Tetrapod probably had a three-chambered heart, as do modern amphibians and reptiles, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel — aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise. The difference in density between air and water causes smells (certain chemical compounds detectable by chemoreceptors) to
Azygos vein Azygos vein The azygos vein is a vein running up the side of the thoracic vertebral column draining itself towards the superior vena cava. It connects the systems of superior vena cava and inferior vena cava and can provide an alternative path for blood to the right atrium when either of the venae cavae is blocked. The azygos vein transports deoxygenated blood from the posterior walls of the thorax and abdomen into the superior vena cava vein. The anatomy of this blood vessel can be quite variable. In some rare variations for example, it also drains thoracic veins, bronchial veins