Let’s say you’re at the airport. You’re waiting in line to get through security. You’re in your socks, in public, and you’re wrestling with those plastic bins, and people are yelling at you about dumping liquids and taking your laptop out your bag. But even with a certain amount of chaos, the line moves pretty quickly. I mean, it should, theoretically. Because really, everyone has the same goal: to get to where they need to be.

Now, you probably think that I’m setting you up to talk about stress or anxiety or something. But I am not! Instead, I want you to imagine that security line as being like your blood, moving through your high-pressured circulatory system. And you, my friend, are not just some impatient traveler in a hurry to catch the next plane to Seattle. You are a bit of blood plasma. And the biggest concern of everyone in your queue is just to get delivered to their destination.

But the problem with rushing stuff through a blood vessel -- or a security line -- is that if it moves too fast, you run the risk that something bad might get through. So, in the case of the line at the airport, every now and then, the security people funnel some random people into a slower line for a full pat down and bag check. And, that is kinda what your lymphatic system does.

It takes up some extra fluid and, while it’s ultimately helping it get to its destination -- it runs a background check to make sure it’s legit. It may not be as well-known as some of your more popular organ systems, but your lymphatic system quietly plays a vital supporting role to both your cardiovascular and immune systems. For one thing, it maintains your homeostasis by eventually returning most of the fluid that has been diverted back into your blood. Which is good, because if it didn’t, your ankles would swell up like balloons, your lungs would fill with fluid, and your blood volume and pressure would drop to the point where your body would just give up and you die.

But the lymphatic system isn’t just another security line. It also has a series of checkpoints that are staffed by special security officers that have the ability to take care of small infections on the spot. Or if they really don’t like the looks of what they’ve found, they can alert the whole immune system to take it down. So, for keeping your blood-volume levels where they need to be, for keeping you from collapsing from low blood pressure, and for preventing you from dying from a staph infection every time you scrape your elbow, you have to thank the unsung hero of your anatomy: your lymphatic system.

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Structurally speaking, your lymphatic system consists of three main parts: The first is just... your lymph. It’s a watery fluid that flows through your lymphatic system. Much of your lymph starts out as blood plasma that gets forced out of your capillaries, way down in the capillary beds, where the leakage helps the exchange of nutrients and gases between the blood in the capillaries and the hungry cells in your tissues. The blood capillaries reabsorb some of the fluid, but a lot gets left behind. So, much of the rest of your lymphatic system works to soak it up and get it back to where it needs to go, inspecting it along the way.

For example, you have a network of lymphatic vessels that help reabsorb the fluid, and about 600 or 700 lymph nodes, which are basically checkpoints that monitor and cleanse the lymph as it filters through. Then there are special lymphoid organs, like the spleen, thymus, adenoids, and tonsils, as well as other lymphoid tissues throughout the body. Some of these organs and tissues house special defensive cells of the immune system to fight infection by gobbling up bacteria and other harmful antigens -- all of which we’ll get into a lot more in upcoming lessons. Others, like the thymus and spleen, provide a nursery for maturing immune cells called lymphocytes.

But to understand the special supporting role that your lymphatic system plays in your body, let’s start with its origins in the capillary beds of your circulatory system. You’ll recall that these beds are where the capillaries carry blood from your arterioles -- and your arteries before that -- and feed blood into your venules -- and eventually your veins. This is where the transfer of nutrients and waste takes place, as blood pressures force plasma out of the arteriole end of the capillaries and into the interstitial fluid between the cells of whatever tissue you happen to be in.

And most of that fluid gets drawn right back into the capillaries on the venous end, thanks to osmotic pressure, but some is left behind. Of the 20 liters a day that’s forced through the arteriolar end of all the capillaries all over your body, only 17 liters is taken back up by the venous end. Those other 3 liters? That becomes your lymph. It’s almost immediately picked up by nearby lymphatic capillaries that wind their way in and among the blood capillaries, and they’re really well-suited for the job.

Lymphatic capillaries are made of loosely overlapping, endothelial cells that look kind of like roofing shingles, forming little flap-like valves that only open in one direction. So when the pressure in the interstitial space becomes greater than the pressure inside the lymphatic capillary, the flaps push open and take in fluid to relieve the pressure. Once the fluid is inside a lymphatic capillary, we can officially call it lymph. Then it flows through successively larger lymphatic vessels to collecting vessels and then to larger trunks and finally, into one of two big ducts that feed back into the lowest pressure area of the circulatory system.

The right lymphatic duct drains all the lymph collected from the upper right area of the torso, as well as the right arm, and the right sides of the head and thorax, and feeds them all into the internal jugular vein. The larger thoracic duct takes lymph from the rest of the body, and dumps it into the subclavian vein.

Overall, these vessels work a lot like your veins do. They operate under very low pressure, have a series of valves that prevent backflow, and are helped along a bit by smooth muscle in the vessel walls that keep the fluid moving. OK, so the lymphatic system takes extra fluid that’s squeezed out of the blood capillaries and basically mops it up. That sounds handy. But, as a living thing that wants to stay alive, what’s in it for you, exactly?

Well, remember: We’re talking about recovering 3 liters of blood fluid per day -- that’s more than half of your total blood volume. If that weren’t returned to your circulatory system, you’d be looking at a serious drop in blood pressure, and little to no oxygen delivery, or waste pick up. Plus, any blockage in the lymph system -- like from a tumor that obstructs one of your vessels -- can be really dangerous, resulting in edema, or swelling, that can constrain blood flow and lead to serious complications.

And finally: Don’t forget that your lymph also plays a key role in the function of your immune system. Say you scrape your elbow, and some bad stuff -- like the staphylococcus bacteria that’s covering your entire body right now -- infiltrates some of the loose areolar connective tissue under your skin. Since that tissue is full of interstitial fluid that drains into your lymphatics, which ultimately drain to your heart, you’re going to want to vanquish those bad guys before they have a chance to wreak any havoc.

What ever will you do? Well, this is where your lymph nodes come in. Your body’s main lymphoid organs, the lymph nodes are where your lymph gets close and regular inspection by its resident immune cells, lymphocytes. Depending on what kind of troublemaker they find in the fluid, the lymphocytes might trigger the release of resident macrophages to attack unwanted microorganisms, or they might activate the general immune system outside of the node, if they need extra support.

The details of how that happens we will get to next time, but for now just understand that lymphocytes are found in, and mature in, the loose reticular connective tissue that makes up a large part of the nodes and most of your other lymphoid organs.

Now, if a node is overwhelmed by whatever invader it’s trying to quell, it may become inflamed. You can actually feel some of your nodes if you poke around the sides of your neck. They feel like little bumps...yeah, there they are. And noticeably enlarged lymph nodes are often an early diagnostic sign of disease or infection, which is why doctors are always feeling around under your jawline during check-ups.

But your lymph nodes aren’t the system’s only checkpoints. You’ve also got a few special outposts placed at strategic entry points to particularly sensitive tissues, like the those in the respiratory and digestive tracts. These are called mucosa-associated lymphoid tissues, or MALTs, because they can be found in mucous membranes around the body, outside of the lymphatic vessels.

Your tonsils are some of the largest collections of these tissues, which together form a ring around the entrance of the pharynx. There they can inspect everything you eat and breathe and try to remove any pathogens before they enter the GI tract or lungs. It’s a pretty big job, actually, which is why sore throats and swollen tonsils are a common sign of a cold or other infection.

Another collection of MALTs, called Peyer’s patches, are in the distal portion of the small intestine, providing another check point along the GI tract. And contrary to popular myth, your appendix is not useless, because it too contains a bunch of lymphoid tissue and is conveniently located where it can destroy any remaining bacteria before it can breach the intestinal wall during absorption.

Exactly how your lymph and immune cells detect and destroy the unwanted visitors that might otherwise kill you is what we’re gonna be talking about for the remainder of this course. But for now you learned how your unsung lymphatic system supports cardiovascular function by collecting, filtering, and returning interstitial fluid back into the bloodstream via a system of lymphatic vessels. We also talked about the system’s major role in fighting off infection, and how lymph nodes, and other lymphoid organs, and MALT areas house antigen-attacking lymphocytes that provide crucial support to the immune system.

Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon patrons whose monthly contributions help make Crash Course possible, not only for themselves, but for everybody, everywhere. If you like Crash Course and want to help us keep making videos like this one, you can go to patreon.com/crashcourse.

This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written by Kathleen Yale, the script was edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.