WEBVTT 00:00:00.000 --> 00:00:04.140 We’ve been spending a lot of time lately talking about eating, and digesting, and metabolizing food. 00:00:04.140 --> 00:00:08.189 And those are some of my favorite things in the world! It’s been a really great time. 00:00:08.189 --> 00:00:12.740 But, as with all good parties, or brunch buffets, in the end, we’re left with a mess. 00:00:12.740 --> 00:00:16.970 And I’m not talking about spilled beer and Dorito crumbs, I’m talking about toxic levels 00:00:16.970 --> 00:00:20.270 of garbage that need to be cleaned up before they kill you. 00:00:20.270 --> 00:00:24.450 In your body, a lot of the cleanup that comes after metabolism is handled by the liver, which 00:00:24.450 --> 00:00:30.480 plays a tremendous role in directing dead cells and leftover chemicals to the digestive and urinary systems. 00:00:30.490 --> 00:00:33.940 But your liver can’t actually escort waste out of your person. 00:00:33.940 --> 00:00:37.969 Your lungs can lend a hand, exhaling carbon dioxide, and of course your colon will eventually 00:00:37.969 --> 00:00:42.140 poop out unusable stuff and old cell-parts. But much of your chemical waste still needs 00:00:42.140 --> 00:00:46.120 to be sorted and disposed of, so one system steps in to bat clean-up. 00:00:46.120 --> 00:00:48.300 And that, is your urinary system. 00:00:48.300 --> 00:00:52.519 This system -- and specifically your kidneys -- does all sorts of important homeostatic 00:00:52.519 --> 00:00:57.659 stuff, like regulating your water volume, ion salt concentrations, and pH levels, and 00:00:57.659 --> 00:01:00.359 influencing your red blood cell production and blood pressure. 00:01:00.359 --> 00:01:03.269 But its main purpose -- what we’re going to be focusing on for the next two lessons 00:01:03.269 --> 00:01:07.610 -- is how it filters toxic leftovers from your blood -- like the nitrogenous waste made 00:01:07.610 --> 00:01:10.470 by metabolizing protein -- and ferries it out of the body. 00:01:10.470 --> 00:01:14.730 And — spoiler alert! — this all involves the how, and the why, and the what of your pee. 00:01:25.400 --> 00:01:29.780 Now you probably know that kidneys are filters, and you may imagine them as sieves that strain 00:01:29.790 --> 00:01:33.290 out the bad stuff, leaving it sitting like a hairball at the bottom of the bathtub. 00:01:33.290 --> 00:01:37.070 But that is, in fact, kind of the opposite of what you should be thinking. 00:01:37.070 --> 00:01:41.810 Most of what’s in your blood is totally removed by the kidneys. Then your body pulls 00:01:41.810 --> 00:01:46.240 back what it wants to hold onto, before the rest is sent on a one-way trip to the bladder. 00:01:46.240 --> 00:01:50.960 It’s kinda like this: you don’t clean out your fridge by just taking out the rotten fruit and fuzzy leftovers. 00:01:50.960 --> 00:01:54.570 Instead, you’ve got to take everything out, and put it on the counter, and then sort through 00:01:54.570 --> 00:01:57.430 what goes back in the fridge and what goes in the trash. 00:01:57.430 --> 00:02:01.030 That’s how your urinary system cleans you up. And it is really good at its job. 00:02:01.030 --> 00:02:04.280 So this morning I decided to go the healthy route and instead of eating my normal breakfast 00:02:04.280 --> 00:02:07.080 of nothing, I had a big 32-ounce protein smoothie. 00:02:07.080 --> 00:02:10.669 My digestive system did its thing, and all the protein was hydrolyzed into amino acids, 00:02:10.669 --> 00:02:14.719 which were absorbed by my blood, and sent all over my body to build and repair cells. 00:02:14.719 --> 00:02:17.620 It’s a beautiful thing, but not without consequence. 00:02:17.620 --> 00:02:21.040 Because metabolizing nutrients -- especially protein -- makes a mess. 00:02:21.040 --> 00:02:24.860 You may remember that amino acids are unique, in that they have nitrogen in their amine groups. 00:02:24.860 --> 00:02:29.840 And because we can’t store amino acids, extra ones get processed into storable carbs or fats. 00:02:29.840 --> 00:02:33.819 But the amine group isn’t used in those storage molecules, so it’s converted to 00:02:33.819 --> 00:02:37.579 NH3, or ammonia, which happens to be toxic. So the 00:02:37.580 --> 00:02:43.140 liver converts the ammonia into a less-toxic compound, urea, which our kidneys filter out into our pee. 00:02:43.150 --> 00:02:47.170 Once out of the body, urea can degrade back into ammonia, which is why dirty, pee-soaked 00:02:47.170 --> 00:02:49.519 toilets and cat litter boxes smell like ammonia. 00:02:49.519 --> 00:02:54.079 Now this business of taking out the nitrogenous trash is one of the urinary system’s biggest jobs. 00:02:54.080 --> 00:02:58.240 Its other major duty is to regulate the balance of salt and water in your blood, and both 00:02:58.249 --> 00:03:02.579 of these tasks are processed in the whole system of tubes that is your urinary system. 00:03:02.579 --> 00:03:05.659 So let’s take a look at some basic pee-making anatomy. 00:03:05.659 --> 00:03:10.620 Your kidneys are a pair of dark red, fist-sized, bean-shaped organs that sit on each side of 00:03:10.620 --> 00:03:13.200 your spine against the posterior body wall. 00:03:13.200 --> 00:03:18.190 Kidneys are retroperitoneal, which means they lie between the dorsal wall and the peritoneum 00:03:18.190 --> 00:03:22.920 -- the membrane that surrounds the abdominal cavity -- rather than inside the cavity itself, 00:03:22.920 --> 00:03:24.529 like your intestines and stomach do. 00:03:24.529 --> 00:03:28.639 Each kidney has three distinct layers, beginning with the outermost cortex. 00:03:28.639 --> 00:03:31.419 Beneath that is the medulla, a set of cone-shaped masses 00:03:31.419 --> 00:03:34.479 of tissue that secrete urine into tiny sac-like tubules. 00:03:34.480 --> 00:03:39.680 And finally, the innermost layer is the renal pelvis, a funnel-shaped tube surrounded by 00:03:39.689 --> 00:03:45.529 smooth muscle that uses peristalsis to move urine out of the kidney, into the ureter, and into the bladder. 00:03:45.529 --> 00:03:49.700 Because the kidneys’ main job is to filter blood continuously, they end up seeing a lot of it. 00:03:49.700 --> 00:03:54.769 In fact, at any given moment they hold over 20 percent of your total blood volume. 00:03:54.769 --> 00:03:58.749 Oxygenated blood enters the kidneys through the large renal arteries, which deliver nearly 00:03:58.749 --> 00:04:02.590 a quarter of all blood pumped through the heart every minute. That means your kidneys 00:04:02.590 --> 00:04:06.169 filter about 120 to 140 liters of blood EVERY DAY. 00:04:06.169 --> 00:04:11.499 As they enter the kidneys, renal arteries branch many, many times, ending in tons of little capillary groups. 00:04:11.499 --> 00:04:15.639 So a kidney isn’t just one big filter; instead, each one is made up of about a million twisty 00:04:15.639 --> 00:04:18.459 microscopic filtering units called nephrons. 00:04:18.459 --> 00:04:22.210 Structurally and functionally, nephrons are where the real business of blood-processing 00:04:22.210 --> 00:04:27.750 -- which, like, “pee-making” -- begins, in three steps: filtration, reabsorption, and secretion. 00:04:27.750 --> 00:04:32.260 Each nephron consists of a round renal corpuscle that resides up in the cortex, followed by 00:04:32.270 --> 00:04:37.240 a long and winding renal tubule that loops around between the cortex and the medulla. 00:04:37.240 --> 00:04:41.150 The outer part of the corpuscle is a cup-shaped feature called the glomerular capsule, because 00:04:41.150 --> 00:04:45.430 inside it there’s a whole tangle of capillaries called the glomerulus -- that’s from the 00:04:45.430 --> 00:04:48.560 Latin word for “ball of yarn,” which is pretty much what it looks like. 00:04:48.560 --> 00:04:52.500 And the endothelium of these capillaries is very porous. So they allow lots of fluid, 00:04:52.500 --> 00:04:57.389 waste products, ions, glucose, and amino acids to pass from the blood into the capsule -- but 00:04:57.389 --> 00:05:01.530 they block out bigger molecules like blood cells and proteins, so they stay in the blood 00:05:01.530 --> 00:05:05.259 and exit through the peritubular capillaries, also known as the vasa recta. 00:05:05.259 --> 00:05:09.889 Now, all the stuff that get squeezed out of the blood into the glomerulus is called filtrate, 00:05:09.889 --> 00:05:14.530 which is then sent along to the elaborately twisting three-centimeter-long renal tubule. 00:05:14.530 --> 00:05:18.150 Even though it looks like it’s just a tube, it has three major parts, some of which are 00:05:18.150 --> 00:05:20.400 permeable to certain substances, but not others. 00:05:20.400 --> 00:05:25.509 First along is the proximal convoluted tubule, or PCT, which is about as convoluted-looking 00:05:25.509 --> 00:05:30.409 at its name suggests; then the tube drops into a dramatic hairpin turn called the nephron 00:05:30.409 --> 00:05:34.190 loop, or the loop of Henle -- I term I kinda like better, personally -- and finally it 00:05:34.190 --> 00:05:39.509 ends in the distal convoluted tubule or DCT, which empties into a collecting duct. 00:05:39.509 --> 00:05:43.180 All this twisting might make the tubule look, like, super inefficient, but it actually serves 00:05:43.180 --> 00:05:45.500 an important purpose, as you might expect. 00:05:45.500 --> 00:05:49.169 Just like with your small intestines, the long, curly shape of the nephron provides 00:05:49.169 --> 00:05:52.940 more time and space for it to re-absorb whatever useable stuff it can. 00:05:52.940 --> 00:05:57.110 And this meandering path also allows the parts of the tubule that are toward the end, to 00:05:57.110 --> 00:06:01.340 have an affect on processes that take place closer to the beginning, as they pass each other. 00:06:01.340 --> 00:06:05.300 Because a lot of the stuff that winds up in the tube are valuable commodities -- like 00:06:05.310 --> 00:06:09.509 ions and glucose and water -- and we don’t want to just pee all of them out if we can help it. 00:06:09.509 --> 00:06:14.740 So, let’s trace the whole process, starting at the top, with the proximal convoluted tubule or PCT. 00:06:14.740 --> 00:06:18.980 The walls here are made of cuboidal epithelial cells, with big ol’ mitochondria that make 00:06:18.990 --> 00:06:24.669 ATP, to power pumps that pull lots of sodium ions from the filtrate, using active transport. 00:06:24.669 --> 00:06:28.300 These cells also are covered in microvilli that increase their surface area and help 00:06:28.300 --> 00:06:31.379 re-absorb much of the good stuff from the filtrate and back into the blood. 00:06:31.379 --> 00:06:35.530 The remaining filtrate passes from the PCT into the loop of Henle, which starts in the 00:06:35.530 --> 00:06:39.199 cortex, then dips into the medulla before coming back into the cortex. 00:06:39.199 --> 00:06:44.090 And the form of this loop is key to its function, because its primary task is to drive the re-absorption 00:06:44.090 --> 00:06:48.099 of water, by creating a salt concentration gradient in the tissue of the medulla. 00:06:48.099 --> 00:06:52.159 It does this mainly by actively pumping out salts in the ascending limb. This creates 00:06:52.159 --> 00:06:56.379 some very salty interstitial fluid in the medulla, so when new filtrate comes down the 00:06:56.379 --> 00:07:01.699 descending loop in front of it, water passively flows out, and into the super salty interstitial space. 00:07:01.700 --> 00:07:04.860 Since most of this water is picked up by the blood pretty quickly, the saltiness of the 00:07:04.870 --> 00:07:08.900 interstitial space doesn’t get diluted. So it can keep drawing water out of the next 00:07:08.900 --> 00:07:10.659 batch of filtrate in the descending limb. 00:07:10.659 --> 00:07:14.229 Needless to say, this is super important, because if we peed out all the water that 00:07:14.229 --> 00:07:17.530 went into our kidneys, we would die of dehydration really quick. 00:07:17.530 --> 00:07:21.110 But even after all that, we are still only two thirds of the way through the process. 00:07:21.110 --> 00:07:24.830 As we move out of the loop of Henle, into the distal convoluted tubule, and on to the 00:07:24.830 --> 00:07:28.719 collecting duct, the remaining filtrate is now officially urine. But there’s one more 00:07:28.719 --> 00:07:32.879 component that we have to squeeze the most out of before we excrete the stuff. Urea. 00:07:32.879 --> 00:07:37.330 Even though we think of urea as a waste product -- just one more part of that protein shake 00:07:37.330 --> 00:07:39.810 that has to be dumped -- the kidneys actually need it. 00:07:39.810 --> 00:07:43.960 They use it to ramp up the concentration gradient earlier in the process, making the medulla 00:07:43.960 --> 00:07:47.499 even saltier for the filtrate that’s back there going through the ascending limb. 00:07:47.499 --> 00:07:51.879 So in the final steps, after the filtrate leaves the DCT, it enters the collecting duct, 00:07:51.879 --> 00:07:56.599 which runs back into the medulla. And while the salt passively draws even more water out 00:07:56.599 --> 00:08:00.349 of the collecting duct, some urea passively leaves the urine as well. 00:08:00.349 --> 00:08:05.129 Making the medulla even more salty -- and, in turn, more effective at drawing out water 00:08:05.129 --> 00:08:07.090 from the ascending limb a few steps back. 00:08:07.090 --> 00:08:10.979 So there’s essentially a traveling pool of urea that escapes the urine, finds its 00:08:10.979 --> 00:08:14.699 way back into the loop of Henle, and then runs the whole course again back to the collecting 00:08:14.699 --> 00:08:17.789 duct -- an ammonia-scented cycle called urea recycling. 00:08:17.789 --> 00:08:22.259 Now all that’s left is a kind of last call to selectively sneak out any extra waste -- like 00:08:22.259 --> 00:08:26.530 hydrogen, potassium, and certain organic acids and bases -- using active transport. 00:08:26.530 --> 00:08:30.889 This is called tubular secretion, and it transports only select kinds of waste that have already 00:08:30.889 --> 00:08:34.740 made their way into the blood that’s in the peritubular capillaries, ready to leave the kidneys. 00:08:34.740 --> 00:08:39.070 This step is kind of like emptying your pockets of any last wads of tissue or crumpled receipts 00:08:39.070 --> 00:08:41.010 as you’re walking a bag of trash to the curb. 00:08:41.010 --> 00:08:44.540 And that’s how your kidneys clean up the mess left over from the giant party that is 00:08:44.540 --> 00:08:49.040 you metabolizing food. So if you thought that your kidneys were just a kinda fine mesh that 00:08:49.040 --> 00:08:51.660 filtered out bad stuff? Now you know that’s not true. 00:08:51.660 --> 00:08:56.089 If you thought your urinary system was basically a matter of: Water goes in, pee goes out? 00:08:56.089 --> 00:08:57.620 That’s DEFINITELY not true. 00:08:57.620 --> 00:09:01.230 And if you thought we were done talking about your urine, that is also not true, either, 00:09:01.230 --> 00:09:04.639 because next time, we’re going to learn how your body regulates what’s absorbed 00:09:04.639 --> 00:09:08.939 and what’s excreted, and we’ll find out can happen when that regulation goes awry. 00:09:08.940 --> 00:09:13.700 But for now, you learned the anatomy of your urinary system, and how your kidneys filter 00:09:13.700 --> 00:09:17.940 metabolic waste and balance salt and water concentrations in the blood. Specifically 00:09:17.940 --> 00:09:23.000 you learned how nephrons use glomerular filtration, tubular reabsorption, and tubular secretion 00:09:23.000 --> 00:09:27.209 to reabsorb water and nutrients back into the blood, and make urine with the leftovers. 00:09:27.209 --> 00:09:31.560 Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon 00:09:31.560 --> 00:09:35.620 patrons whose monthly contributions help make Crash Course possible, not only for themselves, 00:09:35.620 --> 00:09:39.230 but for everyone. If you like Crash Course and want to help us keep making videos like 00:09:39.230 --> 00:09:41.580 this one, you can go to patreon.com/crashcourse. 00:09:41.580 --> 00:09:45.220 This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written 00:09:45.220 --> 00:09:49.870 by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson. 00:09:49.870 --> 00:09:53.910 It was directed and edited by Nicole Sweeney; our sound designer is Michael Aranda, and 00:09:53.910 --> 00:09:55.670 the Graphics team is Thought Cafe.