Lesson Video: The Lymphatic System Biology

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

13:32

Video Transcript

In this video, we will learn about the lymphatic system and the different roles it plays in the body. We will also learn about the different components of the lymphatic system and why they are important.

Our bodies have several different systems that carry out all of our life processes: our digestive system that breaks down our food, our respiratory system that provides us with energy, our circulatory system that supplies our organs with oxygen and nutrients, our nervous system that controls our responses to stimuli, and our immune system that keeps us healthy and free from infection. But all of these systems don’t work alone. They’re all interconnected and dependent on one another, working together to make sure that all of our life processes are running smoothly.

In this video, we are going to learn about the lymphatic system, which plays an important role in both the circulatory system and the immune system. Let’s start by looking at how the lymphatic system and circulatory system work together in the body.

As you may know, the circulatory system consists of the heart, which acts as a central pump, and a network of blood vessels. The heart pumps blood into vessels called arteries, which carry blood away from the heart and towards the other organs of the body. At the tissues of these organs, the arteries branch into smaller and smaller vessels. The tiniest of these vessels are called capillaries, and these branch out, surrounding the cells of the organs. This forms a capillary bed, as you can see here.

Let’s zoom in to one section of the capillary bed to see what happens here. In the capillary bed, some of the fluid from the capillaries leaks out, filling the spaces between the cells of the organs. These spaces are called interstitial spaces. The fluid that fills the interstitial spaces, which is now called the interstitial fluid, is rich in oxygen and nutrients that the cells of the body need. The cells take up these essential molecules from the interstitial fluid and release their waste products back into it. Most of the interstitial fluid is then reabsorbed into the capillaries. The capillaries then merge into larger vessels, eventually pouring into veins, which carry blood away from the organs and back toward the heart.

However, not all of the interstitial fluid is reabsorbed. A small volume of this fluid remains in the interstitial spaces of the body tissues. So what happens to this remaining fluid? Well, this is where the lymphatic system comes into play. Like the circulatory system, the lymphatic system has tiny lymphatic vessels called lymphatic capillaries. These lymphatic capillaries have a similar structure to veins. And they absorb the fluid from the interstitial spaces, eventually returning it to the circulatory system. Once the fluid has been taken up by the lymphatic capillaries, it is referred to as lymph.

The fluid that makes up interstitial fluid is originally supplied by small blood vessels called arterioles that branch into smaller blood vessels called capillaries. Some blood plasma flows out of the capillaries and into the interstitial spaces to make up interstitial fluid. The nutrients can then be absorbed from the interstitial fluid into the cells of the body tissues to be used as required. The cell’s waste products can then move into the interstitial fluid and then into the capillaries, into small blood vessels called venules, and eventually to the heart.

As lymph is derived from interstitial fluid, it’s also derived from blood plasma, which is 90 percent water. It also carries proteins, fats, salts, and white blood cells, which are also called leukocytes.

Let’s take a look at where the lymph goes once it’s been collected by the lymphatic capillaries. Once the extra fluids have been collected by the lymphatic capillaries, these tiny vessels merge into larger and larger lymphatic vessels. These lymphatic vessels have flap-like structures called valves, which make sure that the fluid only flows in one direction, away from the capillary beds. The lymphatic vessels pass the lymph through other lymphatic organs, such as lymph nodes, which we will discuss in more detail later on in the video.

Eventually, the larger lymphatic vessels lead into two main lymphatic ducts, which are known as collecting ducts. One of these two ducts, the thoracic duct, has been labeled on this diagram. These ducts empty the lymph into veins called the subclavian veins, which make up part of the venous system. Remember, veins generally carry blood away from body tissues and organs toward the heart. They’ve been labeled in this diagram in blue. The subclavian veins can return the blood and the lymph to the vena cava, which then drains into the heart.

There’s an important difference between the lymphatic system and the circulatory system that we can spot in this diagram. The circulatory system forms a closed loop, carrying blood from the heart to the organs via the arterial system and then back to the heart through the venous system. The lymphatic system, on the other hand, only carries lymph in one direction, from the interstitial spaces to the veins surrounding the heart. This is why we call the lymphatic system an open system.

Another interesting function of the lymphatic system is the absorption of fats into the bloodstream. This occurs across the surface of the small intestine. If we were to cut a cross section through a part of the small intestine, it might look something like this. The inner surface of the small intestine is covered with small fingerlike projections called villi. A singular villi is called a villus. Villi serve to increase the surface area of the small intestine for the absorption of nutrients from food that passes through it.

Let’s take a closer look at just one villus. Each villus has a network of blood vessels, as you can see here. These blood capillaries connect the villi to the circulatory system. Most of the nutrients from food that we digest are absorbed directly into the bloodstream, but fat molecules are too large to enter these capillaries. Because of this, each villus in the small intestine also has a single lymphatic capillary. These special lymphatic capillaries in the digestive system are called lacteals.

Lacteals are larger than blood capillaries, which is another key difference between the lymphatic system and the circulatory system. This allows the fat molecules to enter these vessels instead and make their way into the larger vessels of the lymphatic system. Through the network of lymphatic vessels, these fat molecules are eventually transported into the bloodstream.

We mentioned earlier that the lymphatic vessels pass the lymph through the lymphatic organs before returning it to the bloodstream. These lymphatic organs have important functions in the immune system. The first lymphatic organs that we’ll learn about are the lymph nodes. These are small bean-shaped structures about one centimeter in size. If you’ve ever had a cold or sore throat, you might have felt swollen lymph nodes just below your jaw on either side of your neck. The diagram here shows several lymph nodes around the neck, but we actually have hundreds of them in groups all over the body.

Let’s take a closer look at just one of these lymph nodes. When the lymphatic vessels collect fluid from the interstitial spaces, they might take up pathogens, which are biological agents that can cause disease. If pathogens are allowed to enter the bloodstream, they can reach the other organs of the body and cause harmful infections. The role of the lymph nodes is to act like a filter for these pathogens. Lymph nodes contain immune cells called lymphocytes, which are a type of white blood cell. If the lymph that’s delivered to the lymph nodes contains pathogens, they are trapped by the lymphocytes.

This is why the lymph nodes sometimes become swollen. The lymphocytes are activated and generate an immune response against these pathogens. This immune response attempts to destroy or inactivate the pathogens, hopefully preventing them from causing infections. The activated lymphocytes multiply to form large numbers and exit the lymph nodes along with the lymph. They can then circulate around the body, attacking any other similar pathogens.

The second lymphatic organ that we’ll learn about in this video is the spleen. It’s located in the upper-left part of the abdomen, just behind the stomach. Usually, it’s between seven and 14 centimeters long. An enlarged spleen can be a sign of certain infections or other health conditions. Like the lymph nodes, the spleen also behaves as a filter, destroying the pathogens that might be present in lymph. The spleen also filters out old or damaged red blood cells from the bloodstream. It also stores the iron that’s obtained from these damaged red blood cells. The iron is eventually returned to the bone marrow. Hemoglobin is produced in the bone marrow using this iron. Hemoglobin is a protein that contains iron and is responsible for carrying oxygen in red blood cells.

Let’s apply what we’ve learned about the lymphatic system to a practice question.

Which of the following is a primary function of lymph nodes? (A) Lymph nodes circulate the lymph back to the tissue it came from. (B) Lymph nodes are responsible for the transport of lymph from tissue to the circulatory system. (C) Lymph nodes are responsible for initiating an immune response to an infection by a pathogen. Or (D) lymph nodes initiate the conversion of waste material in the lymph into useful substances.

This question asks us about the primary function of lymph nodes. Let’s take a look at the lymphatic system and the role of lymph nodes to figure out the answer. The lymphatic system plays important roles in the circulatory system and the immune system. In the circulatory system, blood is pumped from the heart to the different tissues of the body through blood vessels. When the blood vessels reach the body tissues, they branch into finer vessels, which are called capillaries. The capillaries form a network around the cells of the tissues, forming a capillary bed.

Fluid containing oxygen and other nutrients is released from the capillaries and into the spaces between the cells. The spaces between the cells are called the interstitial spaces. The cells take up the oxygen and nutrients they need and release their waste products back into the interstitial fluid in these spaces. Most of the interstitial fluid remaining is then taken up by the capillaries and fed back into the bloodstream to be returned to the heart. However, a small volume of fluid remains in the interstitial spaces. The lymphatic system is responsible for collecting this remaining fluid. This occurs in lymphatic capillaries, which then drain back into the circulatory system. Once the fluid is taken up by the lymphatic vessels, it is called lymph.

Lymph contains a lot of water and carries proteins, fats, and other materials. It might also carry pathogens, for example, disease-causing microorganisms or viruses, from the tissues. As the lymph travels through the lymphatic vessels on its way back to the bloodstream, it passes through lymphatic organs called lymph nodes.

Lymph nodes are small bean-shaped structures found in groups all over the body. They contain immune cells called lymphocytes. If the lymph passing through the lymph nodes contains pathogens, the lymphocytes become activated. They generate an immune response to the pathogen, destroying or inactivating it. The lymphocytes then exit the lymph nodes along with the lymph to travel through the bloodstream and attack all other similar pathogens throughout the body.

If we take a look at the options in the question, we can see that option (C) correctly outlines the primary function of lymph nodes in the body. The correct answer is therefore option (C). The lymph nodes are responsible for initiating an immune response to an infection by a pathogen.

Let’s review the key points that we’ve learned about the lymphatic system in this video. The lymphatic system collects lymph from the interstitial spaces between the cells in body tissues. The lymphatic system then carries lymph through lymphatic vessels to empty it into the circulatory system. The lymphatic system consists of an open loop of vessels that are wider than blood vessels. Lymph nodes are small lymphatic organs that contain immune cells responsible for initiating an immune response against pathogens. The spleen is another lymphatic organ and possibly the most important in the body. Among its other roles, it contains immune cells called lymphocytes that filter pathogens out of the bloodstream. There are special lymphatic capillaries in the digestive system called lacteals, which help in the absorption of fat molecules.

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