A scientist studying capillary fluid exchange in a laboratory environment can most effectively increase the filtration of fluid out of capillaries by doing which of the following?

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11ecba2d_130b_fc6c_a3bf_6342058a03cd_MD0008_00 The movement of fluid into and out of capillaries is important in both physiology and disease. The semipermeable membrane separating capillaries from surrounding tissue (ie, the interstitial space) permits the movement of fluid between these compartments. Consequently, net fluid filtration (J_v) is determined by the hydrostatic and osmotic pressure within each compartment, where a (usually) positive J_v indicates net movement of fluid out of the capillary. Hydrostatic pressure is created by fluid columns irrespective of spatial orientation (eg, blood vessels throughout the body). Fluid moves away from areas of high hydrostatic pressure and toward areas of low hydrostatic pressure. Consequently, capillary hydrostatic pressure (P_c) promotes the movement of fluid out of capillaries, whereas interstitial fluid hydrostatic pressure (P_if) diminishes the movement of fluid out of capillaries (Choice B). Net fluid filtration is related to the difference in hydrostatic pressures as: 11ecba2d_130b_fc6d_a3bf_7f75639b9b5a_MD0008_00 Osmotic pressure (Π) is created during osmosis by the diffusion of solvent across a semipermeable membrane separating compartments with different solute concentrations. Fluid moves from areas of low osmotic pressure to areas of high osmotic pressure, which is relevant to blood vessel physiology because the osmotic pressure of blood is greater than the osmotic pressure of interstitial fluid. The Starling equation can be used to calculate net fluid filtration and relates membrane permeability (K) to hydrostatic and osmotic pressure within the capillary and interstitial space: 11ecba2d_130c_237e_a3bf_675bd4f66339_MD0008_00 Therefore, only a decrease in the osmotic pressure within the capillary Π_c (eg, reduced concentration of plasma proteins or salts) would result in a further net positive J_v, increasing the movement of fluid out of the capillary. (Choices A and C) Increased blood speed or decreased blood viscosity would both result in greater volumetric blood flow. This would reduce blood hydrostatic pressure and decrease net fluid filtration (ie, less fluid moves out of the capillary). Educational objective: Fluid compartments within the body have unique hydrostatic and osmotic pressures. Blood capillaries and interstitial fluid are separated by a semipermeable membrane, and the Starling equation may be used to determine net fluid filtration between these compartments.

A Scientist Studying Capillary Fluid Exchange in a Laboratory Environment