We’ve been spending a lot of time lately talking about eating, and 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 that 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 of, so one system steps in to bat clean-up.
And that, is your urinary system. This system -- and specifically your kidneys -- does all sorts of important homeostatic stuff, like regulating your water volume, ion salt concentrations, and pH levels, and influencing your red blood cell production and blood pressure. But its main purpose -- what we’re going to be focusing on for the next two lessons -- is how it filters toxic leftovers from your blood -- like the 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. 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’s 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. 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 carbs 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. 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 filter blood continuously, they end up seeing a lot of it. In fact, at any given moment they hold over 20 percent 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 120 to 140 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.
Structurally and functionally, nephrons are where the real business of blood-processing -- which, 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’s 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. Now, all the stuff that get squeezed out of the blood into the glomerulus is called filtrate, which is then sent along to the elaborately twisting three-centimeter-long renal tubule.
Even though it looks like it’s just a tube, it has three major parts, some of which are permeable to certain substances, but not others. First along is the proximal convoluted tubule, or PCT, which is about as convoluted-looking at its name suggests; then the tube drops into a dramatic hairpin turn called the nephron loop, or the loop of Henle -- I term I kinda 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 intestines, the long, curly shape of the nephron provides more time and space for it to re-absorb whatever useable stuff it can. And this meandering path also allows the parts of the tubule that are toward the end, to have an affect on processes that take place closer to the beginning, as they pass each other. 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 want to just pee all of them out if we can help it.
So, let’s trace the 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 in microvilli that increase their surface area and help re-absorb much of the good stuff from the filtrate and back into the blood.
The remaining filtrate passes from the PCT into the loop of Henle, which starts in the cortex, 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 re-absorption 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 the descending loop in front of it, water passively flows out, and into the super salty interstitial space. 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 on to 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. 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 waste that have already made their way into the blood that’s in the peritubular capillaries, ready to leave the kidneys. This step is kind of 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 kinda 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, either, because next time, we’re going to learn how your body regulates what’s absorbed and what’s excreted, and we’ll find out can happen when that regulation goes awry. 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 help make Crash Course possible, not only for themselves, but for everyone. 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, 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 Cafe.
