Lesson Explainer: The Lymphatic System Biology

In this explainer, we will learn how to describe the structure and function of the lymphatic system in humans.

The lymphatic system is an organ system that plays important roles in both the circulatory system and the immune system. It consists of a network of lymphatic vessels, which are responsible for taking up the fluid retained in the spaces between the cells of the body, as well as specialized structures called lymph nodes and lymphatic organs like the spleen.

Capillaries are the smallest type of blood vessels. During the circulation of blood, when the capillary blood vessels reach the tissues of the body, part of the fluid leaks out of the capillaries. This fluid, which is called interstitial fluid, moves into spaces in the tissue between the capillaries and cells, which are called interstitial spaces. Interstitial fluid bathes the cells in a medium rich in oxygen and nutrients, supporting the matrix of cells within the interstitial space. The cells take up the oxygen and nutrients they need and release their waste products back into the interstitial fluid. The interstitial fluid will also contain cellular debris and toxic materials.

While most of this fluid is reabsorbed by the blood capillaries, a small amount of it remains in the tissues. This is where the lymphatic system comes into play. The lymphatic system transports fluid from the tissues of the body into the vessels of the circulatory system. As represented in Figure 1, lymphatic capillaries absorb this fluid, which is called lymph after it has entered the lymphatic capillaries.

Example 1: Interactions between the Lymphatic and the Circulatory Systems

The diagram provided shows a simplified drawing of the interactions of a lymphatic capillary with the surrounding tissue.

1. In the diagram, what is represented by the black arrows?
   1. The movement of lymph
   2. The movement of blood
   3. The movement of urea
   4. The diffusion of ions
2. In the diagram, what is represented by the white arrows?
   1. The diffusion of ions
   2. The movement of blood
   3. The movement of lymph
   4. The movement of urea

Answer

The tissues of the human body are supplied with oxygen and nutrients through the blood vessels of the circulatory system. The capillaries form an intricate network in the tissues where they release nutrient-rich fluid into the spaces between the cells, which is called the interstitial space.

Part 1

The cells release some waste products, cellular debris, and toxic materials into interstitial fluid in this interstitial space. Most of this fluid is taken up by blood vessels that transport blood away from the tissues and toward the heart.

Some fluid, however, is left in the tissues of the body. This is where the lymphatic system comes into play. The fluid, composed largely of water, white blood cells, and biological molecules, is taken up by lymphatic capillaries in the tissues. Once the interstitial fluid has been taken up by lymphatic capillaries, it is referred to as lymph.

Let’s examine the figure. We can see that the black arrows indicate fluid being collected from the tissue by lymphatic capillaries.

The black arrows, therefore, represent the movement of lymph.

Part 2

The tissues of the human body are supplied with oxygen and nutrients through the blood vessels of the circulatory system. The capillaries form an intricate network in the tissues where they release nutrient-rich fluid into the interstitial space.

Let’s examine the figure. We can see that the white arrows indicate fluid being delivered to the tissues by blood capillaries.

The white arrows, therefore, represent the movement of blood.

Let’s take a look at the components of the lymphatic system and the functions they perform.

As we have learned, the interstitial fluid comes from the blood capillaries of the circulatory system. The interstitial fluid contains nutrients, waste products, and other biological molecules.

Once the interstitial fluid is taken up by the lymphatic capillaries, it is referred to as lymph. Lymph is largely composed of water, proteins, lipids, salts, and white blood cells, which are also called leukocytes. Most of these white blood cells are produced in bone marrow and transported into lymph, but lymph contains no red blood cells. It can however also contain nutrients and other biological molecules, such as proteins and fats.

Lymph fluid is derived from a substance called plasma. Plasma is the fluid medium of the blood, which carries the blood’s components. It consists of water!

Lymph is taken up by lymphatic vessels—a network of tubes similar to the blood vessels, but with certain important differences, as we will see. The lymphatic vessels begin as thin tubes called lymphatic capillaries, which merge into larger and larger vessels. The vessels lead to two lymphatic ducts, through which lymph fluid is emptied into the venous system, which is the part of the circulatory system consisting of veins that return blood to the heart. You can see this process occurring in Figure 2 below.

These two lymphatic ducts are the right lymphatic duct and the thoracic duct, both of which connect to veins called the subclavian veins. The subclavian veins lead to the lymph eventually being returned to the vena cava, one of the major blood vessels that drains blood from the tissues of the body to the heart. In this way, the fluid retained in the interstitial spaces is returned to the circulatory system.

As we mentioned previously, the lymphatic system is an important part of both the circulatory system and the immune system. Now that we have understood how the lymphatic system plays a unique role in the circulatory system, what function does it serve in the immune system?

On the route of carrying lymph back to the circulatory system, lymphatic vessels transport the lymph through structures called lymph nodes, which are present in groups at different locations in the body, as shown in Figure 3.

Lymph nodes are small, bean-shaped structures, usually about 1 cm in size. They contain immune cells like lymphocytes (white blood cells), which may be of two types: B lymphocytes and T lymphocytes. These lymphocytes are crucial to the body’s immune response. For example, B lymphocytes produce antibodies that are helpful in dealing with potentially dangerous microorganisms and toxins.

Aside from fluid, white blood cells, and nutrients, lymphatic capillaries may also take up disease-causing microbes, or pathogens, from the tissues of the body. When this pathogen-containing lymph is passed through the lymph nodes, the lymphocytes in these nodes trap the pathogens and become activated, generating an immune response against them. The lymph nodes, therefore, are responsible for initiating immune responses against pathogens in the body.

The activated lymphocytes exit the lymph nodes along with the lymph via lymphatic vessels. They enter the circulatory system to attack and destroy the pathogens in the rest of the body. The lymph nodes at various locations within the body therefore behave as filters, to attack pathogens before the lymph is returned to the circulatory system.

Aside from the lymph nodes, the lymphatic system also involves an organ called the spleen, which is considered to be one of the most important lymphatic organs in the body. Figure 4 shows a diagram of the spleen and its location in the body.

The spleen is located in the upper-left part of the abdomen, behind the stomach, and is usually between 7 cm and 14 cm long. In both structure and function, it is similar to a lymph node. Just like lymph nodes, the spleen also contains an array of different immune cells and is capable of producing lymphocytes. The spleen is responsible for filtering blood in a similar manner to how the lymph nodes filter lymph fluid. Blood is delivered to the spleen where lymphocytes can generate a response against any pathogens carried by the blood and destroy them.

The lymphatic system also plays an interesting function in the absorption of fats from the digestive system.

The inner surface of the small intestine is covered with small, fingerlike projections called villi, as shown in Figure 5. These villi increase the surface area of the small intestine, for the better absorption of nutrients from the food we digest.

Most nutrients absorbed in the digestive system directly enter the bloodstream through the network of blood capillaries supplying each villus, as shown in Figure 5. However, fat molecules are too large to enter these blood capillaries.

Each villus is also supplied with a single lymphatic capillary called a lacteal, as represented in Figure 5. Since lacteals are wider than blood capillaries, fat molecules enter these vessels instead and are transported in the lymph.

Now that we have understood the structures and functions of the lymphatic system, let’s turn our attention to the differences between the lymphatic and circulatory systems.

Blood carried in the vessels of the human circulatory system moves in a closed circular loop—from the heart to the organs of the body and back to the heart. In contrast, the lymphatic vessels carry lymph in just one direction—from the interstitial spaces of the body to the veins supplying the circulatory system. The lymphatic system is therefore an open, linear, and unidirectional system of vessels, rather than a closed loop, as you can see in the diagram in Figure 6 below.

Another important difference between the lymphatic vessels and the blood vessels is their size. As we learnt in the digestive system, blood capillaries are extremely narrow and cannot transport larger substances like fat molecules. Lymphatic capillaries, on the other hand, are wider, which enables them to transport a variety of substances.

Let’s summarize everything we have learned so far about the lymphatic system.