Functional Morphology of the Heart in Fishes

The systemic heart of fishes consists of four chambers in series, the sinus venosus, atrium, ventricle, and conus or bulbus. Valves between the chambers and contraction of all chambers except the bulbus maintain a unidirectional blood flow through the heart. The heart is composed of typical vertebrate cardiac muscle, although there may be minor differences in the distribution of spontaneously active cells, the rate and nature of spread of excitatory waves, and the characteristics of resting and action potentials between different fish and other vertebrates. Cholinergic fibers innervate the heart, except in hagfish which have aneural hearts. Fish hearts lack sympathetic innervation. The level of vagal tone varies considerably, and is affected by many factors. In some fish the heart is essentially aneural (without vagal tone) during exercise and may resemble an isolated mammalian ventricle with increased venous return causing increased cardiac output. There are many mechanisms that could increase venous return in exercising fish. rβ-adrenergic receptors have been located on the hearts of some fish, and changing levels of catecholamines may play a role in regulating cardiac activity. Changes in cardiac output in fish are normally associated with large changes in stroke volume and small cha-nges in heart rate.