Anatomical determinants of O2 flux density at coronary capillaries

Calculations indicate that the PO2 in plasma falls to zero approximately 3 microns from an erythrocyte at O2 consumption (VO2) characteristic of myocardium (Federspiel, W.A., and A. Popel, Microvasc. Res. 32: 164-189, 1986). We measured distances between individual red cells along capillaries in rat hearts rapidly frozen in situ. Cell spacing varied widely even in branches of the same capillary. Plasma gaps between red cells were divided into two populations, those less than 5 microns and those greater than 5 microns. Mean gap lengths were 2.1 and 16.5 microns, respectively. Although the number of long plasma gaps was underestimated, gaps greater than 5 microns accounted for one-third of observed capillary length. Frozen muscles were also viewed in cross section. Because the depth of penetration of light was approximately equal to 3 microns, counts of red cell-containing capillary profiles in cross section depend on cell spacing as well as on number of cell-containing flow paths. Counts varied markedly with arterial O2 partial pressure, indicating that the capillary surface area functional for O2 transport changes in response to stress. The adaptive role of change in O2 flux density (flux per area) is discussed in light of new knowledge of tissue O2 gradients.