Lymphatic and Glymphatic Systems

Lymph is more or less blood, minus certain blood parts, such as red and white blood cells. It's just the fluids in your blood, whatever their chemical composition, without the solids.

When it gets into the tissues, it gets called interstitial fluid. It's the SAME fluid. Fluids circulate throughout your body very similar to Earth's water cycle. The water in the ocean is not different water from the water in clouds or rivers or your garden hose. That's just where it happens to be at the moment, but it won't STAY there.

You're blood is not separate from the other fluids in your body to the degree most people seem to imagine.

This is all the same fluid circulating through the body, with different elements added or removed in different contexts. It's really quite elegant and you can think of it sort of like the water cycle.

Interstitial fluid would be the equivalent of groundwater. Like groundwater, it gets into the tissues via seepage.

The circulatory system would be kind of like rivers and streams. And like rivers and streams, the channels it flows through aren't akin to leak-free metal pipes. They are very porous channels with a great deal of seepage feeding groundwater or interstitial fluid, as the case may be.

Obviously -- and you know this already -- part of this fluid cycle is controlled by the pump action of the heart. When it is in your blood vessels -- and I think also when it is in the lymphatic system -- the heart pumps it, driving the circulation.

But when it is out in the tissues as interstitial fluid, it is beyond the reach of the heart. And this is where things get really fascinating.

It is driven back to the circulatory system via two different methods, depending on where it is. The brain has a separate system from the rest of the body.

The lymphatic system handles it in most of the body. In the brain, it's called the glymphatic system and it works completely differently.

In the brain, the glymphatic system flushes fluids from the tissues while you sleep. This is a primary function of sleep and flushing these fluids out is how the body "takes out the trash," which is likely why sleep deprivation is increasingly shown as associated with both the build-up of certain proteins in the brain and, ultimately, certain brain disorders (including Alzheimer's).

In other tissues, muscle action drives the fluid back to the circulatory system. 

Remember, the heart is just a big muscle. So this makes perfect sense.

As a consequence, physical exercise dramatically increases the rate at which interstitial fluid gets returned to the circulatory system.

This means that one of the functions of exercise is "taking out the trash." And this is likely a huge and overlooked factor in why exercise is so beneficial to your health.

Different sources cite different rates, but exercise may increase the outflow by as much as eight times the normal speed of outflow via seepage. It's really a big difference.

If you have any health issues, understanding this human internal "water cycle" is something I highly recommend as enormously helpful. I sometimes take a walk to help myself feel better and I know that at least part of why that sometimes helps is it clears waste products from my tissues by pumping up the volume -- literally -- on the body taking out the trash.

Footnote 

That's an edited version of this comment by me on Hacker News. I did post some links to information about the glymphatic system in one of my replies there. I'm not bothering to include that here because link rot is a thing and I'm just trying to preserve my own ideas and writing and take ownership of it.

I'm posting it here because I no longer participate on Hacker News and I reference that comment a LOT. It has 123 karma, which only I can see, and never mind I got no respect there because no girls allowed in the old boys club.

I have a blog called Witness to Destruction that I initially wanted to call Confessions of a Forum Killer which is a COOLER sounding name, but not actually accurate and probably has disaster waiting to happen written all over it.

But the reason I wanted to call it that is because multiple forums I participated in died or became a shadow of their former selves after I left and I spent some years feeling like they DIED because I left. 

I no longer feel that way. If I am truly the life of the party and you fools run me off and then your party isn't worth attending anymore, that's on YOU, not ME. 

I ultimately decided that it was more like I tripped across someone dying, provided mouth to mouth resuscitation for a time, wasn't appreciated and finally stopped and, gee, the minute I stopped breathing life into them artificially as an uphill battle while everyone pissed on me for it, they FINALLY officially expired. 

BUT however you want to explain it, I'm not participating on Hacker News anymore and past experience suggests the way to bet is the forum is probably in decline and may cease to exist entirely. So if I VALUE it, I need to make a copy of it elsewhere.

And Google algorithms that will act like I plagiarized my own comment BE DAMNED.