There are currently three extant genera of true lungfish (dipnoans): one in South America (Lepidosiren), one in Africa (Protopterus) and one in Australia (Neoceratodus). Although there are numerous fish that can breathe air, a common feature of the dipnoans is a vascular arrangement with a pulmonary artery, and a pulmonary vein that brings the oxygenated blood directly to the heart.The control systems regulating the blood flow in the dipnoan circulation are not well known. In most vertebrates, adrenergic systems - circulating catecholamines and adrenergic nerves (postganglionic neurons of the sympathetic nervous system) - play essential roles in the control of both cardiac function and vascular resistance. In lungfish there is little evidence for adrenergic nerves, instead there is an impressive presence of chromaffin cells within the atrium of the heart, in the walls of the left posterior cardinal vein (azygos vein) and the intercostals arteries. A cholinergic innervation of the lung, the heart and possi...