WACE Human Biology Unit 3 deep dive: homeostasis and disease for the 2026 exam

How Unit 3 fits the exam

Unit 3 (homeostasis and disease) is half of the WACE Human Biology exam content and the source of many extended answer questions, because its processes (feedback loops, immune responses) lend themselves to step-by-step accounts. This deep dive links every Unit 3 dot point on the site. For exam format and timing, see the exam strategy guide.

1. Homeostasis and negative feedback

Dot point: homeostasis and feedback.

Homeostasis keeps the internal environment (temperature, glucose, water, pH) within narrow tolerance limits. The stimulus-response model has five parts: stimulus, receptor, control centre, effector, response. In negative feedback the response reduces the stimulus.

2. The nervous system and nerve impulses

Dot points: the nervous system, organisation of the nervous system, the nerve impulse and action potential.

• Central nervous system: brain and spinal cord. Peripheral: somatic (voluntary, skeletal muscles) and autonomic (involuntary), which splits into sympathetic ("fight or flight") and parasympathetic ("rest and digest").
• Resting potential: about $-70$ mV, maintained by the sodium-potassium pump and the membrane's greater permeability to $\text{K}^+$.
• Action potential: a stimulus above threshold opens voltage-gated $\text{Na}^+$ channels, $\text{Na}^+$ rushes in and the membrane depolarises; then $\text{K}^+$ channels open, $\text{K}^+$ flows out and the membrane repolarises. The response is all or none.
• Myelin and the nodes of Ranvier allow saltatory conduction, which is much faster.
• Synapse: the impulse triggers release of neurotransmitter from vesicles; it diffuses across the cleft and binds receptors on the next neuron. Transmission is one-way.
• Reflex arc: receptor, sensory neuron, interneuron (in the spinal cord), motor neuron, effector. It is fast because it does not wait for the brain.

3. The endocrine system

Dot point: the endocrine system.

Endocrine glands secrete hormones into the blood; only cells with the matching receptors respond. Compared with nervous control, hormonal control is slower to start, longer lasting and more widespread. The hypothalamus links the two systems through the pituitary gland.

4. Regulating temperature, glucose and water

Dot points: regulation of body temperature, regulation of blood glucose, regulation of body fluids and the kidney.

Blood glucose. High glucose: pancreatic beta cells release insulin, which increases glucose uptake by cells and glycogen formation in the liver and muscles. Low glucose: alpha cells release glucagon, which stimulates the liver to break glycogen down to glucose (glycogenolysis).

Water balance. Dehydration raises blood solute concentration. Osmoreceptors in the hypothalamus detect it, the posterior pituitary releases more ADH, and ADH increases the permeability of the distal tubules and collecting ducts to water, so more water is reabsorbed and urine becomes concentrated. Thirst is also stimulated.

5. When homeostasis fails

Dot point: disruption of homeostasis.

• Type 1 diabetes: autoimmune destruction of beta cells, so little or no insulin is produced; treated with insulin.
• Type 2 diabetes: cells respond poorly to insulin (insulin resistance); linked to lifestyle factors; managed with diet, exercise and medication.
• Hypothermia and hyperthermia: body temperature moves outside tolerance limits when heat loss or gain overwhelms the feedback system.

6. Pathogens and transmission

Dot point: pathogens and disease transmission.

Pathogens include bacteria, viruses, fungi, protists (protozoa), parasitic worms and prions. Transmission can be direct (contact, droplets, body fluids) or indirect (contaminated food or water, vectors such as mosquitoes, fomites). Incidence counts new cases in a period; prevalence counts all existing cases at a time.

7. The immune response

Dot point: pathogens and the immune system.

• First line: barriers such as skin, mucous membranes, cilia, stomach acid and lysozyme.
• Second line (non-specific): inflammation, fever, phagocytes (neutrophils, macrophages) and complement.
• Third line (specific): B lymphocytes produce antibodies (humoral immunity) after clonal selection, becoming plasma cells and memory cells. T lymphocytes provide cell-mediated immunity: helper T cells coordinate the response, and cytotoxic T cells destroy infected cells.

8. Immunisation and disease control

Dot point: immunisation and disease control.

• Active immunity: your own immune system makes antibodies and memory cells, naturally (infection) or artificially (vaccination). It is long-lasting.
• Passive immunity: you receive ready-made antibodies, naturally (across the placenta, in breast milk) or artificially (antibody injection). It is immediate but short-lived, with no memory cells.
• Herd immunity: when enough people are immune, transmission chains break, protecting those who cannot be vaccinated.
• Antibiotics act on bacteria, not viruses. Overuse and incomplete courses select for antibiotic-resistant bacteria.

Common mistakes

Check your knowledge

1. Which part of the autonomic nervous system increases heart rate during stress? (Answer: the sympathetic division.)
2. Which ion enters the neuron during depolarisation? (Answer: sodium ions.)
3. A mother's antibodies cross the placenta to her baby. Which type of immunity is this? (Answer: natural passive immunity.)

Then try the Unit 3 practice quiz.