The Distribution Of Oxygen In The Circulation

Data on longitudinal oxygen distribution along the circulation and the microcirculation show that arteriolar blood vessels of most tissues release a significant amount of oxygen, with the result that in general the oxygen supplied by capillaries is secondary to that provided by the arterioles.This arteriolar release of oxygen determines a longitudinal oxygen gradient in the circulation, as shown by Tsai et al. (2003). It has been well established, since the studies of Duling and Berne (1970), that the circulation strives to maintain this pattern of oxygen distribution by adjusting flow via a change in caliber of the resistance arterioles. Vasodilatation increases blood availability while maintaining a relatively constant rate of oxygen exit, and vice versa. Although the specific nature of the microvascular oxygen sensor is not yet fully established, adrenergic enervation in the arteri-olar wall is an important component of the mechanism that controls arteriolar tone, modulating flow in such a fashion that the pattern of oxygen distribution remains approximately normal. It is notable that the steepest portion of the oxygen dissociation curve corresponds to the location of the highest adrenergic nerve endings in the microcirculation (Saltzman et al., 1992).

Changes in the composition of blood, its viscosity, and the oxygen dissociation curve for hemoglobin generate signals that are important in designing and using blood substitutes. A decrease in the oxygen-carrying capacity due to hemodilution induces vasodilatation, and the restoration of the lost red blood cells with an oxygen carrier can produce an additional set of signals depending on whether the carrier has a high or low affinity for oxygen. A high oxygen affinity (low P50) results in oxygen being preferentially unloaded in the capillaries, and vice versa. An additional factor that regulates the delivery of oxygen to the tissue is the consumption of oxygen by the vessel wall. Such consumption is usually assumed to be similar to that of the parenchyma, but several studies show that it may be extraordinarily high, and that it increases with vasoconstriction (Ye et al., 1990; Friesenecker et al., 2004).

Figure 7.1 shows the normal pattern of oxygen distribution in the microcirculation of the tissue in the chamber window preparation of the hamster. It also shows, for comparison, the pattern of oxygen distribution after treatment with arginine vasopressin, a vasoconstrictor, and the same distribution after treatment with a vasodilator that reduces the vessel wall oxygen consumption to the theoretical lower limit, being similar to that of the surrounding tissue.The significance of these configurations is that the tissue is not supplied with oxygen by blood but by the blood vessels, and therefore their oxygen consumption is a factor.

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