General structure of the two-compartments model.

Plasma and interstitial compartments are separated by the porous capillary membrane, in which 3 types of pores are present, LP, SP, UP (large, small and ultrasmall pores). Bi-directional transport of fluid and proteins happens across the membrane through the pore system (but UP are not permeable to proteins), and is complemented by a reabsorption of fluid and solute through the lymphatic vessels. Water is removed from vascular compartment by the HD machine.