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Voiceover: If you have a dog or if ever played fetch with a dog, you've probably noticed that after you've really abused that poor animal and run it into the ground, you've probably noticed that it started to do something interesting. It started panting. Did you ever stop to ask yourself why dogs pant? Well, here's your chance to answer that question. See if you could come up with a reason. It turns out that the main reason that dogs pant is so they don't overheat. Let's take a look at how that works. Let's say here we've got our favorite little puppy and it's just been running around for an hour and it's panting and it's got it's tongue lolling out like they do. What's going to happen when that dog breathes is it's going to breathe in cool, outside air. Let's say on that day it happened to be 70 degrees Fahrenheit and it's going to breathe out hot air because the air that it breathed in is going to equilibrate with its body. If you don't believe me, put up a hand to your mouth as your breathe out and feel how hot the air is that you're breathing out. What this means is that the dog is expelling heat everytime it breathes because it's taking in something cool and putting out something hot and that heat has to come from somewhere. For dogs this is really important because they can't sweat. They have fur coats and they couldn't very well soak those fur coats and sweat everytime they want to lose heat. This is really their best way of getting rid of heat. Now, we humans - and I'm going to try to draw one of we humans over here - we humans can sweat. This is a human with an Afro, apparently. We humans can sweat and so this method of losing heat by breathing heavily is definitely less important for us, but it actually still happens. Just like the dog when we breathe in, or when the air that we breathe in, which technically comes from both the nose and the mouth, that air goes into our lungs and in our lungs and in our lungs it divides into all those little alveoli that you've heard about that have a huge amount of surface area. Let's me just draw a couple of them here. All these tiny alveoli have a huge amount of surface area and that allows all this cool air that you breathe in, which I'm drawing here, to equilibrate with the temperature of the blood that's passing by these capillaries. So just like the dog, when we breathe in air that's let's say 70 degrees Celsius, we're going to breathe out air that's pretty much body temperature. That should not be Celsius, that should be Fahrenheit. When we breathe an air that's 70 or maybe cooler, we are going to breathe out air that's body temperature. If this guy is not having a fever that would be about 98.6 degrees Farenheit. Therefore, we can see that getting rid of extra heat is another thing that the lungs can do. We have a fancy word for this. We call it Thermo, thermo meaning heat just like a thermometer measures temperature, Thermoregulation. So you could say it's regulation of the temperature of our bodies. That's another things that the lungs can do. Lungs can thermoregulate. You'll note that this is kind of convenient because times when you might be overheating, just like this dog, times when you might be overheating or when you've done a lot of exercise and when you've done all that exercise you need a lot of oxygen to feed your muscles and at the same time you want to get rid of heat. These two functions of providing oxygen and getting rid of excess heat coincide nicely in the case of exercise because what happens when you exercise is that you start breathing really heavily. At rest, you might be breathing about five liters of air per minute. Meaning that a total of five liters of air is coming in and out of your lungs every minute, but while you're doing exercise, let's say heavy exercise, let's say you're running a mile, trying to get a really good time, you can easily go up to 50 liters per minute and clearly, that is a lot more, 10 times more. What that means is you're going to be putting out 10 times more heat through your lungs than when you're resting. So it's convenient that when you exercise you get more oxygen by breathing heavily and you also get rid of more heat by breathing heavily.