QCE Biology Unit 2 Maintaining the Internal Environment: deep-dive 2026 guide

How Unit 2 fits into QCE Biology

Unit 2, Maintaining the internal environment, takes the cell-level biology of Unit 1 and scales it up to whole-organism control. Topic 1 is homeostasis: how a multicellular body keeps internal conditions stable. Topic 2 is infectious disease and immunity: how the body defends that internal environment against pathogens. The negative feedback loop is the central model of Topic 1 and reappears in Unit 3 ecosystem stability, so mastering it pays off twice. This guide develops the high-value subject matter with worked examples and a Check your knowledge section.

Homeostasis and the set point

Homeostasis is the maintenance of a relatively constant internal environment despite changes in the external environment. "Constant" does not mean unchanging: each regulated variable oscillates within a narrow range around a set point. Homeostasis is a dynamic steady state, not a fixed value.

Human set points worth knowing:

• Core body temperature: about 37 degrees Celsius.
• Blood pH: 7.35 to 7.45.
• Blood glucose: about 4 to 6 mmol per L between meals.
• Blood osmotic pressure: about 300 mOsm per kg.

If a variable drifts too far from its set point, enzymes denature, membrane transport fails and cells die. The body therefore needs control systems that detect departures and trigger corrective responses.

The negative feedback loop

Every homeostatic control loop contains the same five components in the same order.

1. Stimulus. A change in the variable away from the set point.
2. Receptor (sensor). A cell or structure that detects the change. Examples: thermoreceptors, osmoreceptors, chemoreceptors, glucose-sensing pancreatic beta cells.
3. Control centre. Integrates the input and decides on a response. Often the hypothalamus, brainstem or an endocrine cell.
4. Effector. Carries out the response. Usually a muscle or a gland.
5. Response. The action that returns the variable toward the set point.

In negative feedback, the response opposes the change that triggered it: a rise triggers a response that lowers the variable, and a fall triggers a response that raises it. The variable oscillates around the set point. Negative feedback is the dominant mechanism in physiology.

In positive feedback, the response amplifies the change, accelerating it away from the start until an external event ends the loop. It is rare and reserved for processes that need to go to completion, such as childbirth (oxytocin and uterine contractions), blood clotting and the rising phase of an action potential.

Nervous and endocrine control

The body has two communication systems that drive effectors.

• Nervous control uses electrical impulses (action potentials) along neurons, with chemical neurotransmitters carrying the signal across synapses. It is fast, short-lived and precisely targeted. A reflex arc (receptor to sensory neuron to interneuron in the spinal cord to motor neuron to effector) produces a rapid, involuntary protective response such as withdrawing a hand from a hot surface.
• Endocrine control uses hormones, chemical messengers released by glands into the bloodstream. Hormones travel to all cells but act only on those with the matching receptor. Endocrine signalling is slower to start but longer-lasting, suiting sustained, body-wide responses such as growth, metabolism and blood glucose control.

Many homeostatic systems use both: the hypothalamus (nervous) controls the pituitary (endocrine), which controls other glands, linking fast detection to durable hormonal responses.

Thermoregulation

Mammals are endotherms that hold core temperature near 37 degrees Celsius. The hypothalamus is the control centre, integrating signals from skin and core thermoreceptors.

When too hot: sweat glands secrete sweat (evaporation removes heat), and skin arterioles dilate (vasodilation brings warm blood to the surface for heat loss). Behaviour adds to this (seeking shade).

When too cold: skin arterioles constrict (vasoconstriction reduces surface heat loss), skeletal muscles shiver (generating heat), arrector pili muscles raise hairs (trapping insulating air), and adrenaline and thyroid hormones raise metabolic rate.

Osmoregulation and the kidney

Osmoregulation maintains the water and solute balance of body fluids. The kidney is the main effector organ and the hormone ADH (antidiuretic hormone) is the key regulator.

When blood osmotic pressure rises (dehydration), osmoreceptors in the hypothalamus detect it; the posterior pituitary releases ADH; ADH makes the collecting ducts of the kidney nephrons more permeable to water, so more water is reabsorbed into the blood and a small volume of concentrated urine is produced. When blood is too dilute, ADH release falls, less water is reabsorbed, and a large volume of dilute urine is produced. This is a classic negative feedback loop returning blood osmotic pressure to its set point.

Blood glucose regulation

Blood glucose is held near 4 to 6 mmol per L by two antagonistic hormones from the pancreatic islets.

• After a meal (glucose rises): beta cells detect the rise and release insulin. Insulin makes liver and muscle cells take up glucose and store it as glycogen (glycogenesis), lowering blood glucose.
• Between meals (glucose falls): alpha cells release glucagon. Glucagon stimulates the liver to break down glycogen to glucose (glycogenolysis) and release it, raising blood glucose.

The two hormones form opposing arms of a negative feedback loop, keeping glucose within range. In type 1 diabetes the beta cells are destroyed and produce little insulin; in type 2 diabetes the target cells become resistant to insulin, so correction of a glucose rise is slower.

Pathogens and disease transmission

A pathogen is a disease-causing organism or agent: bacteria, viruses, fungi, protists and prions. Diseases spread by routes including direct contact, droplets and aerosols, contaminated food and water, body fluids, and vectors such as mosquitoes. Reducing transmission targets these routes: hygiene, sanitation, quarantine, vector control and vaccination.

Innate and adaptive immunity

The immune system has two arms.

Innate (non-specific) immunity is present from birth and responds the same way to any pathogen.

• First line of defence: physical and chemical barriers (skin, mucus, cilia, stomach acid, lysozyme in tears).
• Second line of defence: internal non-specific responses if a pathogen breaches the barriers, including phagocytosis by macrophages and neutrophils, the inflammatory response (redness, heat, swelling, pain from increased blood flow and capillary permeability), and fever (a raised hypothalamic set point that slows pathogen growth).

Adaptive (specific) immunity is the third line of defence, mediated by lymphocytes that recognise a specific antigen.

• Humoral response: B lymphocytes recognise an antigen, are activated (with helper T cell support), and proliferate into plasma cells that secrete antibodies. Antibodies neutralise pathogens, mark them for phagocytosis (opsonisation) and trigger their destruction.
• Cell-mediated response: helper T cells coordinate the response and cytotoxic T cells destroy infected body cells.
• Memory: some activated B and T cells become long-lived memory cells. On re-exposure they drive a faster, larger secondary response, the basis of long-term immunity.

Vaccination and antibiotic resistance

A vaccine presents a harmless form of an antigen (attenuated or killed pathogen, a subunit, or an mRNA template) so the body mounts a primary adaptive response and forms memory cells without disease. On later exposure to the real pathogen, the secondary response clears it before symptoms appear. Widespread vaccination also produces herd immunity, indirectly protecting unvaccinated individuals by reducing transmission.

Antibiotics kill or inhibit bacteria but not viruses. Antibiotic resistance arises by natural selection: random mutation produces resistant bacteria, antibiotic use kills the susceptible ones, and the resistant survivors reproduce and pass on resistance. Overuse and incomplete courses accelerate the spread of resistance, which is why antibiotics should be used only when needed and courses completed.

Check your knowledge

A mix of recall, short-response and exam-style application questions covering Unit 2 subject matter. Answer all under timed conditions (about 1 minute per mark), then check against the solutions block.

1. Define homeostasis and explain why it is described as a dynamic steady state rather than a fixed value. (3 marks)
2. List the five components of a negative feedback loop in order and define each. (5 marks)
3. Compare nervous and endocrine control with respect to (a) the signal used, (b) speed of response, and (c) duration of response. (3 marks)
4. A person enters a cold environment and their core temperature begins to fall. Describe the negative feedback response, naming the receptor, control centre, and at least three effectors with their responses. (5 marks)
5. Explain how ADH regulates blood osmotic pressure during dehydration, identifying the receptor, the gland that releases ADH, the target and the response. (4 marks)
6. Distinguish between the actions of insulin and glucagon, and explain why type 2 diabetes results in slower correction of high blood glucose. (4 marks)
7. Distinguish between innate and adaptive immunity, giving two examples of innate defences and explaining the role of memory cells in adaptive immunity. (4 marks)
8. Antibiotic resistance is spreading in a hospital. (a) Explain, using natural selection, how a resistant bacterial population arises. (b) State why antibiotics are useless against a viral infection. (c) Identify two practices that reduce the spread of resistance. (5 marks)