Temperature regulationEdexcel A-Level Biology B: Revision notes
Section 1
Endotherms and ectotherms
An endotherm (mammals and birds) produces heat from metabolic processes such as respiration, so its core temperature stays fairly constant whatever the surroundings. This allows enzymes to work at their optimum and the animal to stay active in the cold, but it needs a high food intake.
An ectotherm (reptiles, amphibians, insects) relies on the external environment for heat. Its body temperature varies with its surroundings, so it is inactive when cold but needs far less food.
Section 2
Behavioural control
Both groups use behaviour. Ectotherms bask to warm up and shelter in burrows or shade to avoid overheating. Endotherms also seek shade, bury into burrows, huddle in groups, change posture or migrate. For an ectotherm behaviour is the main method of control; for an endotherm it supplements the physiological responses.
Section 3
Detecting temperature
Thermoreceptors in the skin detect the temperature of the surroundings. Thermoreceptors in the hypothalamus detect the temperature of the blood flowing through it (the core temperature). The hypothalamus acts as the thermoregulatory centre: it compares the information with the set point and sends impulses along autonomic nerves to the effectors. This is a negative feedback system.
The hypothalamus, not the skin, detects core (blood) temperature. The skin detects external temperature.
Section 4
Responses to cold
- Vasoconstriction: arterioles in the skin constrict, so less blood flows near the surface and less heat is lost.
- Raised hairs: the erector pili muscles contract, trapping an insulating layer of air.
- Shivering: rapid contraction of skeletal muscles releases heat from respiration.
- Sweating stops and metabolic rate may rise.
Section 5
Responses to heat
- Vasodilation: skin arterioles dilate, so more blood flows near the surface and more heat is lost by radiation.
- Sweating: sweat glands release sweat; as the water evaporates it takes heat from the skin.
- Hairs lie flat: the erector pili muscles relax, so less air is trapped.
Sweating is less effective in humid air because evaporation is slower. All the responses are coordinated by the hypothalamus through the autonomic nervous system.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Temperature regulation
- A scientist measures the body temperature of a desert lizard and of a mouse at air temperatures of 10 °C and 35 °C. At 10 °C the lizard's body temperature is 11 °C and the mouse's is 37 °C. At 35 °C the lizard's is 34 °C and the mouse's is 37 °C. The mouse uses more oxygen at 10 °C than at 35 °C. In the wild the lizard lies on warm rocks in the morning and shelters in a burrow at midday.Explain why the lizard basks on warm rocks in the morning and shelters in a burrow at midday.2 marks
- A man walks from a warm room into cold air outdoors. Within a few minutes the skin on his hands looks pale, the hairs on his arms stand up and he begins to shiver. His core body temperature remains at about 37 °C.Explain how shivering and the raising of the hairs help the man to maintain his core temperature.2 marks
- On a hot day a marathon runner's core temperature rises towards 38.5 °C. Her skin becomes flushed and she sweats heavily. The air is humid and she notices that the sweat drips off her rather than drying.Describe how the autonomic nervous system brings about the flushed skin and the sweating.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).