It's a hot day,

and you've just downed 
several glasses of water,

one after the other.

Behind the sudden urge that follows
are two bean-shaped organs

that work as fine-tuned internal sensors.

They balance the amount of fluid
in your body,

detect waste in your blood,

and know when to release 
the vitamins, minerals,

and hormones you need to stay alive.

Say hello to your kidneys.

The main role of these organs
is to dispose of waste products

and to turn them into urine.

The body's eight liters of blood
pass through the kidneys

between 20 and 25 times each day,

meaning that, together, these organs filter
about 180 liters every 24 hours.

The ingredients in your blood
are constantly changing

as you ingest food and drink,

which explains why 
the kidneys need to be on permanent duty.

Blood enters each kidney through arteries
that branch and branch,

until they form tiny vessels that entwine
with special internal modules,

called nephrons.

In each kidney,

1 million of these nephrons form 
a powerful array of filters and sensors

that carefully sift through the blood.

This is where we see just how refined

and accurate this internal 
sensing system is.

To filter the blood, each nephron
uses two powerful pieces of equipment:

a blob-like structure called a glomerulus,
and a long, stringy, straw-like tubule.

The glomerulus works like a sieve,
allowing only certain ingredients,

such as vitamins and minerals,
to pass into the tubule.

Then, this vessel's job is to detect

whether any of those ingredients 
are needed in the body.

If so, they're reabsorbed in amounts
that the body needs,

so they can circulate in the blood again.

But the blood doesn't only 
carry useful ingredients.

It contains waste products, too.

And the nephrons have to figure out 
what to do with them.

The tubules sense compounds 
the body doesn't need,

like urea, left over from 
the breakdown of proteins,

and redirects them as urine 
out of the kidneys

and through two long sewers
called ureters.

The tubes empty their contents
into the bladder to be discharged,

ridding your body of that waste
once and for all.

There's water in that urine, too.

If the kidney detects too much of it
in your blood,

for instance, when you've chugged
several glasses at once,

it sends the extra liquid
to the bladder to be removed.

On the other hand,
low water levels in the blood

prompt the kidney to release some
back into the blood stream,

meaning that less water 
makes it into the urine.

This is why urine appears yellower
when you're less hydrated.

By controlling water, your kidneys
stabilize the body's fluid levels.

But this fine balancing act
isn't the kidney's only skill.

These organs have the power 
to activate vitamin D

to secrete a hormone called renin
that raises blood pressure,

and another hormone 
called erythropoietin,

which increases red blood cell production.

Without the kidneys, our bodily fluids 
would spiral out of control.

Every time we ate, our blood would receive
another load of unsifted ingredients.

Soon, the buildup of waste would overload
our systems and we'd expire.

So each kidney not only 
keeps things running smoothly.

It also keeps us alive.

Lucky then that we have two 
of these magical beans.