The transport of albumin: a critique of the vesicular system in transendothelial transport.

Although albumin circulating in the plasma can exchange with the contents of the extensive vesicular system of microvascular endothelium, the mechanism and quantitative significance of vesicular transport remains unclear. It has recently been shown that transport of electron opaque tracers can occur via the vesicular system, but the detailed ultrastructure is inconsistent with the transcytotic shuttling of single vesicles. Although most physiologic measurements show a major degree of convective coupling in the microvascular permeation of perfused tissues, recent reports suggest that convective coupling is low when albumin escapes from the intact circulation. Because albumin clearance in these studies is consistent with values calculated from ferritin-labeling of vesicles in perfused microvessels, the possibility remains that macromolecular transendothelial transport occurs through transient communications within the vesicular system.