Circulatory Pathways

Two fundamentally different circulatory designs exist among animals. In an open circulatory system, seen in arthropods and most molluscs, the heart pumps a fluid called haemolymph into vessels that empty directly into open body cavities called sinuses (collectively the haemocoel), so the fluid bathes the organs directly rather than staying confined to tubes. In a closed circulatory system, seen in annelids, cephalopods and all vertebrates, blood is pumped by the heart and stays confined within blood vessels throughout its journey, which allows much finer control of pressure and distribution. Among vertebrates, every group has a muscular, chambered heart, but the number of chambers - and therefore how completely oxygenated and deoxygenated blood stay apart - has increased through evolution. Fish have a two-chambered heart (sinus venosus, one atrium, one ventricle, and a bulbus/conus arteriosus) and only single circulation, where blood passes through the heart just once per full circuit of the body. Amphibians have two atria but only one ventricle with no dividing septum, and most reptiles (except crocodiles) likewise have two atria and one incompletely septated ventricle, so oxygenated blood from the left atrium and deoxygenated blood from the right atrium mix to some extent in the shared ventricle - this partial mixing is called incomplete double circulation, even though separate pulmonary and systemic circuits are already present. Crocodiles, birds a …