Pulmonary circulation where carbon dioxide is released and oxygen is picked up during respiration. Arteries are further divided into very fine capillaries which are extremely thin-walled. The pulmonary vein returns oxygenated blood to the left atrium of the heart. The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. This blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. From the left ventricle, the blood passes through the aortic valve to the aorta. The blood is then distributed to the body through the
Vein directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system. The peripheral veins carry blood from the limbs and hands and feet. Microscopically, veins have a thick outer layer made of connective tissue, called the tunica externa or "tunica adventitia". There is a middle layer of bands of smooth muscle called tunica media, which are, in general, much thinner than those of arteries, as veins do not function primarily in a contractile manner and are not subject to the high pressures of systole, as arteries are. The interior is lined with endothelial