Maintaining a stable internal environment (internal)
Internal 3 credits Homeostasis Negative feedback Thermoregulation Blood pressure Osmoregulation Blood glucose Adaptive advantageOne control system, covered in four parts the standard names explicitly: its purpose, its components, its mechanism (including feedback), and the effect of disruption. Five systems are eligible — temperature, blood pressure, osmotic balance, blood glucose, and respiratory gases. Merit needs a specific disruption traced to the responses that correct it; Excellence needs adaptive advantage, the underlying biochemistry, or an analysed breakdown.
1 Reading the notation
Cover all four biological ideas the standard names, across whichever of the five systems you choose, or the description is incomplete by definition.Explanatory note 4, in NZQA's own terms
4 terms- Purpose
- What the system is FOR — which variable it holds steady and why that matters for the animal. Enzymes and membranes work only within narrow ranges
- Components
- The parts: receptor, control centre, effectors — named specifically for this animal. "The hypothalamus", not "the brain"
- Mechanism
- How it responds to normal fluctuations, and the interaction and feedback between the parts. This is where negative feedback is explained
- Effect of disruption
- What happens when an internal or external influence pushes the variable outside the normal range. Merit needs a SPECIFIC disruption traced through
2 Negative vs positive feedback
Only negative feedback is homeostatic — know both, so you can say why the other one is NOT the mechanism here.| Negative feedback | Positive feedback | |
|---|---|---|
| Effect on the stimulus | Reduces it | Amplifies it |
| Self-limiting? | Yes — switches off as the variable returns to normal | No — accelerates until something external stops it |
| Used for | Maintaining stability (homeostasis) | Driving a process to rapid completion (labour, clotting, an action potential) |
3 The five homeostatic systems
Any of these five is eligible — the same stimulus→receptor→control centre→effector→response structure applies to all.| System | Detected by | Effector response |
|---|---|---|
| Body temperature | Thermoreceptors (hypothalamus + skin) | Vasodilation/constriction, sweating/shivering, metabolic rate |
| Blood pressure | Baroreceptors (carotid sinus, aortic arch) | Heart rate via autonomic nerves; arteriole diameter |
| Osmotic balance | Osmoreceptors (hypothalamus) | ADH → collecting duct water permeability |
| Blood glucose | Pancreatic islet cells | Insulin (beta cells) / glucagon (alpha cells) |
| Respiratory gases | Chemoreceptors (mainly CO₂-sensitive) | Breathing rate and depth via the medulla |
Blood pressure is the system most often left out — it has no single obvious hormone the way blood glucose does, but the baroreceptor reflex is exactly as eligible as the other four.
4 A specific disruption, traced through
This is the Merit-level move — specificity is what makes the trace explainable. Internal-style A specific disruption: 30 minutes of running raises core temperature 1.5 °C Trace the responses that re-establish stability. STIMULUS: working muscle releases heat as a by-product of respiration; core temperature rises above the ~37 °C set point. RECEPTOR: central thermoreceptors in the hypothalamus detect the rise in blood temperature; peripheral receptors detect skin temperature. CONTROL CENTRE: the hypothalamus compares the value to the set point and initiates heat-loss responses. EFFECTORS: skin arterioles vasodilate, raising blood flow to the surface; sweat glands secrete. RESPONSE: radiation from the warmer skin increases; evaporating sweat removes latent heat — both lower core temperature. NEGATIVE FEEDBACK: as core temperature falls back toward 37 °C the stimulus weakens, so sweating and vasodilation reduce — the correction switches itself off. ✓ Six steps, each naming a component. The last one — the loop switching itself off — is what makes it negative feedback rather than a list of reactions.5 Reaching Excellence: pick ONE of the three routes
The standard offers three, and only one is required.| Route | What it looks like | Best system for it |
|---|---|---|
| Adaptive advantage | Why the system improves survival here — with its cost | Thermoregulation (endothermy vs ectothermy) |
| Biochemistry / biophysics | Equilibrium reactions, membrane permeability, metabolic pathways | Respiratory gases (the CO₂/H⁺ equilibrium); ADH and aquaporins |
| Analysed breakdown | A real failure, which component failed, and why the loop cannot close | Blood glucose (type 1 diabetes); blood pressure (hypertension); temperature (heat stroke) |
One route, done thoroughly, satisfies the standard. Attempting all three shallowly usually satisfies none.
✕Choosing a system with no clear route to Excellence
Before committing to a system, check you can see at least one of the three Excellence routes for it. Blood pressure works well for a breakdown analysis (hypertension) but has less obvious biochemistry to reach for than respiratory gases or osmoregulation.Practice
Recall it first, then use it — testing yourself builds the memory, where rereading the notes above only feels easier.
Recall and application shuffled together — you have to work out what each item wants before you can answer it, which is the part the exam actually tests. Both kinds feed the same spacing schedule, so anything you miss comes back sooner.
Say the answer out loud before flipping — that attempt is where the learning happens, even when you get it wrong.
Exam-style only. Instant feedback, shuffled each attempt, best score saved on this device.