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Hypoxic states originate as a result of an insufficient amount of oxygen supplied to the cardiac cell and the amount actually required by the cell. The dominant factor affecting the supply of oxygen to cardiac structures is governed by the type and overall capacity of the cardiac blood supply. Whereas the heart of adult homeotherms consists entirely of compact musculature with coronary blood supply, the cardiac musculature of poikilothermic animals consists either entirely of the avascular spongious type, supplied by diffusion from the ventricular cavity, or its spongious musculature is covered by an outer compact layer supplied from coronary arteries. As a whole (ie, without distinction between the compact and spongious layer), the adult poikilothermic heart is significantly more tolerant to oxygen deprivation compared with the homeothermic heart, probably because of higher anaerobic capacity and differences in systems responsible for calcium handling. The hearts of chronic hypoxia-acclimated poikilotherms maintain maximum performance longer when faced with severe acute hypoxia and recover better than hearts of normoxic animals following an acute hypoxic insult. Whether the protective mechanisms in poikilothermic and homeothermic animals are the same remains to be clarified in future experiments. Thus, the poikilothermic heart represents a unique model for comparison of the tolerance to oxygen deprivation in two precisely defined, developmentally stable layers of the same heart differing in structure, type of blood supply as well as the capacity for energetic metabolism.