The physiologic basis for nonconventional vascular perfusion.

It is traditionally thought that the exchange of oxygen occurs only at the capillary level, with both arterial inflow and venous outflow required. However, as early as 1970, studies have indicated that significant arteriolar and venular diffusion of O2 and CO2 occurs. The precapillary and postcapillary diffusion of oxygen and carbon dioxide and their potential regulation via the Bohr effect are key elements in understanding the success of nonconventional vascular perfusion (arterialized venous, total arterial, total venous), where the capillary circulation is bypassed. In retrograde "reverse-flow" island flaps, there is an anterograde blood flow through the capillary bed, with nonconventionally reversed flow within the arterial and venous systems, respectively. The reversal of venous flow is best explained as a combination of valvular insufficiency and the existence of vein-vein bypass channels via the venae comitantes.