Number and distribution of capillaries as determinants of myocardial oxygen tension

Abstract

Blood content per gram of tissue was measured in various regions of the myocardium of the dog, by use of I¹³¹-labeled albumin and Cr⁵¹O₄-labeled red cells. The ratio of I¹³¹ to Cr⁵¹O₄ distributions was uniform, indicating that plasma skimming does not increase O₂ delivery to the inner layers. Gradients in blood content were observed from epicardium to endocardium, and in the base-apex dimension of the heart. Both the transmural gradient and the amount of blood per gram of tissue were greatest at the left ventricular apex. In this region the deeper layers contained 1.5 times as much blood as superficial ones. The data permitted estimates of the number of open capillaries, and of intercapillary distances. These estimates indicate that only a fraction of the available capillaries are perfused at rest. Mean tissue oxygen tensions were computed for various conditions of flow, capillarity, and metabolism by use of the Kety modification of the Krogh equation. Results are discussed in relation to the regulation of tissue PO₂.