Whereas most arteries are located in protected and rather deep areas of the body, many of the principal systemic veins are found near the surface (Fig. 11-8). The most important of the superficial veins are in the extremities, and include the following:
* The veins on the back of the hand and at the front of the elbow. Those at the elbow are often used for drawing blood for test purposes, as well as for intravenous injections. The largest of this group of veins are the cephalic, the basilic, and the median cubital veins.
* The saphenous veins of the lower extremities, which are the body’s longest veins. The great saphenous vein begins in the foot and extends up the medial side of the leg, the knee, and the thigh. It finally empties into the femoral vein near the groin.
The deep veins tend to parallel arteries and usually have the same names as the corresponding arteries. Examples of these include the femoral and the external and internal iliac vessels of the lower part of the body, and the brachial, axillary, and subclavian vessels of the upper extremities. Exceptions are found in the veins of the head and the neck. The two jugular veins on each side of the neck drain the areas supplied by the carotid arteries (jugular is from a Latin word meaning “neck”). The larger of the two veins, the internal jugular, receives blood from the large veins (cranial venous sinuses) that drain the head and also from regions of the face and neck. The smaller external jugular drains the areas supplied by the external carotid artery. Both veins empty directly into the subclavian vein on the left and the right. A brachiocephalic vein is formed on each side by the union of the subclavian and the jugular veins (see Fig. 11-8). (Remember, there is only one brachiocephalic artery.)
Figure 11-8 Principal systemic veins.
The Venae Cavae and Their Tributaries
Two large veins receive blood from the systemic vessels and empty directly into the heart’s right atrium. The veins of the head, neck, upper extremities, and chest all drain into the superior vena cava. This vessel is formed by the union of the right and left brachiocephalic veins, which drain the head, neck, and upper extremities. The unpaired azygos vein drains the veins of the chest wall and empties into the superior vena cava just before the latter empties into the heart (see Fig. 11-8) (azygous is from a Greek word meaning “unpaired”).
The inferior vena cava, which is much longer than the superior vena cava, returns the blood from the parts of the body below the diaphragm. It begins in the lower abdomen with the union of the two common iliac veins. It then ascends along the posterior wall of the abdomen, through a groove in the posterior part of the liver, through the diaphragm, and finally through the lower thorax to empty into the right atrium of the heart. Drainage into the inferior vena cava is more complicated than drainage into the superior vena cava. The large veins below the diaphragm may be divided into two groups:
* The right and left veins that drain paired parts and organs. They include the iliac veins from near the groin, four pairs of lumbar veins from the dorsal part of the trunk and from the spinal cord, the testicular veins from the male testes and the ovarian veins from the female ovaries, the renal and suprarenal veins from the kidneys and adrenal glands near the kidneys, and finally the large hepatic veins from the liver. For the most part, these vessels empty directly into the inferior vena cava. The left testicular in the male and the left ovarian in the female empty into the left renal vein, which then takes this blood to the inferior vena cava; these veins thus constitute exceptions to the rule that the paired veins empty directly into the vena cava.
* Unpaired veins that come from the spleen and parts of the digestive tract (stomach and intestine) empty into a vein called the hepatic portal vein. Unlike other lower veins, which empty into the inferior vena cava, the hepatic portal vein is part of a special system that enables blood to circulate through the liver before returning to the heart.
The word sinus means “space” or “hollow.” A venous sinus is a large channel that drains deoxygenated blood, but does not have the usual tubular structure of the veins. One example of a venous sinus is the coronary sinus, which receives most of the blood from the heart wall see Fig. 10-8. It lies between the left atrium and left ventricle on the posterior surface of the heart, and empties directly into the right atrium, along with the two venae cavae. Other important venous sinuses are the cranial venous sinuses, which are located inside the skull and drain the veins from all over the brain (Fig. 11-9). The largest of the cranial venous sinuses are the following:
* The two cavernous sinuses, situated behind the eyeballs, drain the eyes’ ophthalmic veins. They give rise to the petrosal sinuses, which drain into the jugular veins.
* The superior sagittal sinus is a single long space located in the midline above the brain and in the fissure between the cerebrum’s two hemispheres. It ends in an enlargement called the confluence of sinuses.
* The two transverse sinuses, also called the lateral sinuses, are large spaces between the layers of the dura
mater (the outermost membrane around the brain). They begin posteriorly from the confluence of sinuses and then extend laterally. As each sinus extends around the skull’s interior, it receives additional blood, including blood draining through the inferior sagittal sinus and straight sinus. Nearly all of the blood leaving the brain eventually empties into one of the transverse sinuses. Each sinus extends anteriorly to empty into an internal jugular vein, which then passes through a hole in the skull to continue downward in the neck.
Figure 11-9 Cranial venous sinuses. The inset shows the paired transverse sinuses, which carry blood from the brain to the jugular veins.
Figure 10-8 Blood vessels that supply the myocardium. Coronary arteries and cardiac veins are shown. (A) Anterior view. (B) Posterior view.
The Hepatic Portal System
Almost always, when blood leaves a capillary bed, it flows directly back to the heart. In a portal system, however, blood circulates through a second capillary bed, usually in a second organ, before it returns to the heart. A portal system is a kind of detour in the pathway of venous return that transports materials directly from one organ to another. A much larger portal system is the hepatic portal system, which carries blood from the abdominal organs to the liver (Fig. 11-10). The hepatic portal system includes the veins that drain blood from capillaries in the spleen, stomach, pancreas, and intestine. Instead of emptying their blood directly into the inferior vena cava, they deliver it through the hepatic portal vein to the liver. The portal vein’s largest tributary is the superior mesenteric vein, which drains blood from the proximal portion of the intestine. It is joined by the splenic vein just under the liver. Other tributaries of the portal circulation are the gastric, pancreatic, and inferior mesenteric veins. As it enters the liver, the portal vein divides and subdivides into ever smaller branches. Eventually, the portal blood flows into a vast network of sinuslike vessels called sinusoids. These enlarged capillary channels allow liver cells close contact with the blood coming from the abdominal organs. (Similar blood channels are found in the spleen and endocrine glands, including the thyroid and adrenals.) After leaving the sinusoids, blood is finally collected by the hepatic veins, which empty into the inferior vena cava. The purpose of the hepatic portal system is to transport blood from the digestive organs and the spleen to the liver sinusoids, so that the liver cells can carry out their functions. For example, when food is digested, most of the end products are absorbed from the small intestine into the bloodstream and transported to the liver by the portal system. In the liver, these nutrients are processed, stored, and released as needed into the general circulation.
Figure 11-10 Hepatic portal system. Veins from the abdominal organs carry blood to the hepatic portal vein leading to the liver. Arrows show the direction of blood flow.