of urine.
Section 5
What happens when the kidneys fail and what are the treatment options?
Kidney failure occurs when the kidneys lose the ability to filter blood effectively and regulate water, ions and waste removal. This can result from disease, infection, high blood pressure, or diabetes.
Consequences of kidney failure:
- Urea accumulates in the blood (normally removed by filtration)
- Ions and water are not regulated properly, leading to imbalances
- Blood pressure may rise due to water retention
- Anaemia may develop (kidneys normally produce erythropoietin)
- If untreated, kidney failure is fatal
Treatment 1: Dialysis (artificial kidney) Dialysis removes waste products and excess water from the blood without needing functional kidney nephrons.
- Blood is taken from an artery and passed through a dialyser (a tube with a semi-permeable membrane)
- The dialyser contains a dialysate fluid with similar glucose and ion concentrations to healthy blood
- Waste products (urea) diffuse from blood → dialysate (concentration gradient)
- Excess water is removed by ultrafiltration (applying pressure or osmotic potential)
- Cleaned blood is returned to a vein
- Dialysis must occur 2–3 times per week for 3–5 hours per session
Advantages of dialysis:
- Non-invasive (no surgery)
- Extends life for patients with kidney failure
- Can be done at home (peritoneal dialysis) or in hospital (haemodialysis)
Disadvantages of dialysis:
- Time-consuming; requires frequent visits
- Cannot fully replicate all kidney functions (e.g. hormone production)
- Blood clotting risk and infection risk
- Expensive and depends on machine availability
Treatment 2: Kidney transplant A kidney from a healthy donor (living or deceased) is surgically placed into the body to replace a failed kidney.
Advantages of transplant:
- Provides full kidney function
- More freedom (no need for regular dialysis)
- Better long-term survival rates
- Better quality of life
Disadvantages of transplant:
- Surgical risk
- Organ rejection: the immune system may attack the foreign kidney (HLA matching and immunosuppressants reduce this risk)
- Lifelong need for immunosuppressant drugs (which have side effects)
- Shortage of donor organs
- Transplanted kidney eventually fails and may need replacement
Comparison table:
| Feature | Dialysis | Transplant |
|---|---|---|
| Function restored | Partial (waste removal, water regulation) | Full (all kidney functions) |
| Frequency | 2–3 times per week, 3–5 hours | Once; ongoing maintenance |
| Quality of life | Restricted by schedule | Greater freedom |
| Risks | Infection, clotting, incomplete waste removal | Rejection, surgery, immunosuppressant side effects |
| Cost | Expensive and ongoing | High upfront, lower long-term |
| Duration | Indefinite, while kidney function fails | Until rejection (5–10 years average) |
Examiners want you to compare dialysis and transplant by discussing both advantages and disadvantages. Don't just describe the procedures—evaluate which is 'better' depends on individual circumstances (age, health, organ availability).
A 40-year-old patient's kidneys fail due to diabetes. Option 1: Start dialysis 3 times per week; patient is restricted by schedule but avoids surgery. Option 2: Receive a kidney transplant from a deceased donor; patient has surgery risk and needs lifelong immunosuppressants, but gains freedom and better long-term health. Each choice has trade-offs.
Must Know
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Homeostasis is the maintenance of a constant internal environment (temperature ~37°C, glucose 80–120 mg/100 cm³, water content); this ensures cells can function optimally and enzymes work at maximum efficiency.
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Thermoregulation: the hypothalamus detects temperature changes and coordinates responses—sweating and vasodilation cool the body when hot; shivering and vasoconstriction warm the body when cold. All responses work via negative feedback.
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Blood glucose regulation: the pancreas secretes insulin (when glucose is HIGH) to lower it by increasing uptake and storage, and glucagon (when glucose is LOW) to raise it by promoting glycogen breakdown. Both are hormonal negative feedback.
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Kidney structure and function: the cortex performs ultrafiltration (small molecules forced into Bowman's capsule by pressure); the medulla performs selective reabsorption (useful molecules actively transported back into blood); the ureter carries urine to the bladder. The renal artery brings blood in; the renal vein takes filtered blood away.
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ADH (produced by hypothalamus, released by posterior pituitary) controls water reabsorption in the collecting duct. High ADH = more water reabsorbed = dilute urine. Low ADH = less water reabsorbed = concentrated urine. This maintains blood osmotic potential via negative feedback.
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Kidney failure treatments: Dialysis uses a semi-permeable membrane to remove waste and water but requires frequent visits and only partially restores function. Transplant surgically replaces the kidney, restoring full function and providing better quality of life, but requires surgery and lifelong immunosuppressants to prevent rejection. Choice depends on age, health, and donor availability.
That's the notes covered.
Carry on to the next subtopic.