Nerdfightria Wiki - Urinary System Part 1: Intensive Anatomy and Physiology Course #38 (2023)

Lately, we've been spending a lot of time talking about eating, digesting, and metabolizing food, and those are some of my favorite things in the world. It was a great moment.

But like any good party or brunch buffet, there's disaster at the end... and I'm not talking about spilled beer and Dorito crumbs. I'm talking about toxic levels of junk that need to be cleared before they kill you.

In your body, much of the cleaning takes place after metabolism is controlled by the liver, which plays an important role in funneling dead cells and chemical waste to the digestive and urinary systems. But your liver really can't eliminate waste from your body. Your lungs can help by exhaling carbon dioxide, and of course your colon will end up excreting junk and old cell debris.

But a lot of your chemical waste still needs to be sorted out and disposed of, so a system steps in to help clean it up, and that's your urinary system.

This system, and your kidneys in particular, perform all kinds of important homeostatic jobs, such as regulating water volume, ions, salt levels, and pH levels, as well as influencing red blood cell production and blood pressure.

But its main purpose, which we'll focus on in the next two lessons, is how it filters toxic wastes from the blood, such as the nitrogenous waste produced by protein metabolism, out of the body, and spoiler alert, all of which includes the how and why. and what's in your urine.

(Introduction)

By now, you probably already know that the kidneys are filters, and you can think of them as filters that filter out the bad stuff and leave it like a hairball at the bottom of the bathtub.

But this is really the opposite of what you might think. Most of what's in your blood is completely removed by your kidneys. Your body then pulls out what it wants to keep before sending the rest on a one-way trip to your bladder.

It goes something like this: you don't just clean your fridge out of the rotten fruit and fluffy leftovers. Instead, you have to take everything out and put it on the counter, and then sort out what goes in the fridge and what goes in the trash.

This is how you clean your urinary system. And it does its job very well.

(2:02)

So this morning I went the healthy route and instead of having a simple breakfast I had a large 32oz protein shake. My digestive system worked and all the protein was hydrolyzed into amino acids, which were absorbed into my blood and sent throughout my body to build and repair cells.

It's a beautiful thing, but not without consequences. Because the metabolism of nutrients -especially proteins- is messed up.

You must remember that amino acids are unique because they contain nitrogen in their amino groups. And since we cannot store amino acids, the extras are processed into storable carbohydrates or fat. But the amino group is not used in these storage molecules, so it becomes NH₃, ammonia, which turns out to be toxic.

The liver then converts the ammonia into a less toxic compound, urea, which our kidneys filter out in our urine.

Once urea is removed from the body, it can be broken down back into ammonia, which is why dirty, urine-soaked bathrooms and litter boxes smell like ammonia.

Now, this business of getting rid of nitrogenous waste is one of the major tasks of the urinary system. Its other main function is to regulate the salt and water balance in the blood, and both functions are processed along the passages of the urinary system.

So let's take a look at the basic anatomy of pee.

(3:05)

Your kidneys are a pair of dark red, fist-sized, bean-shaped organs that sit on either side of your spine at the back of your body. The kidneys are retroperitoneal, meaning that they lie between the back wall and the peritoneum, the membrane that surrounds the abdominal cavity, rather than in the cavity itself like the intestines and stomach.

Each kidney has three distinct layers, starting with the outermost cortex. Beneath it is the medulla, a series of cone-shaped masses of tissue that secrete urine into small, sac-like tubes. And finally, the innermost layer is the renal pelvis, a funnel-shaped tube surrounded by smooth muscle that peristaltically transports urine from the kidney to the ureter and bladder.

Because the main function of the kidneys is to continually filter blood, they end up seeing too much. In fact, they contain over 20% of your total blood volume at any one time.

Oxygenated blood reaches the kidneys through the large renal arteries, which supply nearly a quarter of all the blood pumped by the heart every minute. That means your kidneys filter around 120 to 140 liters of blood every day.

When entering the kidneys, the renal arteries branch many, many times, ending up in tons of little clusters of capillaries. So a kidney is not just one big filter, each is made up of about a million interconnected microscopic filter units called nephrons.

(4:18)

Structurally and functionally, the real business of processing blood starts with the nephrons, it's like "crying" in three steps: filtration, reabsorption and secretion.

Each nephron consists of a round renal corpuscle located on top of the cortex, followed by a long, tortuous renal tubule that winds between the cortex and medulla.

The outside of the corpuscle is a cup-shaped feature called the glomerular capsule, because inside is a whole tangle of capillaries called the glomerulus. It comes from the Latin word for ball of wool, and that's what it looks like.

And the endothelium of these capillaries is very porous, allowing a large amount of fluid, waste, ions, glucose and amino acids from the blood to enter the capsule.

But they prevent larger molecules, such as blood cells and proteins, from staying in the blood and exiting through the peritubular capillaries, also known as the vasa recta.

(5:07)

The filtrate is whatever is pushed from the blood into the glomerulus and then passes through the three-centimeter-long ingeniously coiled renal tubules.

Although it looks like just one tube, it is actually made up of three main parts, some of which are permeable to certain substances but others are not.

The first is the proximal convoluted tubule, or PCT, which sounds about as complicated as its name might suggest. The tube then drops into a spectacular sharp curve called the loop of the nephron or loop of Henle, a term I personally prefer.

And finally, it ends in the distal convoluted tubule or DCT, which empties into a collecting duct. All that twisting can make the tubule look super inefficient, but it actually serves an important purpose, unsurprisingly.

Like the small intestine, the nephron's long, curly shape gives it more time and space to reabsorb whatever useful things it can.

And this winding path also allows the parts of the tubule located at the ends to act one after the other in processes that occur closer to the beginning.

(6:01)

Because many of the things that go down the tube are valuable products like ions, glucose, and water, and we don't want to pee on all of them if we can help it.

We're going to demonstrate this entire process, starting at the top with the proximal convoluted tubule, or PCT.

The walls here are cubic epithelial cells with large ancient mitochondria that produce ATP to drive pumps that actively transport lots of sodium ions from the filtrate.

These cells are also covered in microvilli, which increase their surface area and help reabsorb much of the good stuff from the filtrate back into the blood.

The remaining filtrate passes from the PCT to the loop of Henle, which starts in the cortex and then sinks into the medulla before returning to the cortex.

And the shape of this loop is critical to its function, as its main role is to promote water reabsorption by creating a salt concentration gradient in the medullary tissue.

This is done primarily by actively pumping salts into the uplink. This creates very salty interstitial fluid in the medulla. Then, as the new filtrate moves down the downstream loop ahead of it, the water flows passively into the supersaline interstitial space.

(7:02)

Because the blood absorbs most of the water quickly, the salinity of the interstitial space is not diluted, allowing it to continue to draw water from the next batch of filtrate in the descending limb.

Needless to say, this is very important because if we urinate all the water out of our kidneys, we will die of dehydration very quickly. But even after all that, we're only two-thirds of the way there.

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

But there is another component that we must take advantage of before excreting it: urea. Although we consider urea a waste product, just another part of that protein shake to throw away, the kidneys really need it.

They use it to increase the concentration gradient early in the process, making the pith even saltier for the filtrate going up that ascending branch back there.

In the final stages, after leaving the DCT, the filtrate enters the collecting channel, which takes it back to the medulla. And while the salt passively pulls more water from the collecting duct, some of the urea also passively leaves the urine, making the medulla even saltier and, in turn, more efficient at drawing water from the ascending limb a few steps back.

(8:07)

So essentially, there's a pool of migratory urea that escapes the urine, finds its way back into the loop of Henle, and then travels back into the collecting duct, an ammonia-smelling loop called urea recycling.

All that's left now is a sort of final call to selectively smuggle in additional wastes like hydrogen, potassium, and certain organic acids and bases via active transport.

This is called a tubular secretion and carries only selected types of waste that have already entered the blood, which is in the peritubular capillaries ready to leave the kidneys.

This is a bit like emptying your pockets of the last few tissues or crumpled receipts while dragging a garbage bag to the curb.

And so your kidneys clean out the leftovers from the big feast you're metabolizing food. So if you thought your kidneys were just some kind of fine mesh that filters out the bad stuff, now you know that's not true.

If you thought your urinary system was basically a water-in, urine-out thing, that's definitely not true.

And if you thought we were done talking about your urine, neither are you, because next time we're going to learn how your body regulates what goes in and what goes out, and we're going to find out what can happen. , if this regulation goes wrong.

(9:09)

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

Specifically, you learned how nephrons use glomerular filtration, tubular reabsorption, and tubular secretion to reabsorb water and nutrients in the blood and convert waste products into urine.

Thanks to our director, Linnea Boyev, and thanks to all of our Patreon patrons whose monthly contributions make Crash Course possible not just for them, but for everyone.

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

This episode was filmed at Dr. Cheryl C. Kenny Crash Course Studio. It was written by Kathleen Yale, edited by Blake de Pastino and our advisor 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.