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HBIO1 > Oedema
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  • Smallest, most numerous blood vessels
    • Carry blood from arteries to veins
    • Blood flows from arterioles → capillaries → venules
    • Size of lumen is ≈diameter of erythrocytes
    • Thin wall is composed of endothelium (single layer of overlapping flat cells)
  • Function: exchange materials between blood and tissue cells (O2, CO2, nutrients, wastes)
  • Capillary distribution varies with metabolic activity of body tissues
    • Skeletal muscle, liver, and kidney have extensive capillary networks
    • They are metabolically active and require an abundant supply of oxygen and nutrients
    • Connective tissue have less abundant supply of capillaries
    • Epidermis of skin and lens, and cornea of eye lack capillaries
  • Flow of blood is controlled by precapillary sphincters
    • Found between arterioles and capillaries
    • Smooth muscle allows them to contract and reduce blood flow
  • Hydrostatic pressure is created by the heart which pumps blood into arteries
  • At the arteriole end
    • Hydrostatic pressure > water potential
    • Plasma proteins lower water potential
    • H2O, small molecules, and fluid are forced out through permeable capillary wall
    • Plasma proteins are not forced out as they are too large
  • At the venule end
    • Water potential > hydrostatic pressure (due to lower volume)
    • Fluid flows back into blood with waste products produced by cells

Blood Plasma

  • Plasma is a liquid containing proteins, inorganic salts, amino acids, vitamins, hormones
  • Main plasma proteins are albumin, globulins, and fibrinogen
  • Albumin maintains osmotic pressure and acts as a transport protein for various substances
  • Globulins are mainly antibodies
  • Fibrinogen is involved in clotting process


  • Accumulation of tissue fluid in interstitial space
  • ↑capillary hydrostatic pressure due to increased blood volume
    • Heart failure
    • Impaired venous valves of leg (backflow of blood)
  • ↑capillary permeability due to inflammation
  • Impaired fluid return to blood due to low protein levels in blood plasma
    • This ↓colloid osmotic pressure of plasma
      • Fluid fails to return to blood at venous end
      • Fluid accumulates in interstitial space
    • Caused by
      • Low protein absorption from gut
      • Liver disease (proteins not produced)
      • Kidney disease (proteins leak into urine)
  • Blockage of lymphatic vessels
    • Small amounts of plasma protein leak out of blood
    • Usually transported back into the circulation by lymphatic vessels
    • Blocked vessels causes accumulation of protein in interstitial space
    • This ↑colloid osmotic pressure
    • Fluid diffuses from blood into interstitial space
  • Oedema increases diffusion pathway of nutrients and oxygen between blood and cells