Homeostasis — a constant internal environment
Homeostasis keeps conditions inside the body within narrow limits despite changes outside it. It matters because cells, and the enzymes working inside them, only function efficiently over a narrow range of conditions. B13.3 covers two worked examples: blood glucose concentration, controlled by the liver with insulin and glucagon, and body temperature, held near a set point of about 37 °C.
Negative feedback — the four-stage loop
Negative feedback is the mechanism that delivers homeostasis. A receptor detects a change away from the set point, a coordinator processes the information, an effector produces a response, and that response opposes the change and restores the set point. The same four stages fit both factors in this leaf — only the named parts change: brain and skin effectors for temperature, pancreas and liver for blood glucose.
Insulin and glucagon — antagonistic, acting on the liver
The pancreas makes both hormones; the liver does the work. Insulin is released when blood glucose is too high and makes the liver convert glucose into glycogen, so the level falls. Glucagon is released when blood glucose is too low and makes the liver convert glycogen back into glucose, so the level rises. Opposite effects on the same target organ hold blood glucose close to its set point.
Drawn from real examiner reports.
Negative feedback ≠ homeostasis
Homeostasis is the state being maintained — a constant internal environment. Negative feedback is the mechanism that maintains it. An answer to "define negative feedback" that only says "keeping conditions constant" has defined homeostasis and scores nothing. Say instead that a change away from the set point is detected and that the response opposes it, restoring the set point.
Digest 0654 B13: Jun 2022 P41 Q10ciii; Jun 2023 P41 Q1aiii — homeostasis definition given for negative feedback.
Glucagon (hormone) ≠ glycogen (store)
Glucagon is the hormone released by the pancreas that raises blood glucose. Glycogen is the storage carbohydrate held in the liver and muscles. Glucagon acts on the liver to release glucose from glycogen. The two words differ by only a few letters and examiners mark the spelling here, so writing the wrong one loses the marking point outright.
Digest 0654 B13: Jun 2022 P41 Q10d; Jun 2023 P41 Q1bi — glucagon/glycogen spelling confusion.
The liver stores glucose, not the pancreas
The pancreas is a gland: it makes insulin and glucagon and releases them into the blood. The liver is the target organ: it stores glycogen and carries out the glucose to glycogen conversions in both directions. Naming the pancreas as the storage organ, or writing that insulin converts glucose inside the pancreas, is the recorded error.
Digest 0654 B13: Jun 2022 P41 Q10d; Jun 2023 P41 Q1bi — organ confusion, pancreas given instead of liver.
Vasodilation is arterioles widening
Vasodilation means the arterioles supplying the skin capillary network increase in internal diameter, so more blood flows near the surface and more heat is lost by radiation. Blood vessels do not physically move position, and the capillaries themselves do not widen or rise toward the surface — both of those descriptions are refused by the mark scheme.
Digest 0654 B13: Jun 2022 P41 Q10a/b; Jun 2023 P41 Q1aii — "vessels move" and "capillaries widen or rise" given for vasodilation.
The brain is the coordinator, not an effector
In temperature control the brain monitors the temperature of the blood flowing through it and sends out impulses — that is the coordinator stage. The effectors are the sweat glands, the skin arterioles, the hair erector muscles and the skeletal muscles that shiver. Writing the brain into the effector slot collapses two stages of the loop into one.
Digest 0654 B13: Jun 2022 P32 Q1 records "the brain is the coordinator, not the effector" for the reflex arc; the same stage distinction is required here.
Fatty tissue insulates, not hair
The main insulating layer in the skin is the fatty (adipose) tissue beneath the dermis. Raised hairs do trap a layer of air next to the skin, but that is a secondary effect and is not what is wanted when a question asks for the insulating layer. Naming hair as the insulator instead of fatty tissue is the error the reports record.
Digest 0654 B13: Jun 2022 P41 Q10a/b; Jun 2023 P41 Q1aii — hair named as the insulating layer rather than fatty tissue.
Answer with the four-stage chain
For any "explain how the body controls..." question, write receptor, coordinator, effector, opposing response, naming the real structures at each stage. Each stage is credited separately, so skipping the detection or the restoring stage loses marks even if the middle is right.
Always say the response opposes the change
Listing sweating and vasodilation describes what the body does but does not demonstrate negative feedback. One extra clause — "so the level is brought back toward the set point" — is normally a marking point of its own, and it is the one candidates leave out.
Match the number of points to the tariff
A 5-mark homeostasis question wants five distinct, separately creditable statements, not one long sentence that blurs them together. Count the marks first, then count the points you have actually made before moving on to the next part.
Name the vessel and the direction
In temperature answers write "the arterioles supplying the skin" and state whether they widen or narrow, then give the effect on blood flow near the surface and on heat loss. "The blood vessels change" without a direction, or "the capillaries widen", will not be credited.
Define vasodilation and vasoconstriction of the skin arterioles.
Define homeostasis, and give one example of a factor controlled by homeostasis in the human body. (2 marks)
