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Capillary Vessels

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What are capillaries?

  • These are the smallest blood vessels in the body, taking part in micro circulation, by connecting areterioles and venules.

{Micro circulation is the network of arterioles, venules and capillaries embedded within organs responsible for blood circulation among tissues}

  • The capillary wall is a one cell layer thick structure, made of capillary endothelial cells.
  • The thin structure enables exchange of substances between blood and surrounding tissues

Examples: Water, oxygen, CO2, nutrients and  waste material

  • The structure of the capillary walls vary from organ to organ.


There are three kinds of capillaries

  1. Continuous capillaries
    • Endothelial cells provide uninterrupted lining, no gaps between cells
    • Permeability is very minimum
    • Permits passage of only small molecules (muscle, brain)
  1. Fenestrated capillaries
  • Have pores in cytoplasm of endothelial cells
  • Permeability is comparatively higher.
  • Permits passage of larger molecules (kidneys)
  1. Sinusoidal / Discontinuous capillaries
  • Type of fenestrated capillaries with larger openings in endothelium between the cells
  • Permits passage of even larger molecules (liver)



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