The pulmonary system is composed of four vessels, two from each lung, which empty into the left atrium. These are the only veins in the body that carry freshly oxygenated blood.

The portal system consists of the veins that drain venous blood from the abdominal part of the digestive tract-the spleen, pancreas, and gallbladder, but not the lower rectum-and deliver it to the liver. There, it is distributed by a set of venous capillaries. The blood in the portal system conveys absorbed substances from the intestinal tract to the liver for storage,

Figure 1-36.-Principal vessels of the venous system.

alteration, or detoxification. From the liver the blood flows through the hepatic vein to the inferior vena cava.

The systemic system is divided into the deep and superficial veins. The superficial veins lie immediately under the skin, draining the skin and superficial structures. The deep veins, usually located in the muscle or deeper layers, drain the large muscle masses and various other organs. Deep veins commonly lie close to the large arteries that supply the various organs of the body and typically have the same name as the artery they accompany.

VEINS OF THE HEAD, NECK, AND BRAIN.-The superficial veins of the head unite to form the external jugular veins. The external jugular veins drain blood from the scalp, face, and neck, and finally empty into the subclavian veins.

The veins draining the brain and internal facial structures are the internal jugular veins. These combine with the subclavian veins to form the innominate veins, which empty into the superior vena cava (fig. 1-36).

VEINS OF THE UPPER EXTREM- ITIES.-The veins of the upper extremities begin at the hand and extend upward. Avein of great interest to you is the median cubital, which crosses the anterior surface of the elbow. It is the vein most commonly used for venipuncture. Also found in this area are the basilic and cephalic veins, which extend from the midarm to the shoulder.

The deep veins of the upper arm unite to form the axillary vein, which unites with the superficial veins to form the subclavian vein. This vein later unites with other veins to form the innominate and eventually, after union with still more veins, the superior vena cava (fig. 1-36).

VEINS OF THE ABDOMEN AND THO- RACIC REGION.-The veins from the abdominal organs, with the exception of those of the portal system, empty directly or indirectly into the inferior vena cava, while those of the thoracic region eventually empty into the superior vena cava (fig. 1-36).

VEINS OF THE LOWER EXTREM- TIES.-In the lower extremities (fig. 1-36), a similar system drains the superficial areas. The great saphenous vein originates on the inner aspect of the foot and extends up the inside of the leg and thigh to join the femoral vein in the upper thigh. The great saphenous vein is used for intravenous injections at the ankle.

The veins from the lower extremities unite to form the femoral vein in the thigh, which becomes the external iliac vein in the groin. Higher in this region, external iliac unites the internal iliac (hypogastric) vein from the lower pelvic region to form the common iliac veins. The right and left common iliac veins unite to form the inferior vena cava.