Systemic Arteries

The systemic arteries begin with the aorta, the largest artery, which measures about 2.5 cm (1 inch) in diameter. This vessel receives blood from the left ventricle then travels downward through the body, branching to all organs.

The Aorta and Its Parts

The aorta ascends toward the right from the left ventricle. Then it curves posteriorly and to the left. It continues downward posterior to the heart and just anterior to the vertebral column, through the diaphragm, and into the abdomen (Figs. 11-4 and 11-5). The aorta is one continuousartery, but it may be divided into sections:
* The ascending aorta is near the heart and inside thepericardial sac.
* The aortic arch curves from the right to the left andalso extends posteriorly.
* The thoracic aorta lies just anterior to the vertebral column posterior to the heart and in the space behind the pleura.
* The abdominal aorta is the longest section of the aorta, spanning the abdominal cavity.
The thoracic and abdominal aorta together make up the descending aorta.

Branches of the Ascending Aorta and Aortic Arch The first, or ascending, part of the aorta has two branches near the heart, called the left and right coronary arteries, which supply the heart muscle. These form a crown around the heart’s base and give off branches to all parts of the myocardium. The arch of the aorta, located immediately beyond the ascending aorta, ivides into three large branches.
* The brachiocephalic artery is a short vessel that upplies the arm and the head on the ight side. After extending upward omewhat less than 5 cm (2 inches), it divides into the right subclavian artery, which extends nder the right clavicle (collar one) and supplies the right upper xtremity arm), and the right common carotid artery, which supplies the right side of the neck, head and brain. Note that the brachiocephalic artery is unpaired.
* The left common carotid artery extends upward from the highest part of the aortic arch. It supplies the left side of the neck and the head.
* The left subclavian artery extends under the left clavicle and supplies the left upper extremity. This is the last branch of the aortic arch.

Branches of the Thoracic Aorta
The thoracic aorta supplies branches to the chest wall, esophagus, and bronchi (the subdivisions of the trachea), and their treelike subdivisions in the lungs. There are usually 9 to 10 pairs of intercostal arteries that extend between the ribs, sending branches to the muscles and other structures of the chest wall.

Figure 11-4 The aorta and its branches.

Figure 11-5 Principal systemic arteries.

Branches of the Abdominal Aorta As in the case of the thoracic aorta, there are unpaired branches extending anteriorly and paired arteries extending laterally. The unpaired vessels are large arteries that supply the abdominal viscera. The most important of these visceral branches are as follows:
* The celiac trunk is a short artery about 1.25 cm (1/2 inch) long that subdivides into three branches: the left gastric artery goes to the stomach, the splenic artery goes to the spleen, and the hepatic artery carries oxygenated blood to the liver.
* The superior mesenteric artery, the largest of these branches, carries blood to most of the small intestine and to the first half of the large intestine.
* The much smaller inferior mesenteric artery, located below the superior mesenteric artery and near the end of the abdominal aorta, supplies the second half of the large intestine.
The paired lateral branches of the abdominal aorta include the following right and left vessels:
* The phrenic arteries supply the diaphragm.
* The suprarenal arteries supply the adrenal (suprarenal) glands.
* The renal arteries, the largest in this group, carry blood to the kidneys.
* The ovarian arteries in women and testicular arteries in men (formerly called the spermatic arteries), supply the sex glands.
* Four pairs of lumbar arteries extend into the musculature of the abdominal wall.

The Iliac Arteries and Their Subdivisions

The abdominal aorta finally divides into two common iliac arteries. Both of these vessels, which are about 5 cm (2 inches) long, extend into the pelvis, where each one subdivides into an internal and an external iliac artery. The internal iliac vessels then send branches to the pelvic organs, including the urinary bladder, the rectum, and some reproductive organs. Each external iliac artery continues into the thigh as the femoral artery. This vessel gives rise to branches in the thigh and then becomes the popliteal artery, which subdivides below the knee. The subdivisions include the posterior and anterior tibial arteries and the dorsalis pedis, which supply the leg and the foot.

Arteries That Branch to the Arm and Head
Each common carotid artery travels along the trachea enclosed in a sheath with the internal jugular vein and the vagus nerve. Just anterior to the angle of the mandible (lower jaw) it branches into the external and internal carotid arteries. You can feel the pulse of the carotid artery just anterior to the large sternocleidomastoid muscle in the neck and below the jaw. The internal carotid artery travels into the head and branches to supply the eye, the anterior portion of the brain, and other structures in the cranium. The external carotid artery branches to the thyroid gland and to other structures in the head and upper part of the neck. The subclavian artery supplies blood to the arm and hand. Its first branch, however, is the vertebral artery, which passes though the transverse processes of the first six cervical vertebrae and supplies blood to the posterior portion of the brain. The subclavian artery changes names as it travels through the arm and branches to the arm and hand. It first becomes the axillary artery in the axilla (armpit). The longest part of this vessel, the brachial artery, is in the arm proper. The brachial artery subdivides into two branches near the elbow: the radial artery, which continues down the thumb side of the forearm and wrist, and the ulnar artery, which extends along the medial or little finger side into the hand. Just as the larger branches of a tree divide into limbs of varying sizes, so the arterial tree has a multitude of subdivisions. Hundreds of names might be included. We have mentioned only some of them.

Anastomoses
A communication between two vessels is called an anastomosis. By means of arterial anastomoses, blood reaches vital organs by more than one route. Some examples of such end-artery unions are as follows:
* The circle of Willis (Fig. 11-6) receives blood from the two internal carotid arteries and from the basilar artery, which is formed by the union of the two vertebral arteries. This arterial circle lies just under the center of the brain and sends branches to the cerebrum and other parts of the brain.
* The superficial palmar arch is formed by the union of the radial and ulnar arteries in the hand. It sends branches to the hand and the fingers.
* The mesenteric arches are made of communications between branches of the vessels that supply blood to the intestinal tract.
* Arterial arches are formed by the union of branches of the tibial arteries in the foot. There are similar anastomoses in other parts of the body.

Figure 11-6 Arteries that supply the brain. The bracket at right groups the arteries that make up the circle of Willis.

Arteriovenous anastomoses are blood shunts found in a few areas, including the external ears, the hands, and the feet. In this type of shunt, a small vessel known as a metarteriole or thoroughfare channel, connects the arterial system directly with the venous system, bypassing the capillaries (Fig. 11-7). This pathway provides a more rapid flow and a greater blood volume to these areas, thus protecting these exposed parts from freezing in cold weather.

Figure 11-7 Capillary network showing an arteriovenous shunt (anastomosis). A connecting vessel, known as a thoroughfare channel or metarteriole, carries blood directly from an arteriole to a venule, bypassing the capillaries.