Comparative Aspects of Cardiac Adaptation

Abstract

Transition from water to land, necessity of thermoregulation, and physical activity essential for survival of individual species represent serious changes in requirements imposed on cardiac function during phylogenetic development. As a result, the heart size in different species of vertebrates, expressed as a ratio of heart weight to body weight, varies considerably. The maximum acceleration of the heart growth during phylogeny occurs when the metabolic activity of animal tissues has substantially increased, i.e., during transition from poikilothermy to homeothermy. Phylogenetic differences in cardiac size, performance, and energy demand are reflected in the construction of an oxygen pathway from blood to mitochondria. The heart of cold-blooded animals is either entirely spongious, supplied by diffusion from ventricular cavity, or the inner avascular layer is covered by an outer compact layer with vascular supply. The compact heart of adult homeotherms is supplied by capillaries from coronary vessels. It was found that the thickness of the compact layer in poikilotherms increases with increasing heart and body weight, suggesting that the compact layer is necessary for the maintenance of the higher blood pressure in the larger hearts. The structural differences between spongious and compact musculature are accompanied by significant metabolic differences: the spongy myocardium is better equipped for aerobic metabolism than the compact tissue. It is obvious that the responses to different types of increased work load in individual species of lower vertebrates differ according to the structural, functional, and metabolic properties of their cardiac muscle. They vary from the isolated enlargement of the individual myocardial layers, i.e., spongious and compact musculature, to the enlargement of the whole heart, predominantly by combination of hypertrophy, and hyperplasia of muscle cells.