Control of the heart in fish

The single circulatory system of all fish consists of a four-chambered heart (sinus venosus, atrium, ventricle and bulbous arteriosus), in series with the branchial and systemic vascular beds (Randall, 1968; Farrell and Jones, 1992). The matching of rates of water and blood flow over the functional countercurrent at the gills, according to their relative capacities for oxygen, is essential for effective respiratory gas exchange (Piiper and Scheid, 1977) and must be capable of rapid adjustment to varying metabolic rates. Cardiac output, and its components stroke volume and heart rate, are robust indicators of metabolism (Farrell and Jones, 1992), and the changes in heart rate with altered metabolic demand requires that the pacemaker activity is controlled. Fine control of heart rate includes beat-to-beat modulation by the respiratory cycle that can be manifested as one-to-one cardiorespiratory synchrony (Taylor, 1992).