[♪ INTRO] Hank was having a conversation  with some of our SciShow friends on the podcast, SciShow Tangents.

And the episode was about food,  but as the podcast’s name suggests, they went on a tangent about  crocodiles that kind of blew my mind. So, let’s get straight to the heart of the matter… the crocodile heart: it has an extra valve.

And although researchers are still trying  to understand why, they have some ideas. Crocodile and human hearts  do have a lot in common. For instance, there are really only  two places our hearts send blood: They either send it to the lungs  or to the rest of the body.

Ultimately, that’s because we’re  both using our circulatory systems to move around oxygen. Blood picks up oxygen from our lungs,  and then gets propelled through the heart to deliver that oxygen to  all the cells in our bodies. Then, by the time blood returns to the  heart, your cells have used up that oxygen, so your heart shuttles the blood  back to the lungs to pick up more.

And when all of that works as it  should, it helps to keep you alive. But human hearts and crocodile hearts  have a very important difference. Under certain conditions, crocodiles  don’t send their blood back to the lungs when it returns from the rest of the body.

They can instead send that blood back  out to the body through an extra valve that humans don’t have. They don’t do this all the time. For them to use this extra  valve, two things have to happen.

One, they have to have enough blood  pressure built up in the chamber leaving the heart to open the exit valves. And two, they have to be lacking adrenaline. That’s because scientists found  that adrenaline injections can close up the extra valve on command.

In nature, this means that when the  crocodile’s fight or flight response kicks in, they’re directing blood to pick up more oxygen to prepare for whatever they  are fighting or fleeing. So the extra valve is probably  used more in calm times when the crocodile is just  hanging out doing it’s thing. But why have this extra valve  in the first place?

Kind of surprisingly, one group of  scientists found that this valve doesn’t seem to help these animals  be bigger, better crocodiles. They think it might just exist because it  hasn’t been evolutionarily selected against. In other words, it’s not causing  any harm, so it hasn’t evolved away.

It’s just kind of along for the ride. Alternatively, another research group  suggested that it might have something to do with diving, because the valve opens  more when crocodiles go underwater. Since crocodiles aren’t breathing  while they’re underwater, the amount of oxygen in their  lungs goes down over time.

So, some scientists think that  they may be using this extra valve to re-circulate their blood and pull  as much oxygen from it as possible, before they send the blood  back to the lungs for more. Since we don’t do nearly as much  swimming and diving as crocodiles do, this might explain why they  have this valve and we don’t. On the other hand, maybe the  answer is something else entirely.

Yet another team found that more blood  gets sent through this extra valve during digestion, so maybe  it helps with that somehow. More specifically, when researchers  closed off the extra third valve, crocodiles that had just  eaten made less gastric acid than the other crocodiles with all  of their functioning heart valves. Crocodiles use gastric acid to digest their food, and a key ingredient in making  this acid is carbon dioxide.

And CO2 isn’t just one of the ingredients,  but the first to run out in this process. So researchers have suggested that  this extra heart valve could be sending carbon dioxide to a  crocodile’s digestive system, allowing them to make more gastric  acid and digest their food better. In the end, the reason for  this difference between humans and our scaly crocodile friends is  just not conclusive, at least for now.

Like, it might not even be an adaptation. This might have just been a divergence  that stuck around without rhyme or reason. But it also could be helping them do  daily activities like diving or digesting.

Either way, sometimes the discovery on  its own is enough to be mind-blowing, even when we haven’t yet  figured out why it is that way. Thanks for watching this episode of SciShow! And thank you to this month’s  President of Science, Matthew Brant.

Your generous support helps us keep  making free science videos like this one. We appreciate it, and we appreciate you! If you’d like to support us and join  our awesome community of patrons, you can head over to patreon.com/scishow. [♪ OUTRO]