Double Circulation

Double circulation is the arrangement, first described by William Harvey in 1628, in which blood passes through the heart twice for every complete circuit of the body - once through the right side and once through the left - and it is most complete in mammals because their heart chambers are fully partitioned (see Figures 7.11 and 7.12). In the systemic circuit, oxygenated blood leaves the left ventricle through the aorta and is carried by a branching network of arteries, arterioles and capillaries to every tissue in the body; the resulting deoxygenated blood is then collected by venules, veins and the venae cavae and emptied into the right atrium. In the pulmonary circuit, that deoxygenated blood is pumped from the right ventricle to the lungs via the pulmonary artery, and the freshly oxygenated blood returning from the lungs is emptied into the left atrium via the pulmonary vein. Keeping these two circuits completely separate has a real physiological advantage: the lung capillaries must be extremely thin to allow efficient gas exchange, and thin capillaries under high pressure would leak fluid or rupture, flooding the surrounding lung tissue and increasing the diffusion distance for gas exchange; the systemic circuit, in contrast, needs to push blood through much longer vessels and so benefits from higher pressure. A fully divided double circulation lets the pulmonary and systemic circuits each run at the pres …