It seems like it only comes out to play when you’re sick — you put your hand over your

neck or in your armpit to discover a handful of bumpy, swollen lymph nodes.

As if the mucus dripping from my nose wasn’t enough of a sign I was sick, awesome, thanks

lymph nodes!

But even if we tend to feel negatively towards this system or at best, neutral, the lymphatic

system is incredibly important in maintaining a balanced amount of fluid around our bodies.

Those lymph nodes act like checkpoints around the body.

Constantly policing fluid taken from the blood to make sure everything’s okay.

Without our lymphatic system, our immune response would be entirely different.

Plus our feet and ankles would swell up all the time and we’d walk around like giant

marshmallow people.

So in today’s video, we’ll talk about what makes the lymphatic system so important,

and learn why our lymph nodes seem to only rear their ugly heads when we get sick.

This is your lymphatic system.

It’s an enormous network of lymph vessels but also includes larger organs like the spleen,

thymus, and tonsils.

At first glance, it looks kind of like your cardiovascular system, what with all the branching

vessels and everything.

And that’s no coincidence.

One of the main jobs of the lymphatic system is to drain fluid from the cardiovascular

system and act like an emergency overflow channel.

Before we can understand the lymphatic system, we need to know how blood moves around the

body.

Here’s your cardiovascular system.

The heart pumps oxygenated blood through the arteries, which branch off into smaller and

smaller tubes until they arrive at the body’s different tissues — muscles, organs, anything

that consumes oxygen.

Once arteries get to the tissues, they thin out into extremely tiny capillaries.

These vessels are so thin, and the conditions just right so that red blood cells drop off

their oxygen molecules to those tissues, and then return to the lungs to pick up more oxygen

and the cycle continues.

If you want even more background on this process, check out our episode on the respiratory system.

Now, that oxygen-delivery system works perfectly well almost all of the time.

But every now and then, those little capillaries can leak out fluid.

This is the first place we see the lymphatic system shine!

As a drainage network.

When those capillaries start leaking, the watery part of blood plus a few proteins starts

collecting in the interstitial space, or the space between cells.

When that fluid travels from the interstitial space to the lymphatic vessels, we call that

fluid lymph, a murky white liquid with a whole lot of dissolved proteins and white blood

cells floating around.

And our bodies process a lot of lymph, upwards of three liters a day.

Now, any good drainage system needs a gutter, and in this case our gutters are the lymphatic

capillary vessels.

Look, I know it seems unfair to call these hard working vessels gutters.

To reduce these absolute heroes of fluid balance to a drainage system, but that’s what they

are.

They are well built gutters though.

The cells that make up these lymphatic capillaries are arranged in a way that lets fluid in but

not out.

From there, lymph flows into bigger vessels that have some cool hardware.

When you get to larger lymphatic vessels, you’ll find a similar setup to arteries:

a smooth inner layer, a tough outer layer, and middle layer with some built in muscle.

But unlike arteries, you’ll find a series of one way valves that prevent backflow.

The space between each valve is called a lymphangion.

This ends up being a huge deal for fluid flow.

See, the lymphatic system doesn’t have a central pump, unlike the circulatory system

which has the heart.

So to make sure lymph keeps moving through the vessels, the smooth muscle around each

lymphangion squeezes fluid through, segment by segment.

It’s similar to what happens when you swallow food.

Muscles contract around your esophagus to push food into your stomach.

It’s one of the reasons astronauts can still swallow food in space.

The best analogy I could come up with is that water snake toy where you squeeze it and it

moves in one discrete direction.

Even though you’re applying even pressure, the snake moves in one direction only.

One of these days I’ll make a reference that doesn’t make it painfully obvious I

grew up in the 90s.

But today is not that day.

Right, so some other factors move the lymph along as well, like moving our big skeletal

muscles.

After passing through some lymph nodes where the fluid is cleaned by immune cells, the

lymph heads to the veins.

But you’ll notice that the lymph doesn’t return to the capillaries in the cardiovascular

system where it came from.

It’s a one way ticket from cardiovascular capillaries to lymphatic vessels to veins.

And it’s a very good thing that they drain into veins, and not arteries.

The veins are really elastic compared to the arteries, plus they’re under less pressure.

So when the lymphatic vessels drain into veins, they don’t create as dramatic of a blood

pressure bump.

Now, if your capillaries leak more fluid than your lymphatics can drain, you end up with

edema, or fluid that accumulates in the tissues.

In everyday language, this is what we call swelling.

Edema is a normal part of inflammation or injury, but too much edema can get dangerous.

Like edema in the brain increases the pressure on your brain.

Too much pressure inside your skull and you reduce the blood flow to the brain and thus,

the oxygen that it receives.

See, now we’re starting to appreciate these lymphatics a little more, huh?

So that’s the first job of the lymphatic system, to take care of fluid balance.

It also plays a supporting role in the immune system.

That’s where those other lymph organs including the lymph nodes come in.

In total, we’re talking about five to six hundred lymph nodes around the entire body,

and you can easily find some of them on your own body like in the neck and armpits.

These lymph nodes are bundles of lymphoid tissue that filter out some of the nasty pathogens

that collected in your lymph before you returned it to circulation.

Although I think of them less like filters and more like colosseums of death for pathogens.

Cuz they’re not going through a nice gentle filter, they’re about to do battle with

some of the most specialized and destructive immune cells we have — lymphocytes.

Let’s take a look at what happens when a bacteria gets in your body.

First off, that pathogen usually hangs out in the interstitial fluid, not the blood.

And that’s a good thing.

Once bacteria get into your bloodstream they spread around the body way faster.

Luckily, as a first line of defense you’ve got a few cells like macrophages and dendritic

cells.

Sometimes they can engulf the pathogen by themselves.

And these things are colossal snitches.

They’re like little hall monitors.

After they take care of the pathogen, they bring evidence of their good work to the higher

ups or T cells.

That means taking an antigen, in this case the protein on the outside of the invading

cell, and presenting it to the immune cells in the lymph nodes to learn how to beat that

pathogen in the future.

This gives them the special title of APC, or antigen presenting cell.

If you want to learn more about that process, check out our video on B and T cells.

But sometimes that first line of defense isn’t enough, so we shuttle some of those live bacteria

into the lymph and towards the lymph nodes.

That’s where they enter the Thunderdome of immune cells.

We’ve got even more macrophages and dendritic cells, but also many more concentrated B and

T cells.

Some of them are naive T cells.

In this case, the scientific use of “naive” kinda means the same thing as the day to day

use.

A naive T cell is one that doesn’t know how to do its job yet.

And there are lots of naive T cells in the lymph nodes for the APCs to present antigens

to.

This helps our immune system learn what the new pathogen is and prepare defenses against

it.

Usually this is enough to get rid of the pathogen, but if your lymph nodes are working extra

hard to get rid of an infection, they can become inflamed.

And that’s when they start to swell and we start paying attention to them.

And the location of the swollen nodes can give you a good idea of where the infection

is coming from too.

So the lymph nodes in your neck can swell up from the flu or a sinus infection, while

the lymph nodes in your groin can swell from sexually transmitted infections like gonorrhea.

Now, there are a few spots around the body that need a little extra lymphatic protection.

Mucous membranes like the tissue under your tongue tend to let in more pathogens than

the less permeable membranes like our skin, so it’s a good idea to have some extra-concentrated

lymphatic tissue in those spots.

We call that tissue MALT, or mucosa-associated lymphoid tissue.

For instance, the Peyer’s patches in our small intestine are especially important to

protect us from ingested pathogens.

MALT structure differs from tissue to tissue, so those Peyer’s Patches look different

than the MALT in your eyes or lungs.

Pretty cool to see how this system is more complex than just a handful of swollen nodes.

In the last few videos we’ve been focused on how our bodies protect us from pathogens.

Next time, we’ll shift gears a little bit and start talking about neurons.

That’s right, we’re finally getting to the nervous system.

Thanks for watching this episode of Seeker Human, I’m Patrick Kelly.