Decreased oncotic pressure

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  1. The key factor that restrains fluid loss from the capillaries is the osmotic pressure of the plasma proteins - termed the colloid osmotic pressure or oncotic pressure
  2. The total osmotic pressure of plasma is 6000 mm Hg whereas the oncotic pressure is only about 25 mm Hg; however this small oncotic pressure plays an important role in fluid exchange across the capillary wall because the plasma proteins are essentially confined to the intravascular space while the electrolytes responsible for the major fraction of plasma osmotic pressure are equal in concentration on both sides of the capillary endothelium
  3. Of the plasma proteins, albumin is the main determinant of oncotic pressure (albumin is present at almost twice the concentration as the globulins); albumin retains both Na and Cl in the capillary space and accounts for about 65% of the oncotic pressure (globulins 15% and everything else combined 20%)
  4. In the lungs the mean capillary hydrostatic pressure is 8 mm Hg and the plasma oncotic pressure is 25 mm Hg and interstitial fluid oncotic pressure is about 15 mm Hg so the net force slightly favor reabsorption but in left ventricular failure or stenosis of the mitral valve the pulmonary capillary hydrostatic pressure exceeds plasma oncotic pressure and pulmonary edema occurs
  5. Changes in plasma protein concentration:
    1. increases
      1. dehydration
    2. reduction
      1. nephrosis – loss of protein in the urine
      2. burns
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