2.20 Lymphatic System of Man

The lymphatic system is a vital component of the circulatory and immune systems, consisting of lymph, lymphocytes, lymphatic vessels, lymph nodes, tonsils, spleen, and thymus gland. It plays a crucial role in maintaining fluid balance, which is closely monitored by Baroreceptors and Volume Receptors→.

Lymphatic Vessels and Fluid Transport

The lymphatic pathways begin as tiny capillaries, forming larger vessels that eventually lead to collecting ducts, which unite with veins in the thorax. Unlike the closed circulatory system, the lymphatic system is a one-way system.

a. Lymphatic Capillaries

Lymphatic capillaries are microscopic, closed-ended tubes that extend into interstitial spaces, forming complex networks parallel to blood capillaries. Their walls consist of a single layer of squamous epithelial cells. This thin wall allows interstitial fluid (fluid surrounding tissue cells) to enter the capillary. Once inside, this fluid is called lymph.

A specialized lymph capillary found within each intestinal villus is called a lacteal, responsible for absorbing fats and other substances from the digestive tract. The milky fluid found in these lacteals after a fatty meal is known as chyle.

b. Lymphatic Vessels

Lymphatic vessels are formed by the merging of lymphatic capillaries. Their walls are similar to those of veins. Importantly, they possess flap-like valves that prevent the backflow of lymph, ensuring one-way flow. This is particularly important because the Rate of Blood Flow→ in the venous system is low, and lymph moves even more slowly. Lymphatic vessels typically lead to specialized organs called lymph nodes. After leaving the nodes, they merge to form larger lymphatic trunks.

c. Lymphatic Trunks and Collecting Ducts

Lymphatic trunks drain lymph from relatively large regions of the body and are named according to the region they serve. The lymphatic trunks ultimately join one of two major collecting ducts:

• Thoracic Duct: The larger and longer of the two. It receives lymph from the lower body regions, the entire left side of the body (including the left side of the head and neck, and left arm), and empties into the left subclavian vein (near the left jugular vein junction).
• Right Lymphatic Duct: Receives lymph from the right side of the head and neck, the right arm, and the right thorax. It empties into the right subclavian vein (near the right jugular vein junction).

Lymph Nodes and Spleen: Immune and Filtering Organs

a. Lymph Nodes

Lymph nodes are structures situated along the lymphatic pathways. They contain a large number of lymphocytes and macrophages. The functions of lymph nodes include:

• Acting as filters against foreign particles such as bacteria and viruses.
• Macrophages within the nodes engulf and destroy foreign substances, damaged cells, and cellular debris, thus purifying the lymph.

b. Spleen

The spleen contains white blood cells including lymphocytes and macrophages. Its functions are:

• Immune Defense: Helps eliminate germs as lymphocytes and macrophages attack and destroy them in the blood.
• Blood Filtration: Filters blood by removing cellular waste and getting rid of old or damaged blood cells. It is often called the "graveyard of RBCs."
• Storage: Stores blood and iron for future use.
• Production: Produces white blood cells and antibodies to fight infection.
• Fluid Balance: Maintains the levels of fluid in the body.