The Lymphatic Vasculature as a Participant in Microvascular Exchange

Abstract

Optimal organ function requires the two-way movement of substrates and cell products between the circulatory system and the cells themselves. The common thought is that the circulatory system consists of arteries, arterioles, capillaries, venules and veins and that exchange occurs chiefly across capillaries and the venules. The capillaries, given their thin walls and large number, present a large surface area and are therefore perceived as the primary site for fluid and gas exchange. The venules, also thin walled and numerous, possess only minimal coverage by vascular smooth muscle and are seen as the primary site for the movement of larger solutes up to and including the white cells observed to roll along their walls. In fact, the exchange of solutes occurs across the walls of the arterioles, albeit at lower rates [1], especially in organs such as the heart where significant numbers of cardiac myocytes are far removed from capillaries or venules [2]. Essential elements omitted from the list of constituents of the cardiovascular system are the components of the lymphatic system residing within the organ. These include the blind-end initial lymph sacs, collecting lymphatics, and lymph nodes. As will be discussed further, for materials to move from the vascular and tissue compartments, the barriers must possess a finite permeability. In addition, unlike what is presented in the textbooks, all fluid that filters from the vascular space into the tissue space is not reabsorbed back across the same barrier [3]. Obviously, that fluid and accompanying solute must be transported back into the vascular space, otherwise edema occurs and tissue function is lost.