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Introduction to the lymphatic system

Overview of the anatomy, function and main structures of the lymphatic system.

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Hello everyone! It's Megan from Kenhub here, and in today's tutorial, we're going to take a look at the lymphatic system.

Here is an overview of part of the lymphatic system within the trunk with all of these vessels and nodules you can see in yellow. It looks pretty complicated and kind of random but don't worry because the aim of this tutorial is to take you through and show you that it's not as random or complicated as it may first appear.

The lymphatic system serves a few very important roles in the body, so before we go into more detail about its anatomy and function, it's first helpful to understand why we have one at all. In order to do this, I need to take you down to the cellular level. Here we can see a section of bodily tissue as seen under a microscope. There's no need to worry about the specifics of what's going on here just now but for those of you who are interested, this is a section of skin tissue. I'm just using this as an example to generally represent to you guys what's going on pretty much all over the body.

So each of these little purple blobs you can see in the image here is the nucleus of an individual cell, and the tiny white space between the cells is known as the interstitial space and this space is filled with a fluid that is known as the interstitial fluid. This fluid is comprised of blood plasma, lymphocytes or white blood cells, waste products from surrounding cells and many other substances depending on which tissue in the body is being considered. Most of this gets reabsorbed into the blood through the capillaries but a small proportion does not get reabsorbed in this way. This small leftover proportion of fluid still needs to be reabsorbed somewhere though in order to maintain the composition of the interstitial fluid that's essential for the normal functioning of the nearby cells. Leaving this would be like letting your dreams clog up. Everything can soon grind to a halt.

So, it's essential that these extra substances are cleared and this is where the lymphatic system comes in. This system essentially acts as an alternative return group for the substances that get left behind by the rest of the circulatory system and, as we'll see, also plays an essential role in the normal functioning of the body's immune system. So, without further ado, let's continue the tutorial where we'll look at the function, structure and organization of the lymphatic system. Let's dive in by first looking at the function in more detail.

As we discussed just now, the lymphatic system primarily functions to clear tissue fluid from the interstitial spaces as well as substances that cannot be reabsorbed by the venous capillary beds. It also returns lymphocytes or white blood cells from the lymphatic organs into the bloodstream. This is really essential for the proper functioning of the body's immune system. In addition, the lymphatic system also functions to carry away food lipids or fats that are absorbed in the bowel. These are known as chylomicrons and the lymphatic system serves to carry them elsewhere in the body where they can be further processed.

The fluid transported by the lymphatic system is known as lymph. It's worth noting that the composition of lymph is not the same all over the body. Just like the interstitial fluid, its composition varies depending on the region of the body and tends to be similar to that of the corresponding interstitial fluid. Now that we have a basic understanding of its various functions, we'll now talk about the structure and organization of the lymphatic system.

We can look at the system as having two major divisions. These are the vasculature and the organs, and first we'll take a look at the vasculature. Remember the random network that we saw at the beginning of the video? Well, you'll probably be glad to know that it's not really that random at all. In fact, the lymphatic vasculature runs parallel with the rest of the circulatory system and similar to the rest of the circulatory system is made up of the thin-walled lymphatic capillaries which are the smallest of the lymphatic vessels and collect fluid from the interstitial spaces of cells.

Next, we have the lymphatic vessels which are larger and receive lymph from the lymphatic capillaries. We can see these on the image here, some of which are highlighted in green. Along these vessels is where you'll find the lymph nodes which can also be seen on this image again highlighted in green. The lymph nodes are swellings of the lymphatic system that are found throughout the body. As well as filtering lymph, they are also one of the places in the body where lymphocyte formation occurs.

Lymphocytes or white blood cells as you may know them are the body's frontline soldiers in the battle against disease and infection. Because of this, the lymph nodes are also considered to be a part of the body's immune system. After the lymphatic capillaries, lymphatic vessels and lymph nodes, we have the major lymphatic trunks. These receive lymph from the lymphatic vessels and lymph nodes and their function is to drain the lymph back into the venous system at the two lymphatic ducts. The first of these ducts is the thoracic duct located here as indicated by our blue circle and the second is the right lymphatic duct located here.

If we zoom in on this image, I can show you where these ducts drain into the subclavian veins near their junction with the internal jugular veins. The terms given to the locations where the lymphatic ducts drain into the subclavian veins are the left and right venous angles. So, in our image here, we can see the subclavian veins and the internal jugular veins and these angles – the left and right venous angles – and at these points is where lymph enters the venous system.

So now that I've introduced you to one of two major divisions – the vasculature of the lymphatic system - we'll now move on to a brief overview of the lymphatic organs. From this image here, we can get a pretty good overview of the locations of the lymphatic organs that I'll be introducing you guys to. A major function of these organs is to mount a specific immune response – that is, an immune response that is targeted solely against a particular antigen or disease. For this reason, they are mostly situated at likely ports of entry for infectious microbes. These organs can be divided into the primary lymphatic organs and the secondary lymphatic organs.

The primary lymphatic organs like many things in anatomy have another term and can also be referred to as the central lymphatic organs. These organs are where immune cells are produced, matured and selected. I'll now briefly introduce you to two examples of these organs.

The first of these is located here on the body and if we take a closer look, we can get a better view of what is called the thymus. This organ is the site of T-lymphocyte maturation and selection. So, now, it's probably a good time to mention that there are different types of lymphocyte produced around the body and T lymphocytes are one of these. I won't go into detail about the different types during this particular tutorial though. So, at this point, just know that the thymus is the site of maturation and selection of T-lymphocytes.

Another primary organ of the lymphatic system is the bone marrow. Here we can see the femur highlighted in green and if we were to zoom in and look within the bone, we can see here an example of the marrow inside. This is the site of B-lymphocyte development, maturation and selection. So, the thymus is associated with T-lymphocytes and the bone marrow is associated with B-lymphocytes.

The secondary lymphatic organs which are also known as the peripheral lymphatic organs are concerned with a whole variety of other immune processes such as antibody formation, lymphatic proliferation and antigen presentation and are populated with immunocompetent lymphocytes. The first of the secondary lymphatic organs is located here in the body and is known as the spleen. We can see it better in this image here with its surrounding structures. The spleen is, in fact, the only immune organ that is directly integrated into the bloodstream. It's similar in structure to a large lymph node and acts primarily as a filter for the blood.

Another secondary organ which I introduced to you earlier in the tutorial are located throughout the body and if I zoom in here, we can see that I'm referring to the lymph nodes. Here, specifically, we see the axillary lymph nodes highlighted in green. Lymph nodes vary a lot in size depending on their location, however, when there's an infection, these tend to swell and can sometimes be felt under the skin. If you've ever felt those tender lumps that sometimes appear beneath your jaw when you have a cold or flu, you'll know exactly what I'm talking about.

Next, we have the pharyngeal lymphatic ring also known as Waldeyer's ring. This is located here within the blue circle on your circle and is the name given to the group of structures more commonly known as your tonsils. Unfortunately, we humans, can't always be trusted not to consume something that could be bad for us. As you'll know, babies always put the most disgusting things into their mouths which is perhaps part of the reason why we have evolved to have quite a few lymphatic organs situated here. Specifically, Waldeyer's ring includes the pharyngeal tonsils which are the highest of the structures highlighted here in green as well as the palatine tonsils which are the middle structures highlighted here in green. Because of their location, these tonsils can be relatively easy to see for yourself as demonstrated in this image here.

And then finally, we have the lingual tonsils which are located at the back of the tongue as we can see in this image here as the lowest structure highlighted in green. If you aren't already aware, it might be useful for you to know that the term "lingual" means tongue. So even if we've never heard of a name of a structure before, if it has the term lingual in its name somewhere, we can then safely assume that it has something to do with the tongue. This occurs frequently in anatomy and we can often get quite a bit of information about a structure from its name alone even if we haven’t heard of it before.

Finally, the last organ of the lymphatic system that I'm going to introduce you to today is the mucosa-associated lymphoid tissue abbreviated as MALT which is found in the mucosal surfaces. In our image on the right, we can see a lingual tonsil and the patches that are highlighted in green represent the mucosa-associated lymphoid tissue. This type of lymphatic tissue can be found in the mucosal surfaces all over the body and can be defined according to where in the body it's found. For example, bronchus-associated lymphoid tissue abbreviated as BALT is found in the lungs and gut-associated lymphoid tissue abbreviated as GALT is found funnily enough in the gut. This is also where Peyer's patches and the vermiform appendix – this squiggly structure we see here highlighted in green – are located.

So just to round off the tutorial, I want to give you a quick summary of what we talked about today. I started by introducing you to the lymphatic system and gave you an overview of some of its major functions which included clearing fluid from the interstitial spaces which, remember, are essentially just the fluid-filled spaces found between the cells, returning lymphocytes from the lymphatic organs back into the bloodstream and carrying away digested food lipids known as chylomicrons.

Next, we talked about the overall structure and organization of the lymphatic system. First, I introduced you to the vasculature which is similar to the rest of the circulatory system, in that, it's made up of lymphatic capillaries which receive the interstitial fluid and drain into the lymphatic vessels which are also the site of the lymph nodes which I mentioned are swellings found in various locations around the body and in varying sizes.

And, finally, the major lymphatic trunks. These served to drain the lymph back into the venous system at the two lymphatic ducts which are the thoracic duct and the right lymphatic duct. These ducts drain into the venous system at the left and right venous angles.

We then looked at the lymphatic organs which can be divided into the primary and secondary organs. The primary organs are the site of immune cell production, maturation and selection and include the thymus and the bone marrow. The secondary lymphatic organs have several functions including antibody formation, lymphatic proliferation and antigen presentation and are populated with immunocompetent lymphocytes. The examples of the secondary lymphatic organs which I talked to you about included the spleen as well as the lymph nodes and Waldeyer's ring otherwise known as your tonsils. This ring is comprised of three major structures specifically the pharyngeal tonsils, the palatine tonsils and the lingual tonsils.

And, finally, the last secondary lymphatic organ that I talked to you about was the mucosa-associated lymphoid tissue or MALT. Variations of this depend on which mucosal surface of the body is being considered. For example, gut-associated lymphoid tissue is abbreviated as GALT, and this is where we can find Peyer's patches and the vermiform appendix highlighted here in green.

So that concludes our tutorial today introducing you to the lymphatic system. I hope you find it useful and thanks for watching.

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