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It's a hot day,

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and you've just downed 
several glasses of water,

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one after the other.

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

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that work as fine-tuned internal sensors.

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They balance the amount of fluid
in your body,

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detect waste in your blood,

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and know when to release 
the vitamins, minerals,

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and hormones you need to stay alive.

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Say hello to your kidneys.

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

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and to turn them into urine.

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

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between 20 and 25 times each day,

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

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The ingredients in your blood
are constantly changing

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as you ingest food and drink,

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

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Blood enters each kidney through arteries
that branch and branch,

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

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called nephrons.

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In each kidney,

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

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that carefully sift through the blood.

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This is where we see just how refined

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and accurate this internal 
sensing system is.

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

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

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

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

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Then, this vessel's job is to detect

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whether any of those ingredients 
are needed in the body.

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

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so they can circulate in the blood again.

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But the blood doesn't only 
carry useful ingredients.

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It contains waste products, too.

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

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The tubules sense compounds 
the body doesn't need,

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like urea, left over from 
the breakdown of proteins,

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and redirects them as urine 
out of the kidneys

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and through two long sewers
called ureters.

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

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ridding your body of that waste
once and for all.

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There's water in that urine, too.

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

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

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

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On the other hand,
low water levels in the blood

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

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meaning that less water 
makes it into the urine.

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

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

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

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These organs have the power 
to activate vitamin D

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to secrete a hormone called renin
that raises blood pressure,

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and another hormone 
called erythropoietin,

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which increases red blood cell production.

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

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

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

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So each kidney not only 
keeps things running smoothly.

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It also keeps us alive.

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Lucky then that we have two 
of these magical beans.