Lymph travels through a network of small and large channels that are in some ways similar to the blood vessels. However, the system is not a complete circuit. It is a oneway system that begins in the tissues and ends when the lymph joins the blood (see Fig. 12-1).
The walls of the lymphatic capillaries resemble those of the blood capillaries in that they are made of one layer of flattened (squamous) epithelial cells. This thin layer, also called endothelium, allows for easy passage of soluble materials and water (Fig. 12-3). The gaps between the endothelial cells in the lymphatic capillaries are larger than those of the blood capillaries. The lymphatic capillaries are thus more permeable, allowing for easier entrance of relatively large protein particles. The proteins do not move back out of the vessels because the endothelial cells overlap slightly, forming one-way valves to block their return. Unlike the blood capillaries, the lymphatic capillaries arise blindly; that is, they are closed at one end and do not form a bridge between two larger vessels. Instead, one end simply lies within a lake of tissue fluid, and the other communicates with a larger lymphatic vessel that transports the lymph toward the heart (see Figs. 12-1 and 12-2).
Some specialized lymphatic capillaries located in the lining of the small intestine absorb digested fats. Fats taken into these lacteals are transported in the lymphatic vessels until the lymph is added to the blood.
Figure 12-2 Pathway of lymphatic drainage in the tissues. Lymphatic capillaries are more permeable than blood capillaries and can pick up fluid and proteins left in the tissues as blood leaves the capillary bed to travel back toward the heart.
Figure 12-3 Structure of a lymphatic capillary. Fluid and proteins can enter the capillary with ease through gaps between the endothelial cells. Overlapping cells act as valves to prevent the material from leaving.
The lymphatic vessels are thin walled and delicate and have a beaded appearance because of indentations where valves are located (see Fig. 12-1). These valves prevent back flow in the same way as do those found in some veins. Lymphatic vessels (Fig. 12-4) include superficial and deep sets. The surface lymphatics are immediately below the skin, often lying near the superficial veins. The deep vessels are usually larger and accompany the deep veins. Lymphatic vessels are named according to location. For example, those in the breast are called mammary lymphatic vessels, those in the thigh are called femoral lymphatic vessels, and those in the leg are called tibial lymphatic vessels. At certain points, the vessels drain through lymph nodes, small masses of lymphatic tissue that filter the lymph. The nodes are in groups that serve a particular region. For example, nearly all the lymph from the upper extremity and the breast passes through the axillary lymph nodes, whereas lymph from the lower extremity passes through the inguinal nodes. Lymphatic vessels carrying lymph away from the regional nodes eventually drain into one of two terminal vessels, the right lymphatic duct or the thoracic duct, both of which empty into the bloodstream.
Figure 12-1 The lymphatic system in relation to the cardiovascular system. Lymphatic vessels pick up fluid in the tissues and return it to the blood in vessels near the heart.
Figure 12-4 Vessels and nodes of the lymphatic system. (A) Lymph nodes and vessels of the head. (B) Drainage of right lymphatic duct and thoracic duct into subclavian veins.
The Right Lymphatic Duct The right lymphatic duct is a short vessel, approximately 1.25 cm (1/2 inch) long, that receives only the lymph that comes from the superior right quadrant of the body: the right side of the head, neck, and thorax, as well as the right upper extremity. It empties into the right subclavian vein near the heart (see Fig. 12-4 B). Its opening into this vein is guarded by two pocket-like semilunar valves to prevent blood from entering the duct. The rest of the body is drained by the thoracic duct.
The Thoracic Duct The thoracic duct, or left lymphatic
duct, is the larger of the two terminal vessels, measuring approximately 40 cm (16 inches) in length. As shown in Figure 12-4, the thoracic duct receives lymph from all parts of the body except those superior to the diaphragm on the right side. This duct begins in the posterior part of the abdominal cavity, inferior to the attachment of the diaphragm. The first part of the duct is enlarged to form a cistern, or temporary storage pouch, called the cisterna chyli. Chyle is the milky fluid that drains from the intestinal lacteals, and is formed by the combination of fat globules and lymph. Chyle passes through the intestinal lymphatic vessels and the lymph nodes of the mesentery (membrane around the intestines), finally entering the cisterna chyli. In addition to chyle, all the lymph from below the diaphragm empties into the cisterna chyli, passing through the various clusters of lymph nodes. The thoracic duct then carries this lymph into the bloodstream. The thoracic duct extends upward through the diaphragm and along the posterior wall of the thorax into the base of the neck on the left side.
Here, it receives the left jugular lymphatic vessels from the head and neck, the left subclavian vessels from the left upper extremity, and other lymphatic vessels from the thorax and its parts. In addition to the valves along the duct, there are two valves at its opening into the left subclavian vein to prevent the passage of blood into the duct.
Movement of Lymph
The segments of lymphatic vessels located between the valves contract rhythmically, propelling the lymph along. The contraction rate is related to the volume of fluid in the vessel the more fluid, the more rapid the contractions. Lymph is also moved by the same mechanisms that promote venous return of blood to the heart. As skeletal muscles contract during movement, they compress the lymphatic vessels and drive lymph forward. Changes in pressures within the abdominal and thoracic cavities caused by breathing aid the movement of lymph during passage through these body cavities.