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Blood circulation


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Blood transports materials around the body and protects against disease. It consists of cells, solutes and liquid.

Blood is pumped away from the heart at high pressure in arteries and returns to the heart at low pressure in veins.

Heart problems include narrow rigid arteries, leaking heart valves and a faulty pacemaker.


Blood is used to transport materials around the body, and to protect against disease.

It's a mixture of cells, solutes and liquid:

Blood cells

  • Red blood cells transport oxygen.
  • White blood cells protect against disease.
  • Blood platelets help the blood to clot.

Red blood cells

These carry oxygen from the lungs to tissues. Oxygen transport is efficient because:

  • there are huge numbers of red blood cells
  • the cells are tiny to allow them to pass through narrow capillaries
  • the cells have a flattened disc shape to increase surface area allowing rapid diffusion of oxygen
  • they contain haemoglobin which absorbs oxygen in the lungs and releases oxygen in the rest of the body

Higher tier only

Red blood cells have a large surface area to volume ratio to allow rapid diffusion of oxygen. Haemoglobin absorbs oxygen to form bright red oxyhaemoglobin in oxygen rich environments. In oxygen deficient environments this processes is reversed.

White blood cells

White blood cells can engulf bacteria and other pathogens by phagocytosis. They can change shape easily and produce enzymes that digest the pathogens.

Other types of white blood cell secrete antibodies that help destroy pathogens.

Blood plasma

Plasma is a straw-coloured liquid. It transports dissolved substances around the body, including:

  • hormones
  • antibodies
  • nutrients, such as water, glucose, amino acids, minerals and vitamins
  • waste substances, such as carbon dioxide and urea


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