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 and usually occurs simultaneously with situs inversus, asplenia, or polysplenism, persistent left superior vena cava (SVC), and congenital pulmonary venolobar syndrome. Azygos and hemiazygos continuation of the IVC is rare especially when it is not associated with congenital heart disease. The Greek root "zyg" refers to a pair. 'A-' means "not". Thus, "azygos" means "unpaired". The azygos vein is unpaired in that there is only one in the body, mostly on the right side. While there is the hemiazygos vein and its accessory on the left side of the body, they are considered tributaries of the azygos vein rather than