Homeostasis and Response
Homeostasis is the regulation of the internal conditions of the body, such as blood glucose concentration, body temperature and water levels, to maintain a stable internal environment despite changes inside and outside the body. Every homeostatic control system, coordinated by the nervous or hormonal system, is built from a receptor, a coordination centre and an effector working together through negative feedback to return a level to normal.
Method
- Learn the three parts of every control system and practise spotting them in any scenario: a receptor detects the stimulus (the change), a coordination centre (part of the brain, the spinal cord, or a gland) processes the information and decides on a response, and an effector (a muscle or a gland) carries out the response.
- Compare nervous and hormonal communication on three points every time: what is transmitted (electrical impulses along neurons, or chemical hormones in the blood), how fast the response starts, and how long the response lasts and how widely it acts.
- Learn negative feedback as the single idea underlying blood glucose control, thermoregulation and water balance: whenever a level moves away from the normal range, the control system triggers a response that brings it back toward normal.
- For thermoregulation questions, learn the two opposite mechanisms: too hot triggers vasodilation and sweating; too cold triggers vasoconstriction and shivering. Always explain how each mechanism changes the amount of heat lost or generated.
- For blood glucose questions, know that a rise in blood glucose concentration triggers the release of insulin, and a fall triggers the release of glucagon, and that both are released by the pancreas as part of a negative-feedback loop.
- When a question does not name a specific system, use the generic homeostasis vocabulary (stimulus, receptor, coordination centre, effector, negative feedback) rather than guessing at the wrong named system.
Worked example
A person goes for a run on a hot day. Trace the homeostatic pathway that keeps their core body temperature close to 37 degrees Celsius, identifying the receptor, coordination centre and effectors at each stage.
- Identify the stimulus: exercise combined with a hot environment raises the core body temperature above its normal level.
- Identify the receptor: temperature receptors in the skin, and in the thermoregulatory centre of the brain itself, detect that the blood temperature has risen above normal.
- Identify the coordination centre: the thermoregulatory centre in the brain receives this information and compares it with the normal set point.
- Identify the effectors and their response: sweat glands increase sweat production, so evaporation transfers heat energy away from the skin, and blood vessels supplying the skin capillaries dilate (vasodilation), moving more blood near the surface so more heat is lost by radiation.
- Explain the outcome as negative feedback: as the core temperature falls back toward 37 degrees Celsius, the receptors detect this change and the responses are reduced, keeping temperature within a narrow range instead of continuing to fall.
Practice questions
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Q1Name the three parts common to every homeostatic control system.Show answer
Answer: A receptor, a coordination centre and an effector.
Q2State one difference between nervous and hormonal coordination.Show answer
Answer: Nervous impulses are electrical and act very quickly but briefly on a precise area; hormones are chemical, carried in the blood, and act more slowly but for longer over a wider area (accept the reverse phrasing of either point).
Q3Define homeostasis.Show answer
Answer: The regulation of the internal conditions of the body to maintain a stable internal environment, in response to changes inside and outside the body.
Q4Name the gland that monitors and controls blood glucose concentration.Show answer
Answer: The pancreas.
Q5Name the hormone released when blood glucose concentration is too high, and state its overall effect.Show answer
Answer: Insulin; it causes cells, especially liver and muscle cells, to take up glucose from the blood, lowering blood glucose concentration.
Q6Explain, in terms of blood vessels, how the skin helps to cool the body when it is too hot.Show answer
Answer: Blood vessels supplying the skin capillaries dilate (vasodilation), increasing blood flow near the skin surface, so more heat is lost from the blood by radiation.
Q7State what is meant by negative feedback.Show answer
Answer: A control mechanism in which a change away from the normal level triggers a response that brings the level back toward normal.
Q8A patient's core body temperature has fallen to 35 degrees Celsius. State two physical responses their body makes to raise it back toward normal.Show answer
Answer: Shivering (rapid muscle contraction, which releases some heat energy during the respiration needed to power it) and vasoconstriction (blood vessels supplying the skin narrow, reducing blood flow and heat loss near the surface).
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A person's blood glucose concentration rises after eating a meal. Describe how the body responds to bring blood glucose concentration back down to normal.
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Explain two differences between how the nervous system and the hormonal system coordinate a response in the human body.
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A person becomes dangerously cold after falling into open water. Explain how the body detects and responds to this fall in core body temperature, and explain why keeping core temperature close to 37 degrees Celsius matters for the body's cells.
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