Which of the following changes in the lung microenvironment will result in a net overall movement of fluid volume from the intravascular space to the extravascular space leading to edema?
The answer is B.(Chap. 50)
The forces that regulate the disposition of fluid between these two components of the lung extracellular compartment frequently are referred to as the Starling forces. Increases in the hydrostatic pressure within the capillaries and the colloid oncotic pressure in the interstitial fluid tend to promote movement of fluid from the vascular to the extravascular space. By contrast, increases in the colloid oncotic pressure contributed by plasma proteins and the hydrostatic pressure within the interstitial fluid promote the movement of fluid into the vascular compartment.
As a consequence, there is net movement of water and diffusible solutes from the vascular space at the arteriolar end of the capillaries. Fluid is returned from the interstitial space into the vascular system at the venous end of the capillaries and by way of the lymphatics. These movements are usually balanced so that there is a steady state in the sizes of the intravascular and interstitial compartments, yet a large exchange between them occurs. However, if either the capillary hydrostatic pressure is increased (as in left heart failure) and/or the capillary oncotic pressure is reduced, a further net movement of fluid from the intravascular to the interstitial spaces will take place, resulting in pulmonary edema. Pulmonary edema may also develop due to a decrease in lung permeability or protein reflection coefficient as seen in acute respiratory distress syndrome (ARDS). In isolation, atmospheric pressure changes will not have an effect because the pressure is equally distributed to the intra- and extravascular spaces.