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It's a hot day,

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and you've just downed [br]several glasses of water,

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one after the other.

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Behind the sudden urge that follows[br]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[br]in your body,

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detect waste in your blood,

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and know when to release [br]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[br]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[br]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[br]about 180 liters every 24 hours.

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The ingredients in your blood[br]are constantly changing

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as you ingest food and drink,

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

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Blood enters each kidney through arteries[br]that branch and branch,

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until they form tiny vessels that entwine[br]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 [br]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 [br]sensing system is.

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

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

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The glomerulus works like a sieve,[br]allowing only certain ingredients,

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such as vitamins and minerals,[br]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 [br]are needed in the body.

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If so, they're reabsorbed in amounts[br]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 [br]carry useful ingredients.

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It contains waste products, too.

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

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The tubules sense compounds [br]the body doesn't need,

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like urea, left over from [br]the breakdown of proteins,

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and redirects them as urine [br]out of the kidneys

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and through two long sewers[br]called ureters.

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

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ridding your body of that waste[br]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[br]in your blood,

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

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it sends the extra liquid[br]to the bladder to be removed.

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On the other hand,[br]low water levels in the blood

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prompt the kidney to release some[br]back into the blood stream,

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meaning that less water [br]makes it into the urine.

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

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

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

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These organs have the power [br]to activate vitamin D

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to secrete a hormone called renin[br]that raises blood pressure,

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and another hormone [br]called erythropoietin,

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which increases red blood cell production.

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

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

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

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So each kidney not only [br]keeps things running smoothly.

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It also keeps us alive.

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Lucky then that we have two [br]of these magical beans.