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THE LYMPHATIC SYSTEM


Learning Objective:

Identify the parts of the lymphatic system and their function.


All tissues 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 circulatory system by means of 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 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 circulatory system with the remainder of the tissue fluid.

The lymphatic system helps defend the tissues against infections by supporting the activities of the lymphocytes, which give immunity, or resistance, to the effects of specific disease-causing agents.

PATHWAYS OF THE LYMPHATIC SYSTEM

The lymphatic pathway begins with lymphatic capillaries. These small tubes merge to form lymphatic vessels, and the lymphatic vessels in turn lead to larger vessels that join with the veins in the thorax.

Lymphatic Capillaries

Lymphatic capillaries are closed-ended tubes of microscopic size (Fig. 1). They extend into interstitial spaces, forming complex networks that parallel blood capillary networks. The lymphatic capillary wall consists of a single layer of squamous epithelial cells. This thin wall makes it possible for interstitial fluid to enter the lymphatic capillary. Once the interstitial fluid enters the lymphatic capillaries, the fluid is called lymph.

Figure 1.— Circulation plan of lymphatic fluid. This diagram outlines the general scheme for lymphatic circulation. Fluids from the systemic and pulmonary capillaries leave the bloodstream and enter the interstitial space, thus becoming part of the IF (interstitial fluid). The IF also exchanges materials with the surrounding tissues. Often, because less fluid is returned to the blood capillary than had left it, IF pressure increases—causing IF to flow into the lymphatic capillary. The fluid is then called lymph (lymphatic fluid) and is carried through one or more lymph nodes and finally to large lymphatic ducts. The lymph enters a subclavian vein, where it is returned to the systemic blood plasma. Thus fluid circulates through blood vessels, tissues, and lymphatic vessels in a sort of "open circulation."

Image reprinted with permission from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

Lymphatic Vessels

Lymphatic vessels are formed from the merging of lymphatic capillaries. Lymphatic vessels, also known simply as lymphatics, are similar to veins in structure. The vessel walls are composed of three layers: an inner layer of endothelial tissue, a middle layer of smooth muscle and elastic fibers, and an outer layer of connective tissue.

Like a vein, the lymphatic vessel has valves to prevent backflow of lymph. The larger lymphatic vessels lead to specialized organs called lymph nodes. After leaving these structures, the vessels merge to form still larger lymphatic trunks (Fig. 2).

Figure 2.— Structure of a typical lymphatic capillary. Notice that interstitial fluid enters through clefts between overlapping endothelial cells that form the wall of the vessel. Valves ensure one-way flow of lymph out of the tissue. Small fibers anchor the wall of the lymphatic capillary to the surrounding ECM (extracellular matrix) and cells, thus holding it open to allow entry of fluids and small particles.

Image reprinted with permission from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

Lymphatic Trunks and Ducts

Lymphatic trunks drain lymph from large regions in the body. The lymphatic trunks are usually named after the region they serve, such as the subclavian trunk that drains the arm. There are many lymphatic trunks throughout the body. These lymphatic trunks then join one of two collecting ducts, the thoracic duct or the right lymphatic duct (Fig. 3).

Lymphatic trunks from the upper half of the right side of the body converge to form the right lymphatic duct, which empties into the right subclavian vein. Drainage from the remainder of the body is by way of the thoracic duct, which empties into the left subclavian vein.

Figure 3.—Lymphatic drainage. The right lymphatic duct drains lymph from the upper right quadrant (dark blue) of the body into the right subclavian vein. The thoracic duct drains lymph from the rest of the body (green) into the left subclavian vein. The lymphatic fluid is thus returned to the systemic blood just before entering the heart.

Image reprinted with permission from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.
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LYMPH NODES

Lymph nodes, which are frequently called glands but are not true glands, are small beanshaped bodies of lymphatic tissue found in groups of two to fifteen along the course of the lymph vessels (Fig. 4). Major locations of lymph nodes are in the following regions: cervical, axillary, inguinal, pelvic cavity,
abdominal cavity, and thoracic cavity. Lymph nodes vary in size and act as filters to remove bacteria and particles from the lymph stream. Lymph nodes produce lymphocytes, which help defend the body against harmful foreign particles, such as bacteria, cells, and viruses. Lymph nodes also contain macrophages, which engulf and destroy foreign substances, damaged cells, and cellular debris.

Figure 4.—Principal organs of the lymphatic system.

Image reprinted with permission from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.


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David L. Heiserman, Editor

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Revised: June 06, 2015