With prolonged standing, particularly when associated with elevation of Pv in the legs (such as that caused by pregnancy and congestive heart failure), filtration across capillaries is greatly enhanced, exceeding the capacity of the lymphatic system to remove the filtrate from the interstitial space and thus leading to edema.
The concentration of plasma proteins may change in several pathological states and thus alter the osmotic force and movement of fluid across the capillary membrane. The plasma protein concentration is increased in conditions of dehydration (e.g., water deprivation, prolonged sweating, severe vomiting, diarrhea). In this condition, less water moves by osmotic force from the tissues to the vascular compartment, thereby decreasing the volume of the interstitial fluid. In contrast, the plasma protein concentration is reduced in some renal diseases because of its loss in urine, and edema may occur.
When capillary injury is extensive, as in severe burns, intravascular fluid and plasma protein leak into the interstitial space in the damaged tissues. The protein that escapes from the vessel lumen increases the oncotic pressure of the interstitial fluid. This greater osmotic force outside the capillaries leads to additional fluid loss and possibly to severe dehydration.
Balance of Hydrostatic and Osmotic Forces. The pressure sure and the role of these forces in regulating fluid passage across the capillary endothelium were expounded by Frank
Starling in 1896. This relationship constitutes Starling’s • Fig. 17.28 Schematic representation of the factors responsible for filtration and absorption across the capillary wall and the formation of lymph.
Albumin is the most important plasma protein that determines oncotic pressure. Its molecular weight is 69,000 D. Albumin exerts an osmotic force greater than can be accounted for solely on the basis of its concentration in plasma. Therefore, it cannot be replaced on a mole-bymole basis by inert substances of appropriate molecular size, such as dextran. This additional osmotic force becomes disproportionately great at high concentrations of albumin (as in plasma), and this force is weak to absent in dilute solutions of albumin (as in interstitial fluid). The reason for this activity of albumin is its negative charge at normal blood pH and the attraction and retention of cations (principally Na+) in the vascular compartment (Gibbs-Donnan effect).
hypothesis. It can be expressed as follows:
Equation 17.20 k = filtration constant for the capillary membrane πi= interstitial fluid oncotic pressure Filtration occurs when the algebraic sum is positive; absorption occurs when it is negative.
Traditionally, filtration was thought to occur at the arterial end of the capillary, and absorption was thought to occur at its venous end because of the gradient of hydrostatic pressure along the capillary. This scheme is true for an idealized capillary (see