Unit 3 - Changes in Capillary Blood Flow for Heat Loss (H)
When we exercise, the skin looks red due to increased blood flow in the skin capillaries.

Unit 3 - Changes in Capillary Blood Flow for Heat Loss (H)

Have you ever wondered how the body maintains its ideal temperature of 37 degrees C? Well, it's all down to capillary blood flow, which changes depending on how hot or cold we are. This GCSE Biology quiz, Changes in Capillary Blood Flow for Heat Loss, looks not only at how a capillary can be use for heat loss, but also for heat retention.

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This centre has receptors sensitive to the temperature of the blood flowing through the brain. If the core body temperature is too high, the thermoregulatory centre causes the blood vessels supplying the skin capillaries to expand so that more blood flows through them. Since the blood flows through the entire body, it can take the excess heat from the core of the body to the skin, where it can be lost. When a blood vessel of any sort increases in diameter, we say that it dilates. Because the dilation is related to the vascular (blood) system, this process is called vasodilation.

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Vasodilation means that more blood will flow through the capillaries in the surface layers of the skin, which is why most people look redder during exercise. The extra volume of blood heats up the surface of the skin more, so more heat is lost from the body by radiation and convection. During exercise, the sweat glands release more sweat onto the surface of the skin to cool the body down. Because the capillaries are carrying more blood, heating the surface of the skin, the sweat evaporates faster which cools the body more effectively.

If the core temperature of the body drops, exactly the opposite happens (vasoconstriction) in order to try to prevent hypothermia. This process prevents the blood from carrying precious body heat to the surface of the skin where it would be lost. At high altitudes, mountaineers have much lower levels of oxygen in their blood than normal which means that they can only move slowly. This, together with the low temperatures found on mountains, makes it difficult for them to maintain their body temperature and so vasoconstriction occurs. It is a big problem for their toes and fingers as these become intensely cold. The blood flow virtually disappears and if it continues for a long period of time, without the vital oxygen, the cells begin to die. This is called frostbite and the dead tissue turns black and either falls off naturally or is removed surgically. Many mountaineers have lost fingers and toes to frostbite.

How do changes in the size of capillaries help with heat loss and heat retention? Which part of the brain controls our body temperature? Find out if you know by playing this quiz.

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  1. Which of the following encourages heat loss by evaporation?
    Evaporation of water in sweat cools the skin due to the change of state from a liquid to a gas
  2. Name the special cells responsible for detecting temperature changes.
    Chemoreceptors detect chemical changes, for example carbon dioxide levels
  3. What is the core temperature for humans, in degrees C?
    Enzymes start to become denatured at temperatures above 37 degrees C
  4. When we exercise, the skin looks red due to...
    This helps to get rid of excess heat from the core of the body
  5. Thermoregulation is an example of...
    Homeostasis is maintaining a constant internal environment. Internal body temperature needs to be kept within narrow limits
  6. Skin contains thermoreceptors which send messages to which of the following, to start body responses to maintain body temperature?
    This is an example of a feedback system in the body
  7. To allow for more heat loss, the vessels supplying the skin capillaries undergo...
    Skin looks red due to vasodilation. This increases blood flow to the surface of the skin, allowing greater heat loss
  8. When we are cold, muscles shiver to increase...
    Shivering means that the muscles are working harder and therefore respiration will be greater, generating more heat
  9. Name the part of the brain that controls body temperature in mammals.
    Receptor cells detect whether the blood is abnormally hot or abnormally cold
  10. When blood vessels tighten and have a smaller lumen, we call this...
    This reduces the volume of blood that can pass through the blood vessels

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