Urinary System, Part 1: Crash Course Anatomy & Physiology #38

Summary

This script from Crash Course delves into the urinary system's critical role in filtering metabolic waste and balancing blood's salt and water concentrations. It explains how kidneys, composed of a million nephrons, perform filtration, reabsorption, and secretion. The journey of blood through the nephrons, from the glomerular capsule to the renal tubule, is detailed, highlighting the importance of the loop of Henle in water reabsorption and the role of urea in concentrating the medulla. The script also sets up the next lesson on bodily regulation and potential imbalances.

Takeaways

• 🍽️ The liver plays a crucial role in metabolism by directing waste to the digestive and urinary systems but cannot expel waste from the body.
• 💨 The lungs assist in waste removal by exhaling carbon dioxide, while the colon eliminates solid waste and old cell parts.
• 🔍 The urinary system, particularly the kidneys, is responsible for filtering out toxic substances like nitrogenous waste from the blood.
• 🧘 The kidneys perform homeostatic functions such as regulating water volume, ion salt concentrations, pH levels, and influencing red blood cell production and blood pressure.
• 🚰 The process of urine formation involves filtration, reabsorption, and secretion, which are carried out by nephrons, the functional units of the kidneys.
• 🌀 Each kidney is composed of about a million nephrons, which filter blood continuously, handling over 20% of the body's total blood volume at any given time.
• 💧 The renal tubule, part of the nephron, has a twisting structure that allows for the reabsorption of valuable substances like ions, glucose, and water.
• 🔄 The loop of Henle creates a concentration gradient in the medulla by actively pumping out salts, which aids in the reabsorption of water from the filtrate.
• ♻️ Urea, a waste product from protein metabolism, is recycled by the kidneys to enhance the concentration gradient in the medulla, aiding in water reabsorption.
• 🔋 Tubular secretion is the final step where the kidneys selectively remove extra waste substances like hydrogen, potassium, and certain organic compounds from the blood.

Q & A

What is the primary role of the liver in the body's metabolic process?

The liver plays a significant role in directing dead cells and leftover chemicals to the digestive and urinary systems for cleanup after metabolism.

How does the urinary system contribute to the body's cleanup process?

The urinary system, specifically the kidneys, filters toxic leftovers from the blood, such as nitrogenous waste from metabolizing protein, and excretes it from the body.

What happens to the amine groups from amino acids during metabolism?

During metabolism, the amine groups from amino acids are converted into NH3, or ammonia, which is toxic. The liver then converts ammonia into a less-toxic compound, urea, which is filtered out by the kidneys into urine.

What are the main functions of the kidneys?

The kidneys' main functions include filtering metabolic waste, balancing salt and water concentrations in the blood, and regulating blood pressure and red blood cell production.

How does the urinary system regulate the balance of salt and water in the blood?

The urinary system regulates the balance of salt and water in the blood through the process of filtration, reabsorption, and secretion within the nephrons of the kidneys.

What is the structure of a nephron, and what are its key components?

A nephron consists of a renal corpuscle in the cortex and a renal tubule that loops between the cortex and the medulla. Key components include the glomerular capsule, glomerulus, and the renal tubule which is divided into the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.

What is the role of the glomerular capsule in the nephron?

The glomerular capsule is a cup-shaped structure that contains a network of capillaries called the glomerulus. It allows fluid, waste products, ions, glucose, and amino acids to pass from the blood into the capsule while blocking larger molecules like blood cells and proteins.

How does the loop of Henle contribute to the concentration of urine?

The loop of Henle creates a salt concentration gradient in the medulla by actively pumping out salts in the ascending limb, which helps draw water out of the filtrate in the descending limb, thus concentrating the urine.

What is the purpose of the urea recycling process in the urinary system?

The urea recycling process helps to increase the concentration gradient in the medulla, making it saltier and more effective at drawing out water from the filtrate in the ascending limb of the loop of Henle.

What is tubular secretion, and how does it function in the urinary system?

Tubular secretion is the process of selectively removing extra waste such as hydrogen, potassium, and certain organic acids and bases from the blood in the peritubular capillaries back into the renal tubule for excretion.

How much blood does the kidneys filter every day, and what percentage of total blood volume do they hold at any given moment?

The kidneys filter about 120 to 140 liters of blood every day, and they hold over 20 percent of the total blood volume at any given moment.

Outlines

00:00

🧘‍♂️ Metabolism and the Urinary System

This paragraph discusses the importance of the urinary system in the body's metabolic processes. It explains how the liver handles waste from metabolism but cannot remove it from the body. The lungs and colon assist in waste removal, but the urinary system, particularly the kidneys, play a crucial role in filtering out toxic leftovers from the blood, such as nitrogenous waste from protein metabolism. The kidneys also regulate water volume, ion salt concentrations, pH levels, and influence red blood cell production and blood pressure. The paragraph introduces the concept of how kidneys filter blood, not by simply straining out bad substances, but by removing most of the blood's content and selectively reabsorbing what the body needs before sending the rest to the bladder.

05:02

🔬 The Anatomy and Function of the Kidneys

This paragraph delves into the anatomy of the kidneys and their function in the urinary system. It describes the kidneys as bean-shaped organs with three layers: the cortex, the medulla, and the renal pelvis. The kidneys filter a significant portion of the body's blood, with each kidney containing about a million nephrons, which are the microscopic filtering units responsible for blood processing. The nephrons consist of a renal corpuscle and a renal tubule, with the corpuscle containing a glomerulus, a tangle of capillaries that filter blood into the capsule. The paragraph explains the three-step process of blood filtration in nephrons: filtration, reabsorption, and secretion, highlighting the role of the glomerular capsule in the initial filtration of blood into the renal tubule.

Keywords

💡Metabolism

Metabolism refers to the set of life-sustaining chemical reactions in organisms that allow for growth, repair, and maintenance of cells and overall functionality. In the context of the video, metabolism is highlighted as a process that generates waste products, which must be managed to prevent toxicity. The script mentions that the liver and kidneys play crucial roles in metabolite clearance.

💡Liver

The liver is a vital organ that performs numerous functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. The script emphasizes the liver's role in processing waste products and directing them towards elimination through the digestive and urinary systems.

💡Kidneys

Kidneys are essential in filtering the blood of substances that may be harmful, regulating electrolytes, maintaining acid-base balance, and managing fluid balance in the body. The video script explains that kidneys filter out nitrogenous waste and help in maintaining homeostasis by regulating water and salt concentrations in the blood.

💡Nephrons

Nephrons are the functional units of the kidney, each consisting of a renal corpuscle and a renal tubule. They are responsible for filtering the blood, reabsorbing necessary substances, and secreting waste. The script describes nephrons as the starting point of blood processing, involving three steps: filtration, reabsorption, and secretion.

💡Glomerular Filtration

Glomerular filtration is the first step in the nephron's process where blood is filtered, allowing fluid and small molecules to pass into the renal tubule while retaining larger molecules and blood cells. The script uses the analogy of a sieve to explain this process, although it clarifies that the kidneys do not simply strain out bad stuff but actively manage what is reabsorbed and what is excreted.

💡Tubular Reabsorption

Tubular reabsorption is a process where useful substances like glucose, ions, and water are reabsorbed from the filtrate back into the bloodstream. The script mentions that this process occurs in the renal tubule and is crucial for conserving essential nutrients and maintaining fluid balance.

💡Tubular Secretion

Tubular secretion is the active transport of certain substances, such as hydrogen, potassium, and organic compounds, from the blood into the renal tubule to be excreted in the urine. The script likens this to emptying one's pockets of any last unwanted items before disposing of the trash.

💡Urea

Urea is a waste product formed in the liver from ammonia, which is a byproduct of protein metabolism. The kidneys filter urea from the blood into the urine for excretion. The script explains that urea plays a role in the concentration gradient within the kidneys, aiding in water reabsorption.

💡Loop of Henle

The Loop of Henle is a U-shaped segment of the nephron that extends into the medulla of the kidney. It plays a critical role in creating a concentration gradient that helps in the reabsorption of water from the filtrate. The script describes how the loop contributes to the kidney's ability to conserve water and manage urinary concentration.

💡Urinary System

The urinary system is responsible for the production, storage, and elimination of urine. It includes the kidneys, ureters, bladder, and urethra. The video script focuses on the urinary system's role in waste removal and regulation of body fluids, emphasizing the kidneys' function in filtering the blood and the膀胱's role in storing urine.

💡Homeostasis

Homeostasis is the maintenance of a stable internal environment within organisms by regulating physiological processes. The script discusses how the urinary system, particularly the kidneys, contributes to homeostasis by regulating water volume, ion salt concentrations, and pH levels.

Highlights

The liver plays a crucial role in directing dead cells and leftover chemicals to the digestive and urinary systems for waste disposal.

The lungs and colon assist in waste removal by exhaling carbon dioxide and excreting unusable substances.

The urinary system, particularly the kidneys, is responsible for filtering out toxic leftovers from the blood.

Kidneys regulate water volume, ion salt concentrations, pH levels, red blood cell production, and blood pressure.

The process of urine formation involves filtration, reabsorption, and secretion within the nephrons of the kidneys.

Each kidney is composed of about a million nephrons, which are the functional units of the kidney.

Glomerular filtration is the first step where blood is filtered into the renal corpuscle, removing larger molecules.

The renal tubule has three major parts: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.

The loop of Henle creates a salt concentration gradient in the medulla to drive water reabsorption.

Urea, a waste product, is recycled by the kidneys to increase the medulla's saltiness and enhance water reabsorption.

Tubular secretion is the final step where specific waste products are actively transported out of the blood and into the urine.

The kidneys filter about 120 to 140 liters of blood every day, holding over 20 percent of the body's total blood volume at any given moment.

The kidneys' main job is to filter blood continuously, which involves a complex process of reabsorbing useful substances and excreting waste.

The urinary system's efficiency is crucial for maintaining homeostasis and preventing dehydration.

The process of urine formation is a complex cycle that involves the interplay of various physiological mechanisms.

The kidneys' function is not just a simple filtration but a sophisticated process of selective reabsorption and secretion.

Understanding the urinary system's anatomy and function is essential for grasping how the body manages waste and maintains balance.

Transcripts

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.