Name: The Excretory System: From Your Heart to the Toilet - CrashCourse Biology #29
Uploaded: 2014-09-19T02:27:56.000Z
Duration: 12 min 21 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

One of the coolest and most important things

that our bodies do is maintain this thing called homeostasis,

the regulation of a stable internal environment,

no matter where we are or what we're doing.

After all, we put our bodies through a lot every single day:

We're always adding food and liquid and chemicals,

and we're constantly changing temperature

All of our organ systems have some hand in maintaining homeostasis.

I mean, it's basically the thing that makes us not dead.

But the excretory system, aka the urinary system,

which includes the kidneys, the ureters, the bladder,

and the urethra, is the star quarterback of the homeostasis team

is responsible for maintaining the right levels of water

This is called osmoregulation, and it's how our bodies

get rid of the stuff we don't need, like the byproducts

of metabolizing food, while also making sure we don't get dehydrated.

It's the body's greatest balancing act, and your body is doing

it right now, and all the time, as long as you're not dead.

As with other organ systems we've talked about,

not all excretory systems in the animal kingdom are created equal.

Different animals excrete waste different ways

based on their evolutionary history what environments they live in,

and what their hobbies and interests are.

These factors all influence how an animal regulates water,

and most metabolic waste needs to be dissolved

The problem is, a main byproduct of metabolizing food is ammonia,

which comes from breaking down proteins, and it's pretty toxic.

So, depending on how much water is available to an animal

and how easy it is for the animal to lug a bunch of water

around inside it, animals convert this ammonia

Mammals like us, as well as amphibians, and some marine animals

like sharks and sea turtles, convert ammonia into urea,

a compound made from combining ammonia and carbon dioxide,

The advantage of urea is its very low toxicity.

It can hang out in your circulatory systems for a while

But you have to have some extra water available

I mean peeing isn't a huge inconvenience, I mean, is it?

if you a bird or an insect or a lizard livings in the desert.

Animals that have to be light enough to fly

or don't have a bunch of spare water hanging around,

convert ammonia into uric acid, which can be excreted

as a kind of paste, so not a lot of water is needed.

If you haven't taken a close look, next time, do that.

The white stuff in the bird droppings is actually

the uric acid-y pee and the brown stuff is the poop.

So, now that we've established what is and what is not bird poop,

let's get down to the brass tacks of how humans

get all of this urea out of our blood and into our toilets.

The excretory system starts with the kidneys,

the organs that do all the heavy lifting,

from maintaining those levels of water and dissolved materials

in our bodies to controlling our blood pressure.

They start out by filtering out a bunch of fluid

and the stuff dissolved in the fluid out of your blood,

and then they basically re-absorb 99% of it back

before sending that 1% on its way in the form of urine.

On an average day, your kidneys filter out about 180 liters

of fluid from your blood, but only 1.5 liters of that

So most of your excretory system isn't dedicated to excreting

But the system works, obviously, I'm still alive.

Now it is time to get into the nitty gritty details

of how your kidneys do all this, and it's pretty cool.

But there's lots of weird words. So get ready.

Your kidneys do all this work using a network

of tiny filtering structures called nephrons.

Each one of your mango-sized kidneys has about a million of them

If you were, don't do this, but if you were to unravel

all of your nephrons and put them end to end,

This is where all the crazy action happens,

so to understand how they work, we're just going to follow

Blood from the heart enters the kidneys through renal arteries,

and just so you know, whenever you hear the word "renal"

it means you're dealing with kidney stuff.

As the blood enters, it's forced into a system of tiny capillaries

until it enters a tangle of porous capillaries called the glomerulus.

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The Excretory System: From Your Heart to the Toilet - CrashCourse Biology #29

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