Circulatory System

The pigeon's circulatory system meets the very high metabolic demands of flight and also helps regulate body temperature, and is built around a fully four-chambered heart - two auricles and two ventricles - with the right and left sides completely separated by a septum and, unlike in the frog, no sinus venosus. Two precaval veins (superior venae cavae) and one postcaval vein (inferior vena cava) deliver deoxygenated blood into the right auricle, which passes it through the right auriculo-ventricular aperture (guarded by a single-flap valve) into the right ventricle; this side of the heart pumps only venous blood, out through the pulmonary aorta to the lungs. Oxygenated blood returns from the lungs through the pulmonary veins into the left auricle, then through a two-flap valve (anchored by chordae tendineae to papillary muscles) into the left ventricle, which pumps only arterial blood out through a single right aortic arch (only the right arch persists in birds) to the rest of the body. Because the two sides of the heart are fully separated and handle only one type of blood each, the pigeon has a complete double circulation - separate pulmonary and systemic circuits - which is more efficient than the frog's partly-mixed single circuit. From the aortic arch, the arterial system distributes blood via the carotid arteries (to the head), subclavian, brachial and pectoral arteries (to the wing), and, along the dorsal aorta, the co …