We've been spending a lot of time lately talking about eating, digesting, and metabolizing food, and those are some of my favorite things in the world. It's been a really great time.

But as with all good parties or brunch buffets, in the end, we're left with a mess... and I'm not talking about spilled beer and Dorito crumbs. I'm talking about toxic levels of garbage that need to be cleaned up before they kill you.

In your body, a lot of the cleanup comes after metabolism is handled by the liver, which plays a tremendous role in directing dead cells and leftover chemicals to the digestive and urinary systems. But your liver can't actually escort waste out of your person. Your lungs can lend a hand, exhaling carbon dioxide, and, of course, your colon will eventually poop out unusable stuff and old cell parts. 

But much of your chemical waste still needs to be sorted and disposed off, so one system steps in to back cleanup, and that is your urinary system.

This system, and specifically, your kidneys, does all sorts of important homeostatic stuff, like regulating your water volume, ions, salt concentrations, and pH levels and influencing your red blood cell production and blood pressure.

But its main purpose, what we're gonna be focusing on for the next two lessons is is how it filters toxic leftovers from your blood, like nitrogenous waste made by metabolizing protein, and ferries it out of the body, and - spoiler alert - this all involves the how and the why and the what of your pee.

(Intro)

Now you probably know that kidneys are filters, and you may imagine them as sieves that strain out the bad stuff, leaving it sitting like a hairball at the bottom of the bathtub.

But that is, in fact, kind of the opposite of what you should be thinking. Most of what is in your blood is totally removed by the kidneys. Then your body pulls back what it wants to hold onto, before the rest is sent on a one-way trip to the bladder.

It's kinda like this: you don't clean out your fridge by just taking out the rotten fruit and fuzzy leftovers. Instead, you've got to take everything out, and put it on the counter, and then sort through what goes back in the fridge and what goes in the trash.

That's how your urinary system cleans you up. And it is really good at its job. 

(2:02) 

So this morning I decided to go the healthy route and instead of eating my normal breakfast of nothing, I had a big 32-ounce protein smoothie. My digestive system did its thing, and all the protein was hydrolyzed into amino acids, which were absorbed by my blood, and sent all over my body to build and repair cells.

It's a beautiful thing, but not without consequence. Because metabolizing nutrients - especially protein - makes a mess.

You may remember that amino acids are unique, in that they have nitrogen in their amine groups. And because we can't store amino acids, extra ones get processed into storable carb or fats. But the amine group isn't used in those storage molecules, so it's converted to NH₃, or ammonia, which happens to be toxic.

So the liver converts the ammonia into a less-toxic compound, urea, which our kidneys filter out into our pee.

Once out of the body, urea can degrade back into ammonia, which is why dirty, pee-soaked toilets and cat litter boxes smell like ammonia.

Now this business of taking out the nitrogenous trash is one of the urinary system's biggest jobs. Its other major duty is to regulate the balance of salt and water in your blood, and both of these tasks are processed in the whole system of tubes that is your urinary system.

So let's take a look at some basic pee-making anatomy.

(3:05)

Your kidneys are a pair of dark red, fist-sized, bean-shaped organs that sit on each side of your spine against the posterior body wall. Kidneys are retroperitoneal, which means they lie between the dorsal wall and the peritoneum - the membrane that surrounds the abdominal cavity- rather than inside the cavity itself, like your intestines and stomach do.

Each kidney has three distinct layers, beginning with the outermost cortex. Beneath that is the medulla, a set of cone-shaped masses of tissue that secrete urine into tiny sac-like tubules. And finally, the innermost layer is the renal pelvis, a funnel-shaped tube surrounded by smooth muscle that uses peristalsis to move urine out of the kidney, into the ureter, and into the bladder.

Because the kidneys' main job is to continuously filter blood, they end up seeing a lot of it. In fact, at any given moment they hold over 20% of your total blood volume.

Oxygenated blood enters the kidneys through the large renal arteries, which deliver nearly a quarter of all blood pumped through the heart every minute. That means your kidneys filter about a hundred twenty to a hundred forty liters of blood every day.

As they enter the kidneys, renal arteries branch many, many times, ending in tons of little capillary groups. So a kidney isn't just one big filter, instead each one is made up of about a million twisty microscopic filtering units called nephrons.

(4:18)

Structurally and functionally nephrons is where the real business of blood-processing - which is like, "pee-making" - begins, in three steps: filtration, reabsorption, and secretion.

Each nephron consists of a round renal corpuscle that resides up in the cortex, followed by a long and winding renal tubule that loops around between the cortex and the medulla. 

The outer part of the corpuscle is a cup-shaped feature called the glomerular capsule because inside it there is a whole tangle of capillaries called the glomerulus. That's from the Latin word for ball of yarn, which is pretty much what it looks like.

And the endothelium of these capillaries is very porous, so they allow lots of fluid, waste products, ions, glucose and amino acids to pass from the blood into the capsule.

But they block out bigger molecules like blood cells and proteins so they stay in the blood and exit through the peritubular capillaries also known as the vasa recta.

(5:07)

Now all the stuff that gets squeezed out of the blood into the glomerulus is called filtrate, which is then sent along the elaborately twisting three-centimeter long renal tubule. 

Even though it looks like just a tube it actually has three major parts, some of which are permeable to certain substances but not others.

First is the proximal convoluted tubule or PCT which is about as convoluted looking as it's name suggests. Then the tube drops into a dramatic hairpin turn called the nephron loop, or the loop of Henle, a term that I like better personally. 

And finally it ends in the distal convoluted tubule or DCT which empties into a collecting duct. All this twisting might make the tubule look like super inefficient but it actually serves an important purpose, as you might expect.

Just like with your small intestine, the long curly shape of the nephron provides more time and space for it to reabsorb whatever usable stuff it can. 

And this meandering path also allows the parts of the tubule that are toward the ends to have an effect on processes that take place closer to the beginning as they pass each other.

(6:01)

Because a lot of the stuff that winds up in the tube are valuable commodities like ions and glucose and water, and we don't wanna just pee all of them out if we can help it.

Let's demonstrate this whole process starting at the top, with the proximal convoluted tubule, or PCT.

The walls here are made of cuboidal epithelial cells with big ol' mitochondria that make ATP to power pumps that pull lots of sodium ions from the filtrate using active transport.

These cells also are covered with microvilli that increase their surface area and help reabsorb much of the good stuff in the filtrate back in the blood.

The remaining filtrate passes from the PCT into the loop of Henle, which starts in the cortex and then dips into the medulla before coming back into the cortex.

And the form of this loop is key to its function because its primary task is to drive the reabsorption of water by creating a salt concentration gradient in the tissue of the medulla.

It does this mainly by actively pumping out salts in the ascending limb, this creates some very salty interstitial fluid in the medulla so when new filtrate comes down in the descending loop in front of it, water passively flows out and into the super-salty interstitial space.

(7:02)

Since most of this water is picked up by the blood pretty quickly, the saltiness of the interstitial space doesn't get diluted so it can keep drawing water out of the next batch of filtrate in the descending limb.

Needless to say, this is super important because if we peed out all the water that went into our kidneys, we would die of dehydration really quick. But even after all that we are still only two-thirds of the way through the process.

As we move out of the loop of Henle into the distal convoluted tubule and onto the collecting duct, the remaining filtrate is now officially urine.

But there's one more component that we have to squeeze the most out of before we excrete the stuff - urea. Even though we think of urea as a waste product, just one more part of that protein shake that has to be dumped, the kidneys actually need it.

They use it to ramp up the concentration gradient earlier in the process, making the medulla even saltier for the filtrate that's back there going through the ascending limb.

So in the final steps, after the filtrate leaves the DCT, it enters the collecting duct which runs back into the medulla. And while the salt passively draws even more water out of the collecting duct, some urea passively leaves the urine as well, making the medulla even more salty and in turn more effective at drawing out water from the ascending limb a few steps back.

(8:07)

So there's essentially a traveling pool of urea that escapes the urine, finds its way back into the loop of Henle, and then runs the whole course again back to the collecting duct, an ammonia-scented cycle called urea recycling.

Now all that's left is a kind of last call to selectively sneak out any extra waste like hydrogen, potassium and certain organic acids and bases using active transport.

This is called tubular secretion and it transports only select kinds of wastes that have already made their way into the blood that's in the peritubular capillaries ready to leave the kidneys.

This stuff is kinda like emptying your pockets of any last wads of tissue or crumpled receipts as you're walking a bag of trash to the curb.

And that's how your kidneys clean up the mess left over from the giant party that is you metabolizing food. So if you thought that your kidneys were just a kind of fine mesh that filtered out bad stuff, now you know that's not true.

If you thought your urinary system was basically a matter of water goes in, pee goes out, that's definitely not true.

And if you thought we were done talking about your urine, that is also not true because next time we're gonna be learning how your body regulates what's absorbed and what's excreted and we'll find out what can happen when that regulation goes awry.

(9:09) 

But for now, you learned the anatomy of your urinary system and how your kidneys filter metabolic waste and balance salt and water concentrations in the blood.

Specifically, you learned how nephrons use glomerular filtration, tubular reabsorption and tubular secretion to reabsorb water and nutrients back into the blood, and make urine with the leftovers.

Thank you to our headmaster of learning, Linnea Boyev, and thank you to all of our Patreon patrons whose monthly contributions make Crash Course possible not only for themselves, but for everyone.

If you like Crash Course and wanna help us keep making videos like this one, you can go to patreon.com/crashcourse.

This episode was filmed in the Dr. Cheryl C. Kenny Crash Course studio. It was written by Kathleen Yale, edited by Blake de Pastino and our consultant is Dr. Brandon Jackson. 

It was directed and edited by Nicole Sweeney, our sound designer is Michael Aranda, and the graphics team is Thought Café.