Homeostasis — a constant internal environment
Homeostasis is the maintenance of a constant internal environment despite changes outside the body. It matters because cells and their enzymes work properly only across a narrow range of conditions. Section 14 regulates two factors: blood glucose concentration, controlled at Core by insulin — a pancreatic hormone that decreases blood glucose after a meal — and core body temperature, controlled by the skin and the brain.
Negative feedback — insulin and glucagon around a set point
(Extended) Negative feedback reverses any deviation from a set point. A receptor detects the change, a coordinator triggers an effector, and the response opposes the change until the set point is restored and the response switches off. For blood glucose the pancreas detects and the liver acts: insulin converts glucose → glycogen when glucose is high; glucagon converts glycogen → glucose when it is low — antagonistic hormones.
Temperature control — skin structures plus the brain
(Extended) Skin structures to identify: hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels, fatty tissue. Too hot: sweating rises and evaporation removes heat; arterioles vasodilate; hairs lie flat. Too cold: shivering releases heat from extra respiration; arterioles vasoconstrict; hairs are raised to trap insulating air; fatty tissue insulates. The brain detects blood temperature and coordinates the response.
Drawn from real examiner reports.
Brain detects core temp, not skin
The brain detects the temperature of the blood flowing through it — that is core body temperature — and coordinates the corrective response. Skin thermoreceptors detect external temperature only. Writing "the skin detects body temperature and tells the brain to respond" inverts the roles and loses the mark.
Brain as detector of blood temperature: many candidates said the skin detects core body temperature — the brain (hypothalamus) detects and coordinates the response; the skin has thermoreceptors for external temperature. (s22 Papers 11/13 Q27/Q28; s22 Paper 32 Q2b)
Hairs trap air, not keep cold out
When cold, hair erector muscles contract and pull the hairs upright. This traps a thicker layer of air next to the skin; air is a poor conductor, so heat loss by convection and conduction falls. "Hair keeps the cold out" or "stops the cold" names no mechanism and scores nothing. When hot the muscles relax, hairs lie flat and heat escapes.
Hair trapping air (insulation): candidate misconception is that "hair keeps cold air out" rather than "hair traps a layer of warm air, reducing heat loss by convection." (s22 Paper 32 Q2d)
Nervous vs hormonal — finish the contrast
Stating only that nervous control is fast earns partial credit. The mark scheme wants both halves: nervous responses are fast, short-lived and act on one specific effector; hormonal responses (insulin, glucagon) travel in the blood, so they are slower, longer-lasting and reach many target organs.
Hormonal vs. nervous control: nervous is faster, shorter-acting, and specific to one effector; hormonal is slower, longer-acting, and affects many target organs. Candidates stated nervous communication is fast but did not complete the comparison with the slower hormonal system. (w22 Paper 43 Q1c)
Vasodilation cools — it does not warm
Vasodilation widens the arterioles supplying skin surface capillaries, so more blood flows near the surface and more heat is lost — it is the response to being too hot. Vasoconstriction narrows them, so less blood reaches the surface and less heat is lost, conserving warmth. Any answer that reverses this pairing is wrong.
Glycogenesis ≠ glycogenolysis
Insulin drives glycogenesis: the liver converts glucose into glycogen, so blood glucose falls. Glucagon drives glycogenolysis: the liver converts glycogen back into glucose, so blood glucose rises. The two words differ by three letters and the processes run in opposite directions — always name the hormone and the direction together.
The liver is the target, not the source
Both insulin and glucagon are secreted by the pancreas; the liver is the organ they act on. Answers that make the liver secrete either hormone lose the mark, and so do answers that stop at "insulin lowers blood glucose" without naming the liver and glycogen — at Extended the mechanism carries the marks.
Use the five-step feedback scaffold
Every "explain how X is controlled" answer follows one chain: deviation from the set point → receptor detects it → coordinator triggers an effector → the response reverses the change → the response switches off once the set point returns. It fits glucose and temperature equally.
Name the mechanism, not just the term
Vasoconstriction, shivering and sweating earn marks only when the effect is spelled out — less blood near the surface so less heat lost; extra respiration in contracting muscle releasing heat; evaporation of sweat removing heat energy from the skin.
Answer temperature questions in pairs
The three temperature effectors always work as opposites: sweating up versus sweating down, vasodilation versus vasoconstriction, hair erector muscles relaxing versus contracting. Give the matching pair and the reason for each, rather than one half of the response.
MCQ check: which way does the heat go?
If an option claims vasodilation warms the body, or vasoconstriction cools it, the mechanism is backwards — eliminate it. Vasodilation always means more heat LOST; vasoconstriction always means less heat lost.
Homeostasis is the maintenance of a constant internal environment, despite changes in external conditions. Cells and enzymes function best within a narrow range of conditions, so the body must actively regulate factors such as blood glucose concentration and body temperature.
Insulin (Core) is a hormone secreted by the pancreas that decreases blood glucose concentration when it is too high.
(Extended) Negative feedback: a control mechanism in which a change away from a set point (the normal value of a factor) triggers a response that moves the factor back towards the set point — the response opposes ("is negative to") the original change.
State the effect of insulin on blood glucose concentration.
(a) Define homeostasis. (1 mark)
(b) State the effect of insulin on blood glucose concentration. (1 mark)
