venous systems in the body: the pulmonary, portal, and systemic.
The PULMONARY SYSTEM comprises 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 (except the lower rectum), spleen, pancreas, and gallbladder 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, 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. They usually lie close to the large arteries that supply the various organs of the body (fig. 3-31) and usually have the same name as the artery they accompany. The superficial veins of the head unite to form the external jugular veins. They 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. 3-31).
The veins of the upper extremity begin at the hand and extend upward. An extremely valuable vein, the median cubital, crosses the anterior surface of the elbow. It is the vein most commonly used for intravenous injections and infusions.
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 later unites with other veins to form the innominate and eventually, after union with still more veins, the superior vena cava.
In the lower extremity (fig. 3-32), 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. This vein is sometimes used for intravenous injections at the ankle. The superficial venous system of the leg often becomes varicose, or excessively dilated, particularly in persons whose occupations require long periods of standing. When this develops, the venous valves become incompetent, allowing stagnation of blood in the dependent extremity. Under these circumstances varicose ulcers frequently develop. Ligation at several points along the system will force the venous return into the deep venous system and restore normal venous circulation.
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, it unites with the hypogastric vein from the lower pelvic region to form the common iliac vein. The two common iliac veins unite to form the inferior vena cava.
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.
All tissue cells of the body are continuously bathed in interstitial fluid. This fluid is formed by leakage of blood plasma through minute pores of the capillaries. There is a continual interchange of fluids of the blood and tissue spaces with a free interchange of nutrients and other dissolved substances. Most of the tissue fluid returns to the circulation by means of venous capillaries, which feed into larger veins. Large protein molecules that have escaped from the arterial capillaries cannot reenter the circulation through the small pores of the venous capillaries. However, these large molecules, as well as white blood cells, dead cells, bacterial debris, infected substances, and larger particulate matter, can pass through the larger pores of the lymphatic capillaries and thus enter the lymphatic circulation with the remainder of the tissue fluid.
Lymph usually is clear, but following ingestion of a fatty meal the lymph contained in the lymphatic that drain the small intestine appears milky because of the fat globules that have been absorbed. This milky lymph is called CHYLE.
Lymph vessels and lymph nodes form a network throughout the body. Capillaries, like veins,