Ans. B. Reduced plasma colloidal osmotic pressure. (Ref. Ganong 23rd/pg. 592 - 594)# Net fluid movement depends on net driving pressure and two other important factors related to the properties of the membranes. The filtration coefficient ,Kf, depends on the surface area and the permeability of the capillary wall to water (hydraulic conductivity). Leaky capillaries (e.g., due to histamine) have a high Kf. Glomerular capillaries have a high Kf, needed for their function.# The potential causes of pulmonary edema and pleural effusion are increases in capillary hydrostatic pressure, increases in interstitial colloid osmotic pressure, decreased plasma colloid osmotic pressure. Edema due to hydrostatic pressure is called cardiogenic edema. Edema due to osmotic pressure and membrane properties is called non- cardiogenic pulmonary edema.Fluid Filtration Across Capillaries Is Determined by Hydrostatic and Colloid Osmotic Pressures, and Capillary Filtration CoefficientFour primary forces that determine whether fluid will move out of the blood into the interstitial fluid or in the opposite direction. These forces, called "Starling forces " in honor of the physiologist who first demonstrated their importance:1) The capillary pressure (Pc), which tends to force fluid outward through the capillary membrane.2) The interstitial fluid pressure (Pif), which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative.3) The capillary plasma colloid osmotic pressure (Pp), which tends to cause osmosis of fluid inward through the capillary membrane.4) The interstitial fluid colloid osmotic pressure (Pif), which tends to cause osmosis of fluid outward through the capillary membrane.If the sum of these forces, the net filtration pressure, is positive, there will be a net fluid filtration across the capillaries. If the sum of the Starling forces is negative, there will be a net fluid absorption from the interstitial spaces into the capillaries. Net filtration from systemic capillaries is dependent on the Starling forces and capillary permeability. Equation is:Net filtration = Kf where Kf is the filtration coefficient of the membrane, and is directly proportional to capillary permeability, Pcapillary and Ptissue are the hydrostatic pressures in the capillary and tissue (interstitial space), respectively, and pcapillary and ptissue are the osmotic (colloid oncotic) pressures in the capillary and interstitial space, respectively. Increasing central venous pressure increases the capillary hydrostatic pressure (Pcapillary), which increases the filtration of fluid from the systemic capillaries, leading to edema.The movement of fluid between vascular and interstitial spaces is controlled mainly by the opposing effects of vascular hydrostatic pressure and plasma colloid osmotic pressure. Normally, the exit of fluid into the interstitium from the arteriolar end of the microcirculation is nearly balanced by inflow at the venular end; the lymphatics drain a small residual amount of excess interstitial fluid. Either increased capillary pressure or diminished colloid osmotic pressure can result in increased interstitial fluid,n Increased Hydrostatic PressureLocalized increases in intravascular pressure can result from impaired venous return; if the failing heart cannot increase cardiac output, the extra fluid load causes increased venous pressure and, eventually, edema.# Reduced Plasma Osmotic PressureCapillary hydrostatic and osmotic forces are normally balanced so that there is no net loss or gain of fluid across the capillary bed. However, increased hydrostatic pressure or diminished plasma osmotic pressure leads to a net accumulation of extravascular fluid (edema). If the ability of the lymphatics to drain tissue fluid is exceeded, persistent tissue edema results.# Effect of Abnormal Imbalance of Forces at the Capillary MembraneIf the mean capillary pressure rises above 17 mm Hg, the net force tending to cause filtration of fluid into the tissue spaces rises. Thus, a 20 mm Hg rise in mean capillary pressure causes an increase in net filtration pressure from 0.3 mm Hg to 20.3 mm Hg, which results in 68 times as much net filtration of fluid into the interstitial spaces as normally occurs. To prevent accumulation of excess fluid in these spaces would require 68 times the normal flow of fluid into the lymphatic system, an amount that is 2 to 5 times too much for the lymphatics to carry away. As a result, fluid will begin to accumulate in the interstitial spaces, and edema will result.Conversely, if the capillary pressure falls very low, net reabsorption of fluid into the capillaries will occur instead of net filtration, and the blood volume will increase at the expense of the interstitial fluid volume.Note: Approximately 60% of lean body weight is water, two-thirds of which is intracellular and the remainder is in extracellular compartments, mostly as interstitial fluid; only 5% of total body water is in blood plasma.