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Every day, humans literally flush a valuable resource down the toilet: nitrogen. But there are some animals that have figured out a way to recycle the extra nitrogen in their bodies by moving it not to their livers, but to their mouths!

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Physiology of Urea cycle
Biochemistry, Ammonia
One intro line about Urea
Urea metabolism and recycling in ruminants

Every day, you pee out a  valuable resource: nitrogen. Your body uses nitrogen in everything from  making amino acids to expanding blood vessels.

But after processes like digestion,  your body binds it up in urea, a compound we can’t do much with. And  it all goes swirling down the drain. In some ways, it’s our loss.

And it turns out that ruminants like  sheep and cows really have us beat. Because they’ve evolved a way to recycle  their urea so it ends up not in their urine,   but in their spit. Overall, urea isn’t a bad thing.

It might steal some of our sweet, sweet nitrogen,   but it also prevents us and other  mammals from poisoning ourselves. See, some processes, including normal  protein digestion, produce ammonia,  a toxin made of hydrogen and nitrogen. If that ammonia built up in the body,   it would keep brain cells from  efficiently transporting potassium, which could end up causing  severe brain damage or death.

But instead, it gets shuttled off to  liver cells, which turn it into urea. Then, the urea eventually ends up in the bladder. Since we use nitrogen for so much,  peeing out urea might seem like a waste, but for us humans, it’s not all bad.

While recycling urea would be  great from a nutrient perspective, it would also leave us with a bunch of  extra nitrogen that could turn into ammonia. Basically, the extra nitrogen could be nice, but  we don’t always have something to do with it. Cue the ruminants.

Ammonia is dangerous for them, too, but they  can and do recycle urea and the nitrogen in it. They just have something to do with it. They feed it… to their gut microbes.

The process starts like it does in humans, with  ammonia getting turned into urea by the liver. Some of the urea just goes into their urine,   but some of it takes another  journey into their bloodstream, and some of that urea ends  up in their salivary glands. It might sound gross, but it’s not like  they’re directly peeing into their mouths.

And hey: If you want more nitrogen in your  diet, swallowing it is as easy as it gets. As they swallow their food and  saliva, the urea enters the rumen, the first chamber in their stomach. There, it meets a bunch of microbes.

And that’s where the party starts. Those microbes have enzymes that turn  the urea into all kinds of useful things, like amino acids that the  animal can use to run its body. This is called the protein  regeneration cycle, or urea recycling, and it’s kind of like an insurance policy.

Even if ruminants don’t eat  enough nitrogen-rich foods, they have a backup system for  getting the nutrients they need. Granted, the rumen microbes process nitrogen  on the first pass through the gut, too. But some ruminants can recycle nitrogen  around two full times—or more!— before excreting it, which is way  more efficient than what we do.

Today, researchers are learning how to alter this   system to reduce the amount of  nitrogen these animals excrete, since compounds like nitrous oxide  in cow manure are greenhouse gases. But even if we can’t learn how to make this work  for us, it’s an amazing adaptation for them. With a little help from their friends, they’re  getting more from their food than we ever could. ...which brings me to this  week’s President of Space.   SciShow couldn’t get by without a  little help from our friends, either, including today’s President of Space Faisal Saud.

Faisal is one of our patrons on Patreon,   and they help us to continue  making more free content like this. So, our patrons are awesome.   And they can get access to our  monthly livestreams and blooper reels. If you’re not a patron but want to learn  more about what it means to become one,   you can go to ♪outro♪.