keeping the internal environment stable

the movement of water in plants

Homeostasis | HSC Biology Module 8

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

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.

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.

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 11Water balance and the kidneys

The kidneys filter the blood and control how much water is lost in urine (osmoregulation).

Osmoreceptors in the hypothalamus detect that the blood is too concentrated (you are dehydrated).
The pituitary releases ADH (antidiuretic hormone).
ADH makes the collecting ducts in the kidneys more permeable to water, so more water is reabsorbed into the blood.
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 12Plants 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

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Try 3 questions now

Homeostasis is
1. A keeping the internal environment stable
2. B the growth of new cells
3. C the spread of disease
4. D the movement of water in plants
Show answer
A: keeping the internal environment stable
It means keeping internal conditions within a narrow range.
What is normal human core body temperature?
1. A About 30 degrees
2. B About 37 degrees
3. C About 42 degrees
4. D About 25 degrees
Show answer
B: About 37 degrees
About 36.5 to 37.5 degrees.
In negative feedback, the response
1. A makes the change bigger
2. B reverses the change
3. C has no effect
4. D stops all body processes
Show answer
B: reverses the change
It returns the variable toward normal.