BLOOD VESSELS Blood vessels form a closed circuit of tubes that transport blood between the heart and body cells. The several types of blood vessels include arteries, arterioles, capillaries, venules, and veins. Blood Vessel Classifications The blood vessels of the body fall into three classifications:  Arteries and arterioles—distributors  Capillaries—exchangers  Veins and venules—collectors Arteries and Arterioles Arteries are elastic tubes constructed to withstand high pressure. They carry blood away from the heart to all parts of the body. The smallest branches of the arteries are called arterioles. The walls of arteries and arterioles consist of layers of endothelium, smooth muscle, and connective tissue. The smooth muscles of arteries and arterioles constrict and dilate in response to electrical impulses received from the autonomic nervous system. Capillaries At the end of the arterioles is a system of minute vessels that vary in structure, but which are spoken of collectively as capillaries. It is from these capillaries that the tissues of the body are fed. There are approximately 60,000 miles of capillaries in the body. As the blood passes through the capillaries, it releases oxygen and nutritive substances to the tissues and takes up various waste products to be carried away by venules. Venules continue from capillaries and merge to form veins. Veins and Venules Veins and venules form the venous system. The venous system is comprised of vessels that collect blood from the capillaries and carry it back to the heart. Veins begin as tiny venules formed from the capillaries. Joining together as tiny rivulets, veins connect and form a small stream. The force of muscles contracting adjacent to veins aids in the forward propulsion of blood on its return to the heart. Valves, spaced frequently along the larger veins, prevent the backflow of blood. The walls of veins are similar to arteries, but are thinner and contain less muscle and elastic tissue. Arterial System Arterial circulation is responsible for taking freshly oxygenated blood from the heart to the cells of the body (fig. 1-35). To take this oxygenated blood from the heart to the entire body, the arterial system begins with the contraction of blood from the left ventricle into the aorta and its branches. AORTA.—The aorta, largest artery in the body, is a large tube-like structure arising from the left ventricle of the heart. It arches upward over the left lung and then down along the spinal column through the thorax and the abdomen, where it divides and sends arteries down both legs (fig. 1-35). KEY BRANCHES OF THE AORTA.—Key arterial branches of the aorta are the coronary, innominate (brachiocephalic), left common carotid, and left subclavian. The coronary arteries are branches of what is called the ascending aorta. The coronary arteries supply the heart with blood. There are three large arteries that arise from the aorta as it arches over the left lung. First is the innominate artery, which divides into the right subclavian artery to supply the right arm, and the right common carotid to supply the right side of the head. The second branch is the left common carotid, which supplies the left side of the head. The third branch is the left subclavian, which supplies the left arm. ARTERIES OF THE HEAD, NECK, AND BRAIN.—The carotid arteries divide into internal and external branches, the external supplying the muscle and skin of the face and the internal supplying the brain and the eyes. ARTERIES OF THE UPPER EXTREM- ITIES.—The subclavian arteries are so named because they run underneath the clavicle. They supply the upper extremities, branching off to the back, chest, neck, and brain through the spinal column (fig. 1-35). The large artery going to the arm is called the axillary. The axillary artery becomes the brachial artery as it travels down the arm and divides into the ulnar and radial arteries. The radial artery is the artery at the wrist that you feel when you take the pulse of your patient (fig. 1-35). ARTERIES OF THE ABDOMEN.—In the abdomen, the aorta gives off branches to the abdominal viscera, including the stomach, liver, spleen, kidneys, 1-28