Anatomy of Blood Vessels1

Blood and lymph flows through circulatory system and lymphatic system respectively. The circulatory system consists of the heart and blood vessels through which the blood circulates.

The heart pumps blood into vessels that vary in structure, size and function, and there are several types of blood vessels—arteries, arterioles, capillaries, venules and veins (Table 1.1).

Arteries and arterioles are the blood vessels that transport blood away from the heart. They vary considerably in size and their walls consist of three layers of tissue:

Anastomoses are arteries that form a link between main arteries supplying an area, e.g. the arterial supply to the palms of the hand and soles of the feet, the brain, joints and to a limited extent, the heart muscle. When there is a block in the arteries, these arteries ensures the circulation is bypassed through a collateral circulation. End-arteries are the arteries with no anastomoses or those beyond the most distal anastomoses, e.g. the branches from the circulus arteriosus (circle of Willis) in the brain or the central artery to the retina of the eye. When an end-artery is occluded the tissues it supplies die because there is no alternative blood supply.

The smallest arterioles break up into a number of minute vessels called capillaries. Blood cells and large-molecule substances such as plasma proteins do not normally pass through capillary walls. The capillaries form a vast network of tiny vessels, which link the smallest arterioles to the smallest venules (Fig. 1.3). The capillary bed is the site of exchange of substances between the blood and the tissue fluid, which bathes the body cells.

Venules connect the capillaries (arterial system) to the venous system. Venules are very small in diameter and oval-shaped at rest. When pressures increase, they become more circular. While blood vessels typically have three distinct layers, the end venules have only an endothelial layer with a thin layer of collagen fibers (tunica intima).4

As venules increase in diameter and become visible veins, they have the typical three layers mentioned earlier. Veins in the upper extremities and lower extremities are superficial and deep. The two superficial vein extremities are of upper extremities are cephalic and basilic vein. The deep veins are named corresponding to the accompanying arteries or arterial system at the similar level of the vascular system. The basilic vein originates from the dorsal venous network of the hand. It ascends the medial aspect of the upper limb. At the border of the teres major, the vein moves deep into the arm. Here, it combines with the brachial veins to form the axillary vein. The cephalic vein arises from the dorsal venous network of the hand. It ascends the anterolateral aspect of the upper limb, passing anteriorly at the elbow. At the shoulder, the cephalic vein5 travels in the deltopectoral groove and enters the axilla. Within the axilla, the cephalic vein terminates by joining the axillary vein. In cubital fossa, they are connected by median cubital vein, which is common site of intravenous (IV) injection. Another site for IV is dorsal venous arch of hand. Median cubital vein is commonly affected with superficial thrombophlebitis and dorsal venous arch is a common site for cellulitis.

The superficial veins deliver blood to the deep veins, such as the femoral and popliteal veins. However, the superficial system also connects to perforating veins.

In the interstitial space, majority of the tissue fluid drains to their venous end whereas the remainder diffuses through permeable walls of the lymph capillaries and becomes lymph. Lymph is a clear watery fluid, similar in composition to blood plasma, with the important exception of plasma proteins and identical in composition to interstitial fluid.

The lymphatic system consists of:

Lymph capillaries originate as blind-end tubes in the interstitial spaces. They have the same structure as blood capillaries, i.e. a single layer of endothelial cells, but their walls are more permeable to all interstitial fluid constituents, including proteins and cell debris. The tiny capillaries join up to form larger lymph vessels.

The walls of lymph vessels are about the same thickness as those of small veins and have the same layers of tissue, i.e. a fibrous covering, a middle layer of smooth muscle and elastic tissue and an inner lining of endothelium. Lymph vessels have numerous cup-shaped valves, which ensure that lymph flows in one way only, i.e. towards the thorax. There is no ‘pump’, such as the heart, involved in the onward movement of lymph, but the muscle tissue in the walls of the large lymph vessels has an intrinsic ability to contract rhythmically (the lymphatic pump). In addition, any structure that periodically compresses the lymphatic vessels can assist in the movement of lymph along the vessels, commonly including the contraction of adjacent muscles and the pulsation of large arteries. Lymph vessels are divided into superficial and deep lymphatic vessels. Lymph vessels become larger as they join together, eventually forming two large ducts, the thoracic duct and right lymphatic duct, that empty lymph into the subclavian veins.

Thoracic duct begins at the cisterna chyli, which is a dilated lymph channel situated in front of the bodies of the first two lumbar vertebrae. The duct is about 40 cm long and opens into the left subclavian vein in the root of the neck. It drains lymph from legs, the pelvic and abdominal cavities, the left half of the thorax, head and neck, and the left arm.

Right lymphatic duct is a dilated lymph vessel about 1 cm long. It lies in the root of the neck and opens into the right subclavian vein. It drains lymph from the right half of the thorax, head and neck and the right arm.

Lymph nodes are oval or bean-shaped organs that lie, often in groups, along the length of lymph vessels. The lymph drains through a number of nodes, usually 8–10 nodes, before returning to the venous circulation. Function of lymph nodes includes filtering, phagocytosis and proliferation of lymphocytes.