The systemic heart of fishes consists of four chambers in series, the sinus venosus, atrium, ventricle, and conus or bulbus. The heart is composed of typical vertebrate cardiac muscle, although there may be minor differences in the distribution of spontaneously active cells. Fish hearts lack sympathetic innervation. The level of vagal tone varies considerably, and is affected by many factors. Changes in cardiac output in fish are normally associated with large changes in stroke volume and small changes in heart rate.All fishes except lung fishes have 2 chambered venous hearts.