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Human enzymes usually work best at 37°C, which is the human body temperature.

Maintaining this optimum temperature is called thermoregulation. When this stops working properly, high temperatures can cause dehydration, heat stroke and death if untreated. Low temperatures can cause hypothermia and then also death if untreated.

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. These are:

• trapping more or less warm air to insulate
• sweating and shivering
• control of blood flow (vasodilation or vasoconstriction)

Trapping warm air to insulate

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.

Too cold

When we get too cold:

• hair muscles pull hairs on end
• erect hairs trap air
• blood flow in capillaries decreases

Too hot

When we get too hot:

• hair muscles relax. Hairs lie flat so heat can escape
• sweat secreted by sweat glands - cools skin by evaporation
• blood flow in capillaries increases

Sweating and shivering

Lick the back of one of your hands and blow on it. Now just blow on the other without licking it first. Why did the wet surface feel colder? When liquids evaporate from surfaces they cool them. 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.

Shivering is the involuntary contraction and relaxation of muscles. This occurs very quickly. It releases energy from glucose in respiration. Some of this energy is released as heat to warm us up.

Vasodilation and vasoconstriction

Blood vessels supplying blood to the skin can swell or dilate - 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 - vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.

These diagrams show the processes that take place when vasoconstriction and vasodilation occur.