Urinary System, Part 2: Crash Course Anatomy & Physiology #39

It has filled countless diapers, caused discomfort for any number of airline passengers, and it totally ruined the Dude’s rug, which really tied the room together, man. Anatomists call it micturition and I don’t know why because the rest of us know it as urination, which seems like a fine word. All mammals, and most animals urinate to remove toxins and to help maintain water-volume homeostasis, or blood pressure.

And while some of us spray it around to attract mates or mark territory, or deter predators, as far as I know, only humans actually study pee. In fact, we’ve been doing it for thousands of years. Even before Hippocrates extolled the diagnostic virtues of pee-sniffing, early Sumerian and Babylonian physicians were making urine-related observations.

Medieval doctors, as well, diagnosed diseases based on on smelling, inspecting, or even tasting urine samples. And although they were often totally off-base -- which makes me feel bad for those guys who sipped urine for no reason -- they were kind of on to something. Today, urological tests can help detect a lot of ailments based on the color, smell, clarity, and chemical composition of a sample.

Freshly peed urine is usually about 95 percent water, slightly acidic -- with a pH of around 6 -- a little aromatic, and usually somewhere between clear and dark yellow in color, depending on your level of hydration. Urine also contains over 3000 different chemical compounds, and their varying levels of concentration can tell us a lot about what’s going on in the body. For example, if you give me a urine sample -- I will have no idea what to do with it, but if you give it to a doctor and they can see that it’s cloudy with white blood cells, that’s a good sign you’ve got a urinary tract infection.

If it smells sweet and contains a lot of glucose, you may have diabetes. If it looks pink, then -- unless you’ve erecently aten beets -- you probably have internal bleeding somewhere. And if it is chocked full of proteins, you could be pregnant, or working out too hard, or have high blood pressure, or be headed to heart failure… So as you can see, even if the most thought you’ve given the subject is to wonder if you should pee now or wait until the end of the movie, the whole process of producing, storing, and eliminating pee is no where near as simple as it may seem.

From osmotic pressure to stretch receptors to hormones, our circulatory, nervous, and endocrine systems regulate how much urine we produce, what goes into it, and when to get rid of it. So join me as we journey into the world of pee. Wait.

No. Can we rephrase that? All right, how about: “let’s look at where your pee comes from.

” No, actually, that doesn’t sound good either. Let’s begin by looking at what regulates the production of urine. That works.

Last time, we discussed how your kidneys filter your blood, but the actual production of urine can be affected by a whole host of factors. One thing that might have crossed your mind last time is that the production of urine must -- by its very nature -- be influenced by blood -- specifically, its volume and its pressure. Because, step one in pee-making is the process of glomerular filtration -- where blood is filtered in the little blood-filled balls of yarn that are the glomeruli.

So, just like water in a hose, higher pressure in the blood must push more plasma out of the capillaries and into the glomeruli. But here’s a problem: Your kidneys can only handle so much filtrate at a time, so they have to maintain a constant rate of flow inside of them. This is known as the glomerular filtration rate -- or how much blood passes through the glomeruli every minute.

And your kidneys have ways of regulating this rate, despite changes in blood pressure. If your blood pressure happens to increase, for example, the higher pressure causes the arterioles leading to your glomeruli to stretch. And then the smooth muscle in the walls of the glomeruli respond to this stretching stimulus by constricting -- automatically reducing the amount of blood flow into the glomeruli and leaving the flow rate relatively unchanged.

This kind of intrinsic control, or autoregulation, is helpful in controlling the filtration rate through normal ranges of blood pressures, but the kidneys mostly regulate urine concentration at the other end of the nephron tubules. This kind of regulation I’m sure you’re familiar with. If you’ve ever had too much coffee or gone on a bit of a bender, you may have experienced the pleasure of having to pee every five minutes.

That’s because your endocrine system has a lot to say about your bathroom breaks, so you have some strong hormonal mechanisms that affect when and how often you go. And as it happens, both caffeine and alcohol inhibit the release of one of these hormones -- called antidiuretic hormone, or ADH -- which is secreted by the posterior pituitary gland to help the body retain water and stay hydrated. How ADH works is kind of complex, but first, let’s remember that cell membranes are generally not that permeable to water.

But in the parts of the nephron that reabsorb water, like the descending limb of the loop of Henle, water has to move easily through cells, from the filtrate to the blood. This is possible because of special protein channels in their membranes called aquaporins that are on both the apical, or filtrate-facing side, and the basal, or capillary-facing side of the cells. By contrast, the cells lining the collecting duct only have aquaporins on the basal side, so not a lot of reabsorption takes place there usually.

But ADH triggers those cells to move aquaporins they have in storage, over to the apical side, which allows more water to leave the urine. And since caffeine and alcohol inhibit ADH, that means no moving aquaporins, which means very little water reabsorption, and ultimately tons of peeing, and dehydration. So, leah, lots of factors affect the production of urine.

But once it’s produced, it doesn’t just leave the building. It has to be moved, and stored, until the time is right. Once the urine leaves the kidneys, it enters the ureters, a pair of slender tubes that drop down to the posterior urinary bladder.

Contrary to what you might think, your ureters aren’t just passive tubes, and your pee doesn’t wind up in your bladder because of gravity alone. Rather, like the small intestines, each ureter features a layer of smooth muscle that contracts to move urine using peristalsis. The frequency and strength of these peristaltic waves varies, depending on how fast urine is being produced, and a series of valves prevent pee from backing up, making sure that instead it reaches the bladder.

The bladder is a hollow, collapsible sac that temporarily stores urine. Like the kidneys, it’s retroperitoneal, located posterior to the pubic bone and anterior to the rectum. The bladder wall consists of three layers -- an inner mucosa, surrounded by a thick muscular layer called the detrusor wrapped in a fibrous, protective outer membrane.

The inner mucosal layer consists of transitional epithelium, which allows the bladder to expand so it can hold more urine, a handy feature for social mammals like us who prefer dry underwear and peeing in private. When it’s empty, it collapses into a triangular shape, folding up on itself like a deflated balloon. Then as urine accumulates, the bladder thins and expands into a pear-shape, and all those folds disappear.

A full bladder can comfortably hold around 500 ml of pee, though it can usually expand to hold a maximum of around one liter. At that point, though, you’re pushing your luck, because prolonged overdistention could -- in theory -- lead to a burst bladder, although you’d probably just pee your pants first. But let’s assume for the sake of polite conversation that you have found an appropriate location to relieve yourself: Your urine enters the thin but muscular urethra by passing through the internal urethral sphincter.

Now we don’t actually have voluntary control over this particular sphincter, but the autonomic nervous system keeps it cinched up whenever you’re not peeing to prevent leakage. Once the urine is through the sphincter, it heads down through the urogenital diaphragm which includes the last stop, the external urethral sphincter, which is probably the one you’re familiar with, because it’s made of skeletal muscles and is the one that you control voluntarily. ONLY NOW are we finally ready to explore the act of micturition itself, the actual excretion of urine, urination.

As the pee from your morning coffee builds up, it causes the bladder to push out, activating the stretch receptors in its walls. The resulting nerve impulses zip along afferent fibers to the sacral region of the spinal cord, along interneurons, and toward the brain, eventually exciting the parasympathetic neurons and inhibiting the sympathetic system. This tells the detrusor to contract while the internal urethral sphincter simultaneously opens, and the external sphincter relaxes so that the pee can flow out.

This, you may or not recall, is kind of an acquired skill. When you’re a baby, those stretch-receptor impulses trigger a simple spinal reflex that coordinates this whole process, and you have no real control over when you pee. But within a couple of years of birth, your brain’s circuits have developed the ability to override simple reflexive urination and to choose a different neural pathway.

So how’s that possible? Well, an area of your brainstem, called the pons, contains two different centers that lock down your urination control, or lack of it: there’s the pontine storage area, which inhibits urination, and the pontine micturition center, which gives it the green light. As your bladder fills up, impulses triggered by stretch receptors head to the pons and other higher brain centers that give you that conscious feeling that you have to pee.

If your bladder isn’t full, and you’re too busy to find a bathroom, it mostly activates the pontine storage area that keeps you from peeing, by inhibiting your parasympathetic activity and increasing sympathetic output. Of course, the longer you hold it, the more your bladder fills up, and eventually the need to pee becomes too strong to ignore, at which point the pontine micturition center jumps into action, overriding the previous orders, and opening the sphincters so you can finally tinkle. And that’s how your own personal waterworks … works.

Whether you’re a baby in diapers, or a grown-up science student … or a guy who was sent to “leave a message” on Jeffrey Lebowski’s rug. Today you learned how the urinary system regulates the production of urine, by maintaining a study glomerular flow rate. We also talked about the anatomy of storing and excreting urine -- from the ureters to the urethra -- and we went over the nervous system’s role in controlling the act of urination.

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 you want to help us keep making videos like this, and you want to get thanked at the end of every episode, like I just did for all of our Patreon patrons -- if that’s you then thank you so much -- 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 by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael Aranda, and the Graphics team is Thought Cafe.