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
Axillary vein Axillary vein In human anatomy, the axillary vein is a large blood vessel that conveys blood from the lateral aspect of the thorax, axilla (armpit) and upper limb toward the heart. There is one axillary vein on each side of the body. Its origin is at the lower margin of the teres major muscle and a continuation of the brachial vein. This large vein is formed by the brachial vein and the basilic vein. At its terminal part, it is also joined by the cephalic vein. Other tributaries include the subscapular vein, circumflex humeral vein, lateral thoracic vein and thoraco-acromial