Section 1 of 9What is homeostasis?
Homeostasis is the regulation of the conditions inside the body to maintain a constant internal environment, in spite of changes outside.
It keeps conditions at the optimum for cells and for enzyme action.
Body temperature
The factors controlled in humans include body temperature, blood glucose concentration, water content (and so the concentration of body fluids), ion concentration, carbon dioxide level and blood pH.
The body does not hold these exactly constant; each fluctuates slightly around a set value, but is brought back whenever it strays too far.
Section 2 of 9Why constant conditions matter
Enzymes
Enzymes work best at a particular temperature and pH.
- Human enzymes have an optimum around 37 degrees Celsius.
- If the body is too cold, reactions are too slow;
- if it is too hot, enzymes can denature and lose their shape, so metabolism fails.
Water and ions
Water and ions if the concentration of the blood is too high or too low, water moves into or out of cells by osmosis,
- causing them to shrink or swell and burst.
Glucose
Glucose cells need a continuous supply for respiration. Too little starves the brain; too much damages tissues and affects water balance.
Section 3 of 9The control system
Most homeostatic mechanisms use the same three parts:
The control system
Receptors
Receptors detect a change in a condition (the stimulus), for example temperature receptors in the skin and brain.
Coordination centre
Coordination centre receives the information and decides on the response, for example the brain, spinal cord or pancreas.
Effectors
Effectors carry out the response to restore the condition to normal; these are muscles or glands.
The information may travel by nerve impulses or by hormones. Because the response reverses the original change, the system is negative feedback: if a value rises above normal, the effectors lower it; if it falls, they raise it. Negative feedback therefore keeps the level close to the set point.
Section 4 of 9A worked example of negative feedback
Take body temperature.
1
If the core temperature rises, temperature receptors in the hypothalamus of the brain and the skin detect it.
2
The brain acts as coordinator and sends nerve impulses to effectors: sweat glands produce sweat, and blood vessels near the skin widen.
3
Heat is lost, temperature falls and the system turns off.
4
If the temperature falls, shivering and constriction of the skin blood vessels raise it again.
5
A graph of body temperature over time shows a small wave around 37 degrees Celsius as the mechanisms switch on and off,
- which is typical of a negative feedback system.
| Condition | Controlled by | Effector or organ |
|---|---|---|
| Temperature | Nervous system | Skin, muscles, sweat glands |
| Blood glucose | Insulin and glucagon | Liver, muscles |
| Water content | ADH | Kidneys |
Section 5 of 9Applying the idea in exam questions
When asked to explain a homeostatic response, use the sequence: stimulus, receptor, coordinator, effector, response, and then say how the change is reversed.
1
Mention the term negative feedback.
2
Data questions usually provide a graph with a change in condition followed by a recovery: describe the initial change with figures, say when the response starts and say how long it takes for the level to return to normal.
3
Remember that homeostasis fails in disease: in diabetes the glucose system does not work,
- and in kidney failure the water and ion balance cannot be maintained.
4
Later lessons look at temperature, glucose and water balance in detail.
Section 6 of 9Homeostasis across the body
It is useful to see how many different parts of the body contribute to homeostasis.
Homeostasis across the body
The skin and brain regulate temperature; the pancreas and liver regulate glucose; the kidneys regulate water and ions; the lungs remove carbon dioxide,
- which would otherwise make the blood too acidic.
Each system works by negative feedback,
- but the type of messenger differs: temperature control uses mostly nerve impulses,
- while glucose and water balance use hormones.
This difference is why temperature responses such as sweating begin within seconds, whereas the effects of ADH take minutes to develop.
Several things can disrupt a control system.
1
A receptor could fail to detect the change, the coordinator could be damaged (for example a hypothalamus affected by a tumour),
- or the effector could be unable to respond.
- (as when the pancreas cannot make insulin)
2
Medical treatments often aim to replace or support one part of the loop: insulin injections replace a missing hormone, dialysis replaces the kidney as an effector,
- and rehydration drinks give water and ions.
3
Thinking about which part has failed helps with questions about diabetes, kidney failure and heat illness later in this topic.
4
Practice sketching the loop as boxes and arrows, labelling the receptor, coordinator, effector, response and the arrow showing feedback.
Section 7 of 9Exam-style questions
Explain why it is important that body temperature is kept at about 37 degrees Celsius. [3]3 marks
Enzymes have an optimum temperature at about 37 degrees [1]. If the body is too cold, enzyme-controlled reactions are too slow [1]; if too hot, enzymes can denature so reactions stop [1].
Describe the roles of a receptor, coordination centre and effector in a homeostatic system. [3]3 marks
The receptor detects the change in the condition [1]. The coordination centre processes the information and decides on a response [1]. The effector carries out the response that restores the normal level [1].
Blood glucose rises after a meal and then falls to the normal level. Explain why this is an example of negative feedback. [2]2 marks
The rise is detected and triggers a response that lowers glucose [1]. As glucose returns to normal, the response stops, so the change is reversed [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Homeostasis | Regulation of internal conditions to keep them stable. |
| Negative feedback | A response that reverses a change and returns a condition to normal. |
| Receptor | Detects a stimulus. |
| Effector | A muscle or gland that carries out a response. |
| Enzyme | A biological catalyst; has optimum temperature and pH. |
| Denature | Permanent change to the shape of a protein such as an enzyme. |
| Set point | The normal value of a controlled condition. |
Section 9 of 9Exam technique
1
Saying homeostasis keeps conditions exactly constant; they vary slightly around a set point.
2
Saying enzymes die at high temperature; they denature, as they are not alive.
3
Listing only temperature as an example; glucose, water and ions are also controlled.
4
Writing that negative feedback increases the change; it reverses it.
Marked practiceQuizzes for this lesson
البيولوجيا الجزيئية — أسرار تخليق البروتين
مقارنة اتجاهات الإنزيمات
قواعد اتجاهات البلمرة ومضاد الكودون
- إنزيم بلمرة RNA: يبني دائماً في الاتجاه 5′ → 3′، ويقرأ شريط القالب فقط في الاتجاه 3′ → 5′.
- كودون البدء: AUG يقابله مضاد كودون UAC ويشفر للميثيونين أول حمض أميني.
- كودونات الوقف: UAA, UAG, UGA ليس لها tRNA يقابلها، بل يرتبط بها بروتين عامل الإطلاق (Release Factor) لإنهاء الترجمة.