CLAUS LUNAU / SCIENCE PHOTO LIBRARY CLAUS LUNAU / SCIENCE PHOTO LIBRARY
Primate, reptile and fish blood circulation. Illustration of the circulatory systems of a primate (left), a reptile (centre) and a fish (right). Humans are a type of primate. The common features are the heart and lungs (or gills in fish), with differences in the way oxygenated (red) and deoxygenated (blue) blood is circulated. Fish have a two-chambered heart and a single circulatory route for blood, with the heart pumping blood to the gills, after which it flows round the body and back to the heart. Reptiles and mammals such as primates have a pulmonary circuit, where blood is pumped from the heart to the lungs and back, and a second systemic circuit where blood is pumped to the body and back. In addition, reptiles have a three-chambered heart (and two aortas), while mammals have a four-chambered heart (and one aorta). In reptiles, some mixing of oxygenated and deoxygenated blood occurs in the heart.
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