Comparative Aspects of the Cardiac Blood Supply

Abstract

Phylogenetic differences in cardiac size, performance, and energy demand are reflected in the construction of an oxygen pathway from blood to mitochondria. Vertebrates generally possess four types of myocardial structure and blood supply: (a) spongious musculature only, supplied by diffusion from the ventricular cavity--lacunar blood supply; (b) the inner avascular spongious layer, which is covered by an outer compact layer with a vascular supply; (c) like (b) but with capillaries also present in some trabeculae of spongious musculature; and (d) compact musculature only, supplied by coronary vessels. While the heart of adult homeotherms consists of compact musculature with a coronary blood supply, the ventricular myocardium of most species of cold-blooded vertebrates is of types (a), (b), or (c). The spongy layer is characterized by having more active oxidative metabolism and greater sensitivity to oxygen deprivation, and the toxic effects of catecholamines as compared with the compact one. The fraction of ventricular wall occupied by coronary supplied compact myocardium varies considerably (e.g., in fish between 7% and 73%). All available findings support the hypothesis that the development and growth of the vascular compact layer is determined by overall oxygen consumption and not by phylogenetic position of the animal. It is related to body mass, muscular activity, capacity of oxidative metabolism, and maintenance of body temperature. The compact layer is thus necessary for the maintenance of higher blood pressure in the larger hearts (according to the law of Laplace). This also means that the development of vascularization is a consequence of this evolution since coronary arteries are the only way to supply the compact musculature with blood.