Role of Microvessels in Oxygen Supply to Tissue

Abstract

Since the early work of August Krogh in 1919, capillaries have been assumed to be the sole supplier of oxygen for tissue. Recent studies provide convincing evidence that other microvessels also contribute to tissue oxygenation and that capillaries play a much more complex role than originally proposed by Krogh.

FIGURE 1

Top: Krogh’s model of oxygen supply to tissue considers each capillary to be the unique supplier of oxygen to a tissue cylinder. Bottom: predicted oxygen content as a function of distance along capillary. Because model assumes that all oxygen exchange occurs in capillary network, oxygen content at entrance to capillary must be equal to that in systemic arterial blood. Oxygen diffuses out along length of capillary, resulting in a linear decrease in oxygen content that reaches a value at the end of the capillary equal to that found in systemic venous blood.

FIGURE 2

Oxygen exchange among microvessels. Our current understanding of oxygen supply to tissue requires incorporation of extensive diffusive interactions among microvessels. A vascular network is shown containing arterioles, venules, and capillaries, with diffusion of oxygen indicated by interrupted arrows and convection of oxygen in arterioles, capillaries, and venules indicated by appropriately shaded solid arrows. A significant amount of oxygen has been shown to diffuse out of arterioles. Some of the oxygen that leaves arterioles is consumed by tissue immediately surrounding them, some is shunted to nearby venules, while the remaining portion is picked up by capillaries and subsequently distributed to tissue. Exchange of oxygen within capillary network is affected by oxygen tension in adjacent capillaries as well as nearby arterioles and venules. Thus consideration of various diffusive interactions among all types of microvessels is key to understanding oxygen supply to tissue.

FIGURE 3

Various pathways oxygen takes once it enters microvasculature by convection (Qo_{2 in}) are presented as a flow diagram. Solid arrows indicate convective flow, and interrupted arrows indicate diffusive pathways. Diffusive pathway that oxygen takes is determined by oxygen tension gradient present at a particular location. Oxygen that does not diffuse out of microvessels is returned to systemic circulation by convection (Qo_{2 out}).