Schematic of circulatory system of larval lamprey and hagfish, a model of the protovertebrate

Oxygen-rich blood from cutaneous respiration mixes with the oxygen-poor blood returning to the heart from the muscle and other organs, before the admixture enters the heart. Thus the heart, which lacks a coronary circulation and relies entirely on the oxygen in luminal blood is downstream (efferent) from the gas exchange organ.

Schematic of the circulatory system of a gill-breathing fish

As fishes became larger the skin was no longer adequate as the sole gas exchanger. When gills became the site of gas exchange, replacing the skin, the heart was left upstream (efferent) the gas-exchanger. Thus, oxygen-poor blood returning to the heart from the muscle and other organs is not enriched. These fish may be limited in their aerobic performance a potential selective pressure for the evolution of a coronary circulation.

Schematic of the circulatory system of Amia calva, a basal air breathing fish.

Oxygen-rich blood from the lung mixes with the oxygen-poor blood returning to the heart from the muscle and other organs, before the admixture enters the heart. Thus the heart, which lacks a coronary circulation and relies entirely on the oxygen in luminal blood is downstream (efferent) from the gas exchange organ. Lung breathing fishes with this type of circulatory arrangement (e.g., the Australian lungfish, Neoceratodus forsteri, the gar, Lepisosteus, and tarpon, Megalops) are very active fish and airbreath while active independent of the tension of oxygen in the water.

Schematic of circulatory system in the South American lungfish, Lepidosiren paradoxa

These fish are obligate airbreathers. Imagine fish that drown when held underwater! They live at times in oxygen-poor water and their gills have degenerated, especially the filaments of the 3rd and 4th gill arches, which prevents oxygen-rich blood that has passed through the lung and flows through these gill arches from losing oxygen to the water. The 5th and 6th gill arches receive oxygen-poor, carbon dioxide-rich blood. The streams of blood are kept separate by septation of the atrium into a right and left side, by a partial septum in the cardiac ventricle, and by a spiral valve in the conus arteriosus. The gill filiments of the 5th and 6th arches are used to remove carbon dioxide from the blood. One half of the heart lacks enrichment of luminal blood with oxygen, and these fishes are not highly active.