Why accumulation of fluid in the interstitial compartment occurs when there is an excessive loss of albumin in urine.

Explain why accumulation of fluid in the interstitial compartment occurs when there is an excessive loss of albumin in urine. 

 Flow of fluid to interstitial space depend on
1. Starling forces
2. Capillary permeability
3. Available surface area.
 When capillary permeability increases flow of fluid increase.
  And when available surface area increases, again flow of fluid increases.
 Starling forces are,
1. Filtration
2. Osmosis.
 Filtration depends on hydrostatic pressure.
 That is pressure exerted by fluid.
 At arteriolar end, capillary hydrostatic pressure is 37mmHg.
 At venular end, 17mmHg.
 Hydrostatic pressure in interstitial fluid is 1mmHg.
 Therefore there is net pressure towards interstitial fluid.
 Arteriolar end = 37-1 = 36mmHg.
 Venular end = 17-1 = 16mmHg.
 Osmosis depends on colloid osmotic pressure.
 That is the pressure exerted by colloid substance, specially by plasma proteins.
 Colloid osmotic pressure towards capillary is 25mmHg.
 Colloid osmotic pressure towards interstitial space is 0mmHg.
 Because there are no proteins in interstitial space.
 Therefore, net pressure towards capillary is 25mmHg.
 So, the net movement of fluid in arteriolar end is into the interstitial space.
 In the venular end into the vessels.
 More fluid comes out of the arteriolar end, than the amount of fluid entering the venular end.
 Excess fluid is carried by the lymphatics.
 Albumin is the major contributor to the colloid osmotic pressure.
 Due to its,
1. Low molecular weight
2. Abundance.
  Albumin is lost in urine.
 Albumin concentration along the capillary falls.
 Colloid osmotic pressure falls.
 More fluid comes out in the arteriolar end.
 And less fluid enters the venular end.
 When the excess fluid that moves out cannot be compensated by lymphatics, fluid accumulates in interstitial space.