Negative feedback is a loop in which the body detects a change and responds to reverse it. Four steps carry the marks: change, detection, response and return to normal.
This lesson opens the cluster on homeostasis and the urinary system. The next lesson shows how the kidney carries out the response step for water.
What are the four steps?
- Change: a factor moves away from its normal range.
- Detection: a receptor or control centre notices the change.
- Response: an effector acts to reverse the change.
- Return to normal: the factor moves back into range.
“Negative” means the response works against the change. A rise triggers a fall, and a fall triggers a rise.
Worked example with an invented case
An invented student drinks very little during a hot afternoon, so the water content of the blood falls.
Question: Explain how the body restores the water content of the blood.
- Change: blood water content falls, so the blood is more concentrated.
- Detection: the hypothalamus in the brain detects the higher concentration.
- Response: the pituitary gland releases more ADH, and the kidneys reabsorb more water into the blood. Less urine forms, and it is more concentrated.
- Return to normal: the blood water content rises towards its normal range.
A full answer: “When blood water content falls, the hypothalamus detects the change. More ADH is released, so the kidneys reabsorb more water, and the blood water content returns towards normal.”
The mistake that loses marks
The common slip is to name the organ and stop, as in “The kidney controls water.” That gives no sequence.
| Step | Naming only | Full explanation |
|---|---|---|
| Change | Missing | Blood water content falls |
| Detection | Missing | Hypothalamus detects it |
| Response | “Kidney controls water” | More ADH, more water reabsorbed |
| Return | Missing | Water content rises to normal |
The fix is to write the four labels in the margin and fill in each one before adding detail.
A routine for any feedback question
- Write what changed and in which direction.
- Name what detects it.
- Name what responds and what it does.
- Close with the factor returning towards normal.
The biological data interpretation trainer lets you practise the same steps on graphs of a factor rising and falling.
Check yourself
Invented case: after a long run, a student drinks a large bottle of water and the blood water content rises. Write the four steps of the body’s response.
Answer
Change: blood water content rises, so the blood is more dilute. Detection: the hypothalamus detects the dilute blood. Response: less ADH is released, so the kidneys reabsorb less water and more dilute urine forms. Return to normal: the blood water content falls back towards its normal range.
The direction is opposite to the worked example, but the four steps are the same.
What to study next
Move to the place where the response happens in tracing filtration, reabsorption and urine formation. For the nerve and hormone signals behind detection, see coordination and response.
If you want a teacher to hear you explain a loop, see online one-to-one Biology tuition.