Biology Unit 4

Biology Unit 4

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  • Created by: hanah
  • Created on: 04-12-11 20:34

The hypothalamus is the processing centre in the brain that controls body temperature. It does this by triggering changes to effectors, such as sweat glands and muscles controlling body hair. Heat stroke can happen when the body becomes too hot; and hypothermia when the body becomes too cold.

Temperature control

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

Our body can only stay at a constant temperature if the heat we generate is balanced and equal to the heat we lose.

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

How our body maintains a constant temperature

Temperature receptors in the skin detect changes in the external temperature. They pass this information to the processing centre in the brain, called the hypothalamus.

The processing centre also has temperature receptors to detect changes in the temperature of the blood. The processing centre automatically triggers changes to the effectors to ensure our body temperature remains constant, at 37°C.

The effectors are sweat glands and muscles.

If we are too hot or too cold, the processing centre sends nerve impulses to the skin, which has two ways to either increase or decrease heat loss from the body's surface.

Cross section of skin showing hairs and the muscles that control them.  (

Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.

  1. Hairs on the skin trap more warmth 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 in the skin secrete sweat onto the surface to increase heat loss by evaporation. This cools the body. Sweat secretion slows when the body temperature returns to normal.

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Maintaing bodys temp

When things go wrong - heat stroke

Heat stroke is an uncontrolled increase in body temperature.


Heat stroke is caused by:

  • high temperatures which cause an increase in sweating. This can lead to dehydration which reduces sweating which then allows the core body temperature to rise.
  • not drinking sufficient water when hot

As the core body temperature rises, the normal mechanisms for controlling body temperature break down. This can lead to a further rise in the core body temperature.


The symptoms of heat stroke are:

  • increased body temperature
  • hot, dry skin
  • rapid heartbeat
  • increased or decreased blood pressure
  • headache
  • confusion
  • unconsciousness.


  • Move the person to a cool, shady place.
  • Cool the person by covering them with damp sheets or spraying them with water.
  • Cool the person with a fan.
  • Seek professional medical help.
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When things go wrong - hypothermia

Hypothermia occurs when the core body temperature falls below 35°C.

It happens when the body is losing heat faster than it can make it.


Hypothermia is caused by:

  • extreme cold
  • taking sedatives or alcohol when cold
  • certain medical conditions such as heart problems
  • being very young or very old - these people cannot regulate their body temperature very well


The symptoms of hypothermia are:

  • violent shivering, which stops as hypothermia becomes more severe
  • confusion
  • difficulty in moving
  • memory loss
  • tiredness
  • slurred speech
  • slow, shallow breathing
  • weak pulse.


  • Move the person somewhere warm.
  • Change them out of wet clothing.
  • Wrap them in warm clothing.
  • Give them warm drinks, but not alcohol.
  • Give them food that is rich in carbohydrates
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Blood and temperature control - Higher

When we are too hot, blood vessels supplying blood to the skin can swell or dilate (vasodilation). This allows more warm blood to flow near the surface of the skin, where the heat can be lost to the air.

This is why some people's skin looks redder when they feel too hot.

When we are too cold the blood vessels supplying warm blood to the skin become narrow or constrict (vasoconstriction). This reduces the flow of warm blood near the surface of the skin, and reduces heat loss.

This is why some people's skin looks paler when they feel too cold.

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.

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.

Signals along nerves from the hypothalamus control both vasodilation and vasoconstriction.

Muscles attached to our skeleton can also receive signals from the hypothalamus when we feel too cold. They respond by shivering. The rapid contraction of muscles during shivering results in heat being produced during respiration. This heat then warms up surrounding tissues.

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