In order to distribute nutrients and oxygen needed for metabolism, mammals need a highly efficient circulatory system. The main differences in circulatory structure between mammals and most other vertebrates are in the heart and in the red blood cells. The mammalian heart has four chambers (as do birds and crocodilian reptiles) compared to the three chambers found in the reptiles (except the crocodilians).
The additional chamber is the result of a muscular wall (or septum) that divides the ventricle (lower half of the heart) into two chambers. In reptiles there is a single ventricle in which deoxygenated blood from the right atrium mixes with oxygenated blood from the left atrium. In mammals deoxygenated blood enters the right ventricle only and is then pumped to the lungs. Oxygenated blood returns from the lungs to the left atrium and then enters the left ventricle from which it is sent to the systemic circuit and the rest of the body. Thus, mammals have separated the pulmonary and systemic circuits with the result that mammalian blood is more fully oxygenated than the blood of terrestrial vertebrates with three-
The concave shape also increases surface area and places the membrane surface closer to the hemoglobin molecules facilitating gas diffusion. Thus, mammalian blood is capable of carrying more oxygen than reptilian blood. The mammal heart is large, as are the lungs, and together these organs occupy most of the thoracic cavity. In certain taxa, these organs may be even bigger. Bats, for example, have a heart that is three times larger than the average terrestrial mammal of the same size. The mammal lung is sponge-