Why do we need to maintain a constant internal environment?Maintaining a constant body temperature

Body temperature must be controlled within a very narrow range so that the body can function properly. In particular, enzymes in a body's cells must have the correct temperature to be able to catalyse chemical reactions.

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

Core body temperature

Temperature control is the process of keeping the body at a constant core body temperature close to 37°C.

The body can only stay at a constant temperature if the heat generated is balanced and equal to the heat lost.

Although the core temperature must be close to 37°C, fingers and toes can be colder. This is because energy is transferred from the blood as it travels to the fingers and toes.

Responding to changes in temperature

If the body becomes too hot or too cold, various mechanisms in the skin can be turned on or off.

Too cold

• Hair muscles pull hairs on end.
• Erect hairs trap air.
• Blood flow in capillaries decreases.

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.

Skin responses

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 hair follicles 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. The sweat transfers heat away from the skin as it evaporates. Sweat secretion stops when body temperature returns to normal.
3. Blood vessels supplying blood to the capillaries in the skin can widen or dilate – vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. The person's skin appears red.
4. Blood vessels can narrow or constrict – vasoconstriction. This reduces heat loss through the skin. The person's skin appears pale.

Muscle responses

Muscles can also receive messages from the brain when the body temperature decreases. They respond by shivering. Shivering causes the muscles to contract and relax quickly. This process generates heat.

Experiments to investigate temperature change

The processes by which organisms regulate their body temperature can be modelled using regular lab equipment.

A beaker contains warm water at a temperature of 40°C. This represents an organism with a body temperature of 40°C.

There are two methods you can use to model two ways the organism can control its body temperature.

Method 1 - Using a single Bunsen burner to maintain a constant temperature.

When the temperature falls below 40°C, the Bunsen burner is used to heat the water back up.

This represents using a single effector to control body temperature.

Method 2 - Using a Bunsen burner and ice to maintain a constant temperature.

When the temperature falls below 40°C, the Bunsen burner is used to heat the water back up.

If the temperature rises above 40°C, ice is added to the beaker of water to bring the temperature back down.

This method has two ways of controlling temperature, which work oppositely. It represents using effectors antagonistically to control body temperature.