ABSTRACT
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. Cyclostomes, elasmobranchs, dipnoans and some teleosts seem to lack adrenergic innervation of the heart. The heart of cyclostomes is not innervated by the sympathetic nervous system, but the heart contains large quantities of adrenaline and noradrenaline stored in chromaffin cells. The parasympathetic, vagus nerve is the sole nervous regulator of heart rate in elasmobranchs and predominates in teleosts. The role of the autonomic innervation in the regulation of heart rate can be quantified by calculating the autonomic tones. In the teleost fish the heart receives both a cholinergic vagal supply and an adrenergic sympathetic supply. Close beat-to-beat temporal relationships between heart beat and ventilation, or cardiorespiratory synchrony, has long been hypothesized for fish. Studies using isolated gill arches demonstrated a very important difference between fish and the air-breathing vertebrates.
