The lymphatic system, constituted of the lymph vessels and nodes and the lymph, serves several functions.

A partial, parallel venous system

In the circulatory system, the heart pumps blood throughout the body via arteries, then smaller arterioles and finally through tiny capillaries, from which molecules are exchanged with cells and the rest of the body. Due to the pressure of the blood, much plasma is also released. The endothelial walls of the capillaries are just one cell thick and the spaces between them allow fluid to pass, but not the larger erythrocytes (RBCs), which therefore remain in the blood.

Outside the capillaries, the escaped fluid and molecules constitute the extracellular or interstitial fluid. It is composed of WBCs, including lymphocytes, some hormones, glucose and other molecules, such as proteins and lipids. Much of it is reabsorbed, but some remains outside. One of the jobs of the lymphatic system is to carry it back into the blood stream.

The lymphatic system therefore can be considered a parallel path to the venous system, but it carries no RBCs, only WBCs, called leukocytes. The lymph, as the fluid is called when it is inside the lymphatic system, is returned to the veins at two ducts placed strategically where blood pressure is relatively very low and the liquid can therefore be inserted into the vein easily.

Lymph is not pumped by the heart. Smooth muscles in the walls of the lymph vessels, as well as skeletal muscle, squeeze the lymphatic vessels, pushing the lymph through one-way valves which prevent its returning to the capillaries. Similar valves allow the interstitial fluid to enter the lymph vessels but not to leave them. Lymph flows from lymphatic capillaries, or terminal lymphatics, and into bigger lymph vessels before re-entering blood veins in either the right lymphatic duct (which drains the upper right-hand side of the body) or the thoracic duct (which drains all the rest).

Immune system function

The lymphatic system is much more than a simple alternate return route for WBCs. Since it carries lymphocytes, it is crucial for the immune system. Scattered about the body along the lymphatic vessels are some 500-600 special tissues (too small to be called organs) called lymph nodes, in which immune-system cells do much of their work.

There are macrophages present in all tissue, but most of the immune-system cells spend their off-duty hours in the lymph nodes. The complicated process of B-cell and T-cell growth and activation (to be discussed in the next section) is carried out more rapidly in an environment where macrophages or dendritic cells and B and T cells proliferate and this is in the lymph nodes. The large number of such cells, as well as macrophages and others, means that the lymph passing through the node has most if not all its pathogens filtered out and phagocytozed.

Lymphatic vessels, from Openstax College

The spleen is not connected to the lymphatic vessels, but It contains a large number of macrophages and dendritic cells which carry out a similar filtering process not on lymph, but on the blood.

Other secondary lymphoid tissues are:

• lymphoid nodules, such as the tonsils, in the respiratory and digestive tracts, which contain dense clusters of lymphocytes;
• mucosa-associated lymphoid tissue (MALT);
• bronchus-associated lymphoid tissue (BALT).

Function with other molecules

Glucose in the small intestine diffuses out of the intestine, into capillaries and thence throughout the the body through the circulatory system. But lipids leave the small intestine in the form of chylomicrons, which are too large to enter capillaries. However, they can enter the lymphatic system lacteals, lymphatic capillaries in the intestinal villi. The lymphatic system then conveys the lipid-bearing chylomicrons into the blood.

In a similar way, other molecules, such as some hormones, or waste products, being sent to the liver or kidneys, and which are produced not in the blood but elsewhere in the body, may not be able to enter blood capillaries, and so flow into the circulatory system via the lymphatic system.

Now we go on to the immune system.