Blood vessels deliver blood from the heart to the tissues, and return oxygen-depleted blood to the heart. The vascular system is made up of three vessel types. Arteries carry blood away from the heart, veins return blood to the heart, and the tiny capillaries connect arteries to veins, while also suffusing tissues with blood. The largest vessels in the body include arteries and veins.
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Human circulation is subdivided into two categories, notes Dr. Lauralee Sherwood in her book "Human Physiology." Systemic circulation takes oxygenated blood from the heart to the body tissues, and the largest of the systemic circulation arteries is the aorta. Blood enters the aorta from the left ventricle of the heart by way of a one-way valve to prevent backflow. From the aorta, several arterial branches carry blood to the head, later branches carry blood to the arms, and then the aorta proceeds downward through the body as the abdominal aorta.
The second subdivision of human circulation is pulmonary circulation, which carries deoxygenated blood from the heart to the lungs for oxygen, and then returns it to the heart. In his book "Anatomy and Physiology," Dr. Gary Thibodeau notes that the largest of the arteries in pulmonary circulation is called the pulmonary artery, which exits the heart via a valve from the right ventricle. The pulmonary artery, which later branches, is the only artery in the body to carry deoxygenated blood.
The systemic veins return deoxygenated blood from the tissues to the heart. Separate veins drain the lower and upper body, and these veins converge as they approach the heart. As such, the largest veins in the body are the ones that enter the right atrium of the heart. These veins, note Thibodeau, are called the superior and inferior vena cava. The superior vena cava drains the upper body and head, while the inferior vena cava drains the lower body.
After blood picks up oxygen from the lungs, it must return to the heart. The veins of pulmonary circulation return this blood, eventually dumping it into the left atrium of the heart. As with systemic circulation, pulmonary veins converge and grow larger as they approach the heart. The largest of these veins, of which there are four, are called pulmonary veins. Notes Thibodeau, these are the only veins in the body to carry oxygenated blood.