Accessory Pumps in Fishes

A remarkable variety of accessory pumps occurs in the venous circulation of different fishes. These range from the portal heart of hagfishes behind the liver which has cardiac-type muscle and resembles the atrium of the main heart, with an ECG with P and T waves; to the caudal hearts of elasmobranchs and teleosts. These fish also have hemal arch and fin pumps.

Apart from the hagfish portal heart, all of these interesting devices are driven by skeletal muscles. For example, in the shark hemal arch pump venous blood from the myotomes is driven into the caudal vein past ostial valves as the myotomes contract and compress the vascular bed, and so when the fish swims there are cyclical pressure pulses in the caudal vein, and venous blood flow increases (just when cardiac output increases and a higher venous return is required).