Thermoregulation controls body temperature

1. Thermoregulation keeps core body temperature within narrow limits around approximately $37{}^{\circ }\mathrm{C}$.
2. Respiration releases heat inside the body, while heat is transferred to and from the surroundings through the skin.
3. A rise above the normal range activates responses that increase heat loss.
4. A fall below the normal range activates responses that reduce heat loss or generate more heat.
5. As temperature returns towards normal, the corrective response becomes smaller.

The epidermis forms the surface

1. The epidermis is the thin outer layer of the skin and contains no blood vessels.
2. It forms a protective, waterproof barrier that reduces uncontrolled water loss from the body.
3. Sweat released by ducts reaches the epidermal surface and evaporates from it.
4. Heat transferred from the dermis must pass through the epidermis before reaching the surroundings.

The dermis changes heat transfer

1. The dermis contains the structures that detect temperature and alter heat loss through the skin.
2. Sweat glands release more sweat when the body is too hot.
3. Evaporation requires energy, so energy is transferred from the skin to the surroundings and the skin cools.
4. Blood vessels supply capillary networks close to the surface, allowing the amount of warm blood near the surface to be adjusted.
5. Temperature receptors detect changes in skin temperature and send electrical impulses towards the brain.
6. Hair erector muscles contract in cold conditions and pull the hairs upright.
7. Upright hairs trap a layer of still air, which reduces heat transfer by convection, although this response has a small effect in humans.
8. Fatty tissue beneath the dermis insulates the body because fat conducts heat poorly.

The hypothalamus coordinates responses

1. The hypothalamus contains the thermoregulatory centre in the brain.
2. Temperature receptors in the hypothalamus detect the temperature of the blood flowing through it.
3. Temperature receptors in the skin detect changes in the external environment and send electrical impulses to the hypothalamus.
4. The hypothalamus processes both sources of information and compares body temperature with the normal range.
5. It sends nerve impulses to effectors such as sweat glands, hair erector muscles and skeletal muscles.
6. The effectors produce responses that oppose the temperature change, and the hypothalamus reduces the response as normal temperature is restored.

Shivering generates more heat

1. When core body temperature falls, the hypothalamus sends impulses that trigger rapid, involuntary contractions of skeletal muscles.
2. The contracting muscle cells need more energy, so their rate of respiration increases.
3. Respiration transfers energy from glucose, and some of this energy is transferred as heat.
4. The heat warms the blood and surrounding tissues, raising core body temperature towards normal.
5. Shivering therefore increases heat production rather than reducing heat loss.