9.4) Blood

9.4) Blood

Red blood cells:

  • they contain haemoglobin – a red protein that combines with oxygen
  • they have no nucleus so they can contain more haemoglobin
  • they are small and flexible so that they can fit through narrow blood vessels
  • they have a biconcave shape (flattened disc shape) to maximise their surface area for oxygen absorption


White blood cells:

  • Two most numerous types are phagocytes and
  • Phagocytes engulfs (ingest) and digest harmful bacteria and cell debris (process called phagocytosis).
  • Lymphocytes produces antibodies.



  • When tissues are damaged and blood vessels cut, platelets clump together and block the smaller capillaries.
  • The platelets and damaged cells at the wound also produce a substance that acts, through a series of enzymes, on the soluble plasma protein called fibrinogen.
  • As a result of this action, the fibrinogen is changed into insoluble fibrin, which forms a network of fibres across the wound.
  • Red cells become trapped in this network and so form a blood clot. The clot not only stops further loss of blood, but also prevents the entry of harmful bacteria into the wound.


The transfer of materials between capillaries and tissue fluid:

  • The fluid that escapes from capillaries is not blood, nor plasma, but tissue fluid.
  • Tissue fluid is similar to plasma but contains less protein, because protein molecules are too large to pass through the walls of the capillaries.
  • This fluid bathes all the living cells of the body and, since it contains dissolved food and oxygen from the blood, it supplies the cells with their needs.
  • Some of the tissue fluid eventually seeps back into the capillaries, having given up its oxygen and dissolved food to the cells, but it has not received the waste products of the cells, such as carbon dioxide, which are carried away by the bloodstream.
  • The tissue fluid that doesn’t return to the capillaries joins the lymphatic system.
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