Opposing BHP is…
Interstitial Fluid Hydrostatic Pressure (IFHP) pushes fluid from interstitial spaces back into capillaries. In most cases this pressure is small at 1mmHg. Blood Colloid Osmotic Pressure (BCOP) is a force caused by the suspension of large plasma proteins within blood plasma and averages 26mmHg in most capillaries. This force “ pulls ” fluid from interstitial spaces into capillaries.
Opposing BCOP is….
Interstitial Fluid Osmotic Pressure (IFOP) which “ pulls ” fluid out of capillaries into interstitial fluid. This is also a small force at 5mmHg because only tiny amounts of proteins are present in the interstitial fluid. Therefore, whether fluids leave or enter capillaries is entirely dependant on the balance of the above pressures. The dominant force prevails. If pressures that push fluid out of capillaries exceed the pressures that pull fluid into capillaries, fluid will move from capillaries to interstitial fluid = FILTRATION. However, if pressures that push fluid out of interstitial spaces into capillaries exceeds the pressures that pull fluid out of capillaries, fluid will move from interstitial spaces into capillaries = REABSORPTION.
Net filtration Pressure (NFP) indicates the direction of fluid movement.
NFP = (BHP + IFOP) – (BCOP + IFHP)
Filtration
Reabsorption
At the arterial end of a capillary:
NFPa = (35+5) mmHg – (26+1) mmHg
NFPa = 13 mmHg
Therefore, at the arterial end of a capillary, there is a net outward pressure of 13mmHg. This means that fluid/solutes move out of the capillary and into the interstitial fluid (ie: O2).
At the venous end of a capillary:
NFPv = (16+5) mmHg – (26+1)
NFPv = -6 mmHg
Therefore, at the venous end of a capillary, there is a net inward pressure of -6mmHg. This means that fluid/solutes move into the capillary from the interstitial fluid (ie CO2). Edema : If filtration greatly exceeds reabsorption there is an abnormal increase in interstitial fluid volume called Edema. This is usually detectable when interstitial fluid volume raises above 30%
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