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.

Key Term: Interstitial Space

Interstitial space is the structural, interstitial fluid-filled environment between the functional cells of body tissues located outside the blood and lymphatic vessels.

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.

Definition: Lymphatic System

The lymphatic system is the system of the body that transports excess fluid and some important substances from the tissues of the body into the vessels of the circulatory system. It also has an important role in the immune system as it transports white blood cells to fight infection.

Figure 1: A diagram depicting the lymphatic vessels in the capillary beds of the body. In this figure, the white arrows represent the movement of blood, while the black arrows represent the movement of lymph.

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.

Definition: Lymph

Lymph is the fluid transported by the lymphatic system, composed of water, leukocytes, nutrients, and other biological molecules.

Key Term: Leukocytes

Leukocytes, or white blood cells, are a part of the immune system that help the body fight against disease and foreign bodies.

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 90% water!

Key Term: Blood Plasma

Blood plasma is a yellow liquid that the blood cells are suspended in. Blood plasma also carries materials like nutrients, wastes, and minerals throughout the body.

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.

Figure 2: A diagram showing how lymph fluid from tissues are returned to the venous system through lymphatic vessels and ducts (shown in green).

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.

Definition: Lymphatic Vessels

Lymphatic vessels are a network of tubes that transport lymph from the tissues of the body to the circulatory system.

Example 2: The Composition and Function of Lymph

Which of the following best describes lymph?

  1. A fluid produced by lymph nodes that is used to regulate the body’s water content
  2. A fluid comprised of waste products that is transported by lymph vessels to the kidneys to be excreted
  3. A fluid derived from blood, comprised of red blood cells, proteins, and platelets, that is transported in lymph vessels
  4. A fluid derived from blood plasma, comprised of water, nutrients, and other biological molecules, that is transported in lymph vessels

Answer

In the body, the circulatory system functions to deliver oxygen and nutrients to the organs of the body and to remove carbon dioxide and waste products. When the blood vessels reach the tissues of the body, they release the fluid component of blood, called plasma. This fluid is largely composed of water and contains white blood cells, nutrients, and other biological molecules like proteins and fats. The cells take up the nutrients they need and release their waste products back into the fluid.

Most of this fluid is then taken up by blood vessels that carry blood back from the tissues toward the heart. However, some of this fluid is taken up by the lymphatic vessels. Once taken up by the lymphatic capillaries, this fluid is referred to as lymph. Lymph is largely composed of water, white blood cells, and other molecules.

The best description of lymph, therefore, is a fluid derived from blood plasma, comprised of water, nutrients, and other biological molecules, that is transported in lymph vessels.

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.

Figure 3: A diagram of the lymphatic system representing the various locations of lymph nodes in the body.

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.

Definition: Lymph Nodes

A lymph node is a small organ found at junctures throughout the lymphatic system. Lymph nodes house immune cells and filter the lymph fluid.

Key Term: Lymphocytes

Lymphocytes are immune cells that generate immune responses against pathogens in the body. There are two main types of lymphocytes: B cells and T cells.

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.

Example 3: The Function of Lymph Nodes in the Lymphatic System

Which of the following is a primary function of lymph nodes?

  1. Lymph nodes are responsible for initiating an immune response to an infection by a pathogen.
  2. Lymph nodes are responsible for the transport of lymph from tissue to the circulatory system.
  3. Lymph nodes initiate the conversion of waste material in the lymph into useful substances.
  4. Lymph nodes circulate the lymph back to the tissue it came from.

Answer

In the lymphatic system, fluid from the tissues of the body is collected by a network of vessels called lymphatic vessels and delivered to the circulatory system. This fluid is called lymph and is largely composed of water, white blood cells, and biological molecules. It may also contain disease-causing microbes, or pathogens, collected from the different tissues of the body.

As the lymphatic vessels transport the lymph from the tissues to the circulatory system, they pass the lymph through several small, bean-shaped structures called lymph nodes. Lymph nodes are usually about 1 cm in size and contain immune cells called lymphocytes. Here, any pathogens carried in the lymph are trapped, and the immune cells generate a response against them. The activated immune cells then move out of the lymph nodes, along with the lymph, to attack pathogens in the rest of the body.

A primary function of lymph nodes is, therefore, to initiate an immune response to an infection by a pathogen.

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.

Figure 4: A diagram showing the location of the spleen in the body, behind the stomach.

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.

Key Term: Spleen

The spleen is one of the most important lymphatic organs. It contains immune cells and is responsible for filtering pathogens out of the bloodstream.

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.

Figure 5: The lymphatic capillaries called lacteals in the villi of the small intestine.

Key Term: Villi

Villi (singular: villus) are fingerlike protrusions in the wall of the small intestine that increase its surface area for nutrient absorption.

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.

Definition: Lacteals

Lacteals are a special type of lymphatic capillaries found in the villi of the small intestine in the digestive system.

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.

Figure 6: This diagram shows how the circulatory system is a closed loop, while the lymphatic system is an open, linear system.

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.

Key Points

  • The lymphatic system collects fluid, called lymph, from the interstitial spaces of the body and carries it through lymphatic vessels to the circulatory system.
  • Lymph nodes are small structures containing immune cells, responsible for initiating an immune response against pathogens.
  • The spleen is one of the most important lymphatic organs in the body. It contains immune cells (lymphocytes) and filters pathogens out of the bloodstream.
  • There are special lymphatic capillaries in the digestive system called lacteals, which help in the absorption of fat molecules.
  • The lymphatic system is a unidirectional, open, and linear transportation system.

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