A warm room and a cold porch — answer key

1. WQ1 · Practice

Model answer. Homeostasis is the maintenance of a constant internal environment: conditions inside the body, such as the core temperature, are kept close to a set point.

A full answer includes

• The maintenance of a constant internal environment.

Also accept

• maintenance of a constant internal environment

Watch for

• “Homeostasis keeps every condition in the body exactly the same.” — Conditions change a little and are brought back: the core temperature dipped from 37.0 to 36.8 °C in the porch and came back up.

2. WQ2 · Practice

Model answer. The core temperature, deep inside the body, stayed close to 37 °C (36.9 °C after 10 minutes). The skin temperature fell from 34.5 to 26.0 °C.

A full answer includes

• The core temperature.

Also accept

• core temperature

Watch for

• “The skin temperature, because the skin is the outside of the body.” — The skin temperature fell by 8.5 °C. The temperature kept near 37 °C is the core temperature, deep inside.

3. WQ3 · Practice

Model answer. The fall is the starting value minus the final value: 34.5 − 26.0 = 8.5 °C.

Checked automatically

• Fall in skin temperature: 8.5 °C (within ±0.05 °C)
  • Common wrong answer 0.1 : That is the fall in the core temperature. Use 34.5 − 26.0.
  • Common wrong answer 60.5 : That is the sum. Subtract: 34.5 − 26.0.

A full answer includes

• 34.5 − 26.0 = 8.5 °C.

Also accept

• 8.5 °C

Watch for

• “0.1 °C.” — That is the fall in the core temperature. The skin fell much more.

4. WQ4 · Practice

Options as numbered on the pupil sheet: 1 — insulates: slows the loss of heat from the body; 2 — less blood flows near the skin surface, so less heat is lost; 3 — respiration in the muscles releases heat.

Model answer. Vasoconstriction narrows the arterioles supplying the skin surface capillaries, so less blood flows near the surface and less heat is lost. Shivering is rapid muscle contraction: respiration in the muscle cells releases heat. Fatty tissue insulates, slowing the loss of heat.

A full answer includes

• vasoconstriction: less blood near the skin surface, less heat lost; shivering: respiration in muscles releases heat; fatty tissue: insulates, slows heat loss.

Also accept

• vasoconstriction less heat lost; shivering heat released; fat insulates

Watch for

• “Shivering cools the body down.” — Shivering releases heat: the muscles respire faster as they contract.

5. WQ5 · Practice

Model answer. Sweat glands release sweat onto the surface. As the water in sweat evaporates, it takes heat from the skin, so the skin cools.

A full answer includes

• The water in sweat evaporates, and evaporation takes heat from the skin.

Also accept

• evaporation takes heat from the skin

Watch for

• “Sweat is cold, so it cools the skin.” — Sweat comes out at about skin temperature. It cools the skin as it evaporates.

6. WQ6 · Practice

Model answer. In vasoconstriction the arterioles that supply the capillaries near the skin surface become narrower. Less blood flows near the surface, so less heat is lost.

A full answer includes

• The arterioles supplying the skin surface capillaries become narrower, so less blood flows near the surface.

Also accept

• arterioles narrow: less blood near the surface

Watch for

• “The capillaries move deeper into the skin.” — Blood vessels do not move. The arterioles supplying the surface capillaries become narrower.

7. WQ7 · Practice

Model answer. The core temperature is part of the internal environment. It stayed close to its set point of 37 °C even though the air was 25 °C colder: that is homeostasis, the maintenance of a constant internal environment. The skin is at the surface and its temperature is allowed to change a lot.

A full answer includes

• The core temperature is part of the internal environment.
• It stayed close to its set point of 37 °C even though the surroundings changed a lot.
• Homeostasis is the maintenance of a constant internal environment; the skin temperature is allowed to change.

Also accept

• core kept near 37 °C despite the cold: constant internal environment

Watch for

• “It shows that the skin is better at homeostasis than the core.” — The skin temperature changed a lot. The core temperature is the one kept constant.

8. WQ8 · Practice

Model answer. (a) The core temperature fell from 37.0 °C to 36.8 °C: 0.2 °C. (b) The skin temperature fell from 33 °C to 22 °C: 11 °C. The core stayed close to its set point; the skin changed a lot.

Checked automatically

• (a) Fall in core temperature: 0.2 °C (within ±0.05 °C)
  • Common wrong answer 2 : Check the core scale: 37.0 − 36.8 = 0.2 °C.
• (b) Fall in skin temperature: 11 °C (within ±0.5 °C)
  • Common wrong answer 0.2 : That is the core. Read the skin graph: 33 − 22.

A full answer includes

• (a) 37.0 − 36.8 = 0.2 °C.
• (b) 33 − 22 = 11 °C.

Also accept

• (a) 0.2 °C; (b) 11 °C

Watch for

• “(a) 2 °C.” — The core graph runs from 36 to 38 °C: each numbered line is 0.5 °C and each thin line 0.1 °C. 37.0 − 36.8 = 0.2 °C.

9. WQ9 · Practice

Model answer. Vasoconstriction: the arterioles supplying the skin surface capillaries narrow, so less blood flows near the surface and less heat is lost. Shivering: the muscles contract quickly, and respiration in the muscle cells releases heat. Hair erector muscles raise the hairs, trapping a layer of air (a little insulation in humans), and the fatty tissue under the skin insulates.

A full answer includes

• The arterioles supplying the skin surface capillaries narrow (vasoconstriction): less blood near the surface, less heat lost.
• Muscles shiver: respiration releases heat.
• Hair erector muscles raise the hairs, trapping air; or fatty tissue insulates.

Also accept

• vasoconstriction; shivering; hairs raised or fatty insulation

Watch for

• “The blood vessels move away from the skin.” — Vessels do not move: the arterioles supplying the surface capillaries become narrower.

10. WQ10 · Practice

Model answer. In vasodilation the arterioles supplying the skin surface capillaries widen. More blood flows near the surface, so more heat is carried from the core to the skin and lost to the air.

A full answer includes

• The arterioles supplying the skin surface capillaries widen.
• More blood flows through the capillaries near the surface.
• More heat is lost from the skin to the air.

Also accept

• arterioles widen; more blood near the surface; more heat lost

Watch for

• “The capillaries move closer to the surface.” — Capillaries do not move. The arterioles supplying them widen, so more blood flows through them.

11. WQ11 · Practice

Model answer. When the body is too warm, sweat glands release sweat, which cools the skin as it evaporates, and the arterioles supplying the surface capillaries widen (vasodilation). Shivering and raised hairs are responses to cold.

A full answer includes

• Sweat glands release sweat.
• The arterioles supplying the skin surface capillaries widen.

Also accept

• sweating; vasodilation

Watch for

• “Shivering happens when the body is too warm.” — Shivering releases heat: it is a response to cold.

12. WT1 · Apply

Model answer. The air warmed from 24 to 30 °C, but the core temperature rose by only 0.8 °C and then stayed at 37.8 °C between 40 and 60 minutes. It was being kept close to a steady value while running released a lot of heat: that is homeostasis. The skin temperature fell as sweat evaporated.

A full answer includes

• The core temperature rose only 0.8 °C (37.0 to 37.8) while the air warmed by 6 °C.
• The core temperature levelled off at 37.8 °C, so it was being kept near a steady value.
• This is homeostasis: the internal environment is kept nearly constant.

Also accept

• core changed little and levelled off: homeostasis

Watch for

• “No: the core temperature rose, so homeostasis failed.” — Homeostasis keeps conditions close to a value, not exactly the same. A rise of 0.8 °C that levels off is kept under control.

13. WT2 · Apply

Model answer. Raised hairs trap a layer of air next to the skin, and air is a poor conductor of heat, so it insulates. The fox's fur is long and thick, so it traps a thick layer of air. Human hairs are short and sparse and trap very little, so raising them helps humans very little.

A full answer includes

• Raised hairs trap a layer of air, which is a poor conductor of heat (insulation).
• The fox's long, thick fur traps a thick layer of air; human hairs are short and few, so they trap very little.

Also accept

• traps a thick layer of air; human hairs too short

Watch for

• “The fur makes heat.” — Fur makes no heat. It slows the loss of heat by trapping air.

14. WT3 · Apply

Model answer. Sweat cools the skin only when its water evaporates, because evaporation takes heat from the skin. In humid air little sweat evaporates: it drips off instead, carrying little heat away, so the skin and the body cool less.

A full answer includes

• Sweat cools the skin only when its water evaporates, taking heat from the skin.
• In humid air less sweat evaporates (it drips off), so less heat is taken from the skin.

Also accept

• sweat must evaporate to cool; less evaporates in humid air

Watch for

• “The person sweats less on a humid day.” — The person may sweat as much or more; the problem is that the sweat does not evaporate.