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Science

Homeostasis and hormones

Controlling body temperature

Human enzymes usually work best at 37ºC, which is human body temperature. This can be measured in several places, including the ear, finger, mouth and anus.

There are various ways to measure body temperature, including using a clinical thermometer, heat-sensitive strips, digital probes or thermal imaging cameras.

Extremes of body temperature are dangerous:

  • high temperatures can cause dehydration, heat stroke, and death if untreated
  • low temperatures can cause hypothermia, and death if untreated

Control mechanisms

The body’s temperature is monitored by the brain. If you are too hot or too cold, the brain sends nerve impulses to the skin, which has three ways to either increase or decrease heat loss from the body’s surface:

  1. Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.
  2. If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal.
  3. Blood vessels supplying blood to the skin can swell or dilate - called vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. Blood vessels can shrink down again - called vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.

Muscles can also receive messages from the brain when you are cold. They respond by shivering, which warms you up.

Controlling temperature

Too coldToo hot
Diagram shows cross section of skin when cold, with erect hairs (B) caused by tense hair muscles (A), and reduced blood flow to capillaries (C)

Diagram shows a cross section of skin when hot. Hair muscles relax (D) causing hairs to lie flat so that heat can escape. Sweat is secreted (E) from the sweat glands. Blood flow in the capillaries is increased (F)

A - Hair muscles pull hais on end.

B - Erect hairs trap air.

C - Blood flow in capillaries decreases.

D - Hair muscles relax. Hairs lie flat so heat can escape.

E - Sweat secreted by sweat glands. Cools skin by evaporation.

F - Blood flow in capillaries increases.

A very common mistake in exams is to write that the blood vessels move up and down in the skin. The blood vessels do not move during vasodilation and vasoconstriction.

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