1 - The Heart

This chapter discusses different aspects of the cardiac anatomy, morphology, and physiology of fish. The fish heart is a four-chambered organ contained within a pericardial sac. Together, the sinus venosus, atrium, ventricle, and either an elastic bulbus arteriosus, or a contractile conus arteriosus raise the potential and kinetic energy of the blood. The principal tissue component of the sinus wall is connective tissue with bounding inner endothelial and outer epicardial linings. The amount of cardiac muscle present in the sinus venosus varies considerably among species. The ventricle shows considerable species variability with respect to its relative mass, gross morphology, histology, and vascularity. The addition of inner fibers, either as a circular arrangement around the sac-like elasmobranch ventricle, or as coils around the vertices of the pyramidal teleost ventricle, is clearly a characteristic of fish that are more active and perhaps provides a mechanical advantage for developing higher blood pressures. The histological structure of the fish coronary artery is characterized by an external parenchyma surrounding a medial layer of vascular smooth muscle; an internal elastic lamina separates the media from the intima, which normally has a single layer of endothelial cells.