Edema is caused by all of the following mechanisms, EXCEPT:
Correct Answer: Increased plasma proteins
Description: Edema is increased volume of extracellular extravascular fluid due to interference with the normal flow of fluids between blood, lymphatics, and tissue. The lymphatic circulation takes up a significant poion of fluid from the interstitial tissues, along with extravascular proteins, and returns it to the blood. If the lymphatic circulation is obstructed, the lymphatic drainage is interrupted. Affected areas have large accumulation of fluid and are edematous. The vascular system also aids in maintaining fluid volume. The endothelium is the lining of the vessels and serves as a semipermeable membrane for fluids and components, primarily by keeping proteins intraluminal colloidal pressure and causing fluid leakage and edema. Capillary blood pressure (BP), or hydrostatic pressure, is the force in capillaries that drives fluids from the capillaries into the tissues. By increasing the hydrostatic pressure, fluids tend to be forced out of the capillaries into tissues, causing edema. Sodium and water also aid in maintaining fluid balance. Sodium retention causes water retention, leading to an expansion of the extracellular fluid volume both intravascularly and in the tissues with edema. The protein concentration of plasma also functions in the maintenance of fluid balance. The major protein involved is albumin. By decreasing the plasma protein concentrations, there is a decreased colloid osmotic pressure in the blood, so there is a decreased counterforce to the hydrostatic BP. Thus, there is increased fluid escaping from the capillaries and decreased resorption of fluid from tissues. Fluid tends to accumulate extracellularly, with generalized edema. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 31. Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.
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