The fish heart (figure 1a) is much different than the amphibian/reptile/bird/mammal heart (figures 1b and c). Hearts are very complex--they're not just a bunch of random arteries and veins connecting tissue. Fish hearts simply draw in deoxygenated blood in a single atrium, and pump it out through a ventricle. This system is termed "single circulation", as blood enters the heart, gets pumped through the gills and out to the body, Blood pressure is low for oxygenated blood leaving the gills.

3 and 4 chambered hearts have a pulmonary circuit (pathways taking blood from heart to lung and back to heart) that is very complex and must be set up such that blood can travel from the heart to become oxygenated in the lungs and then be properly pumped back the heart and out to the body. The 3 (and 4) chambered heart has "double circulation" (figure 1b and c) and is quite different from "single circulation" (figure 1a) of fishes.

"Double circulation" has an interior circuit within the heart--blood enters the heart, leaves the heart and gets oxygenated, enters the heart again, and then gets pumped out to the body. Because "Double circulation" allows oxygenated blood to be pumped back into the heart before going out to the body, it pumps blood with much more pressure and much more vigorously than "single circulation".


Though the 4 chambered heart has 2 atrium-ventricle pairs, both pairs do not do the same thing. There are 4 steps involved with blood entering the heart: 1) oxygen poor blood enters the first atrium. 2) oxygen poor blood is fed to the first ventricle, which pumps it out to the pulmonary circuit (lungs) where it is enriched in oxygen. 3) Oxygen rich blood just leaving the lungs is pumped back into the second atria. 4) Oxygen rich blood is then fed to the second ventricle, which pumps the oxygen rich blood out of the heart and back into the body for usage.

The 4 chambered heart differs from the 3 chambered heart in that it keeps oxygenated blood completely separate from de-oxygnated blood, because there is one ventricle for deoxgynated blood and one for oxygenated blood. In the 3 chambered heart, a single ventricle pumps both out of the heart, and there is some mixing between fresh and old blood. The 2 ventricle-4 chamber heart prevents mixing allows the blood leaving the heart to have far more oxygen than it would otherwise. This is good for enhancing the more fast paced lifestyle that birds and mammals tend to have, giving an advantage to having a 4 chambered heart.