Homeostasis keeps conditions suitable for metabolism

1. Homeostasis is the maintenance of a stable internal environment within narrow limits.
2. The internal environment is the blood and tissue fluid surrounding body cells.
3. Internal conditions change after eating, exercising and moving between hot and cold surroundings, so the body continually corrects deviations.
4. Conditions fluctuate around a normal level rather than remaining completely constant.

Stable conditions maintain reaction rates

1. Metabolic reactions are controlled by enzymes, whose activity depends on temperature and the chemical conditions around cells.
2. Low temperatures reduce the kinetic energy of enzyme and substrate particles, so successful collisions occur less often and reactions slow down.
3. High temperatures can change an enzyme's active site, so the enzyme becomes denatured and can no longer catalyse its reaction.
4. A suitable blood glucose concentration provides cells with glucose for respiration without allowing the concentration to become dangerously high or low.
5. If blood glucose falls too low, cells receive less glucose for respiration and cannot transfer energy at the required rate.
6. A suitable water balance prevents excessive movement of water into or out of cells by osmosis.
7. Dilute body fluids cause water to enter cells by osmosis, while concentrated body fluids cause water to leave cells.
8. Large osmotic changes can make animal cells swell and burst or shrink, disrupting their normal reactions.

Negative feedback reverses a change

1. A receptor detects a change in an internal condition and sends information to a coordination centre.
2. The coordination centre compares the condition with its normal level and coordinates a response.
3. An effector, such as a muscle or gland, produces the response.
4. The response opposes the original change, so the condition returns towards its normal range and the response is reduced.
5. The normal level is a set point, but the measured condition usually fluctuates slightly above and below it.
6. The corrective response decreases once the deviation has been reduced, preventing a large change in the opposite direction.
7. Nervous signals coordinate rapid responses, while hormones in the blood can coordinate slower and longer-lasting responses.
8. $change\to receptor\to coordination\; centre\to effector\to corrective\; response$

Internal and external changes are controlled

1. An external change, such as a fall in air temperature, can increase heat loss from the body.
2. An internal change, such as increased respiration during exercise, can increase heat production inside the body.
3. Eating a carbohydrate-rich meal raises blood glucose, while sweating heavily lowers the water content of the blood.
4. Several control systems can operate together, so exercise can challenge temperature, blood glucose and water balance at the same time.
5. Homeostatic responses counter these changes so cells continue to work under suitable conditions.