# Homeostasis

> Keeping the internal environment stable with negative feedback, the nervous and endocrine systems, and how the body controls temperature, blood glucose and water.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/homeostasis/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The marathon in the heat
At the Gold Coast marathon on a warm morning, a runner's muscles are burning glucose fast and making heat. Her body temperature starts to climb. Without her thinking about it, the blood vessels in her skin widen and she sweats heavily. As the sweat evaporates, it carries heat away.

At the drinks station, she takes water. Her kidneys have already been saving water, making only a little dark urine. Her liver is releasing stored glucose to keep her blood sugar steady. Dozens of systems are working at once to keep her inside a narrow safe zone. If they fail, she could collapse with heat stroke. This balancing act is called **homeostasis**.

**In short:** Homeostasis is how the body keeps its internal environment stable, like body temperature and blood glucose, so cells work properly. It works by negative feedback: a receptor detects a change, a control centre responds, and an effector reverses it, using nerves and hormones.

## Starting point: what you already know

- Enzymes work best at an optimum temperature and pH (see Enzymes).
- The nervous system carries fast electrical messages. Hormones are chemical messengers in the blood (Year 9 science).
- Cells need glucose and water and must remove wastes.

## What is homeostasis?

**Homeostasis** is keeping the **internal environment** (blood and the fluid around cells) **stable**, within a narrow range, even when the outside world changes.

| Variable | Normal range in humans |
|---|---|
| Core body temperature | About 36.5 to 37.5 degrees |
| Blood glucose | About 4 to 8 mmol/L |
| Blood pH | 7.35 to 7.45 |
| Water and salt balance | Kept steady by the kidneys |

Why does it matter? **Enzymes** only work well in a narrow range of temperature and pH, and cells shrink or burst if the water balance is wrong (osmosis).

## The stimulus-response model and negative feedback

### Key idea: The stimulus-response pathway
**Stimulus** (a change) → **receptor** (detects it) → **control centre** (often the brain) → **effector** (a muscle or gland) → **response** (brings the variable back to normal)

In **negative feedback**, the response **reverses** the change. If temperature rises, the body acts to cool down. If it falls, the body acts to warm up. It works like a thermostat on an air conditioner.

*Diagram: Negative feedback loop: temperature rises, receptors detect it, the hypothalamus responds, effectors cool the body, and temperature returns to normal*
*Negative feedback brings a variable back toward normal. The same loop works for cold, with shivering and narrowed vessels.*

## Two control systems

| | Nervous system | Endocrine system |
|---|---|---|
| Message | Electrical impulses along neurons | Hormones carried in the blood |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Effect lasts | Short time | Longer time |
| Target | Specific muscles or glands | Any cell with the right receptor |

The **hypothalamus** in the brain links the two. It is the control centre for temperature and water balance, and it controls the **pituitary gland**, the "master gland" of the endocrine system.

## Temperature control (thermoregulation)

| Too hot | Too cold |
|---|---|
| **Vasodilation**: skin blood vessels widen, more heat lost | **Vasoconstriction**: skin blood vessels narrow, heat kept in the core |
| Sweating: evaporation removes heat | Shivering: muscles contract and release heat |
| Hairs lie flat | Hairs stand up (goosebumps) |
| Behaviour: seek shade, remove clothes | Behaviour: put on clothes, huddle |
| Lower metabolic rate | **Thyroxine** raises metabolic rate |

**Endotherms** (mammals, birds) make their own heat. **Ectotherms** (reptiles, most fish) depend on outside heat and control temperature mostly by **behaviour**, like a goanna basking on a rock.

## Blood glucose control

The **pancreas** watches blood glucose and releases two hormones.

- Glucose **high** (after a meal) → **insulin** released → cells take in glucose, and the liver stores it as **glycogen** → glucose falls.
- Glucose **low** (exercise, fasting) → **glucagon** released → the liver breaks glycogen into glucose → glucose rises.

### Exam tip: Insulin and glucagon
In**sulin** puts sugar **in** to cells. **Glucagon**: the glucose is **gone**, so release some.

**Mind map**

```text
Homeostasis
  Why
    Stable internal environment
    Enzymes need it
  Model
    Stimulus
    Receptor
    Control centre
    Effector
    Response
  Negative feedback
    Reverses the change
  Systems
    Nervous: fast
    Endocrine: hormones
    Hypothalamus links them
  Examples
    Temperature: sweat, shiver
    Glucose: insulin, glucagon
    Water: ADH
```

### Year 11: Water balance and the kidneys
The kidneys filter the blood and control how much water is lost in urine (**osmoregulation**).

1. **Osmoreceptors** in the hypothalamus detect that the blood is too concentrated (you are dehydrated).
2. The **pituitary** releases **ADH** (antidiuretic hormone).
3. ADH makes the collecting ducts in the kidneys more permeable to water, so more water is reabsorbed into the blood.
4. Less, more concentrated urine is made, and blood concentration returns to normal.

If you drink a lot of water, less ADH is released and you make lots of dilute urine. **Aldosterone** controls salt reabsorption in a similar way.

**How nerves send messages.** At rest, a neurone is negative inside. In an **action potential**, sodium channels open and **sodium ions rush in**, making the inside positive (**depolarisation**). Potassium ions then flow out to return it to rest.

**Crossing a synapse.** The impulse reaches the end of the neurone, which releases a **neurotransmitter**. It diffuses across the gap and binds to receptors on the next cell, starting a new impulse.

**Why nerves for urgent jobs.** Changes like low blood oxygen need a response in seconds. Nerve impulses are far faster than hormones in the blood.

**Negative feedback in hormones.** High thyroxine reduces TSH release from the pituitary, so less thyroxine is made. The hormone switches off its own production.

**Positive feedback** makes a change bigger instead of reversing it. In childbirth, contractions cause more oxytocin, which causes stronger contractions, until the baby is born.

### Year 12: Plants and homeostasis, and when it fails
Plants also keep their internal conditions stable: **stomata** close when water is short, controlled by the hormone **abscisic acid**, and leaves can change angle to avoid overheating.

When homeostasis **fails**, disease follows. **Type 1 diabetes** happens when the immune system destroys insulin-making cells, so blood glucose stays high. **Type 2 diabetes** happens when cells stop responding properly to insulin. **Heat stroke** happens when the body can no longer lose heat fast enough and its temperature passes about 40 degrees. These are disorders studied in the next chapters.

**Kinds of adaptation.**
- **Structural**: a body feature (sunken stomata, thick fur).
- **Physiological**: how the body works (sweating, counter-current blood flow in a penguin's feet).
- **Behavioural**: what the animal does (huddling, basking).

**Sunken stomata** sit in pits where moist air is trapped. This lowers the water vapour gradient, so less water evaporates.

**Metabolism and temperature.** As it gets colder, an endotherm raises its metabolic rate to make more heat. An ectotherm's body cools with its surroundings, so its enzymes slow and its metabolic rate falls.

**Reading a glucose graph.** If blood glucose stays high for hours after a sugary drink, the person is not making enough insulin or their cells are not responding to it, a sign of diabetes.

## Quick summary

- Homeostasis keeps the internal environment stable so enzymes and cells work properly.
- Stimulus → receptor → control centre → effector → response.
- Negative feedback reverses a change.
- The nervous system is fast and short-lived. Hormones are slower and last longer. The hypothalamus links them.
- Too hot: vasodilation, sweating. Too cold: vasoconstriction, shivering.
- High glucose: insulin. Low glucose: glucagon. Dehydration: ADH saves water.

## Quiz

Try these questions from the chapter quiz.

1. Homeostasis is
   - A. keeping the internal environment stable
   - B. the growth of new cells
   - C. the spread of disease
   - D. the movement of water in plants
   - Answer: A. keeping the internal environment stable. It means keeping internal conditions within a narrow range.

2. What is normal human core body temperature?
   - A. About 30 degrees
   - B. About 37 degrees
   - C. About 42 degrees
   - D. About 25 degrees
   - Answer: B. About 37 degrees. About 36.5 to 37.5 degrees.

3. In negative feedback, the response
   - A. makes the change bigger
   - B. reverses the change
   - C. has no effect
   - D. stops all body processes
   - Answer: B. reverses the change. It returns the variable toward normal.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/homeostasis/#quiz