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
Superficial vein Superficial vein Superficial vein is a vein that is close to the surface of the body. This differs from deep veins that are far from the surface. Superficial veins are not paired with an artery, unlike the deep veins, which typically have an artery with the same name close by. Superficial veins are important physiologically for cooling of the body. When the body is too hot the body shunts blood from the deep veins to the superficial veins, to facilitate heat transfer to the surroundings. Superficial veins can be seen under the skin. Those below the level of the heart