Figure 1Physiology of capillary fluid exchange. Capillary hydrostatic pressure decreases along the capillary path and becomes lower than oncotic capillary pressure, which remains stable. Thus, the net filtration pressure (NFP) is positive favorizing transcapillary fluid filtration (transudation) at the arteriolar side while it is negative at the capillary outlet responsible for reabsorption of interstitial fluid into the vascular compartment. For most tissues, there is an imbalance between filtration and reabsorption in favor of an excess of fluid in interstitial space which is balanced by removal of interstitial fluid via the lymphatics according to its flow rate (Qλ) which finally returns to the bloodstream through the thoracic duct.