 Now we're going to talk about thermoregulation, which is how animals keep their body temperatures regulated or relatively constant. So you can remember that because thermo-temperature regulation. There's two types of animals when it comes to temperature regulation. Endotherms and exotherms. And you can tell a little bit about endotherms by looking at the word itself. So endo is a prefix that means inside, and exo is a prefix that means outside. So endotherms regulate their body temperature from inside. They regulate temperature independent of environment. Exotherms regulate their temperature based on the environment, based on their outside surroundings. Endotherms, mammals, dogs, people, birds, exotherms, fish, reptiles, lizards, snakes. So then we're going to talk about how an endotherm regulates its body temperature. This is an example of a person whose set body temperature is 98.6 degrees Fahrenheit. Let's say that person goes outside on a really hot day. Their temperature is going to reach higher than their set temperature. So then their thermoregulatory system is going to kick in and start doing some things to help cool them back to that set point. It works kind of the same way your home heating system and cooling system works. If you set your temperature in your house to 70 degrees and the air outside gets really hot, and it's hotter than 70 degrees in your house, your air conditioning will kick on. So this is like a person's air conditioning system. Vasodilation, which means that your blood vessels that are really close to the surface of your skin, dilate, they get larger. That's a way for more heat to leave your bloodstream. So that's why if you go running or it's just really hot outside, you are going to be flushed. Your skin gets redder because your blood is actually going close. There's more blood going closer to the surface of your skin. So that's why a person gets red in the face when they're hot. Sweating is another way that your body cools you off. Panting, people don't typically pant, but dogs pant because they can't really sweat. So that is the way that your body gets you back to this temperature. But say then you go and you walk into a walk-in freezer. Your environment got really cold, so you're going to get cold, and your body needs to heat you up. Your body doesn't have a heating system like your house does, but it has some ways to heat you up. Vasoconstriction, so your blood vessels actually get smaller closer to the surface of your skin, so they're not losing as much heat. You shiver. That shivering actually produces heat. You're doing movement. You're using up your energy in order to move, and that produces heat. In most animals, their hair stands up on end. That's in order to insulate them better. Humans don't have fur covering them anymore, and so they have goose bumps, which are those erector-pel-eye muscles that are making the hair stand up. You still see that even though you don't have fur to stand up to insulate you. So that's the way that your body switches back to get you back up to 98.6 degrees. So this is how most endotherms regulate their temperature. They have an internal system that switches so that they can get back to a constant temperature. Most ectotherms do things like laying in the sun. If it's a little cold, before they go in their burrows, they'll lay in the sun, soak up as much heat as they can, then they'll go into the underground burrows. If they are too hot, they go lay in the shade, they stop moving, they really almost lay there in torpor so that they're not really making any heat of their own if they're too hot. So ectotherms have to use the environment to adjust their temperature. Endotherms have some internal systems for that. Another type of thermoregulation or a system that helps in thermoregulation is called countercurrent heat exchange. It happens only in some animals, but here's an example of some animals that does happen in dolphins, sharks, hummingbirds and bees. And countercurrent heat exchange is a way to make sure that the heart and the internal organs stay warm, even if an animal is standing on a cold rock or swimming in cold water. And so what happens is the cold venous blood, the blood that's coming from the cold part of the body in the vein and going toward the heart, the vein is very close to this artery and the artery has warm blood, it's coming from the heart and has warm blood and it's going out to the cold flippers or feet or wherever the extremities are that are cold. And so these artery and vein are very close to one another and what happens is the heat from the artery flows into the venous blood. So this blue is used here to represent cold and not to represent deoxygenated blood. All of this blood is deoxygenated as it's going from the body to the heart and again all of this arterial blood is oxygenated, but we're using this to represent heat, this red and blue. So as the artery runs out to the cold flipper, that blood becomes colder because it's giving its heat to the vein. That means that when it gets to that cold extremity, it's not going to lose a bunch of heat to the environment because it's already lost that heat to the blood that is going to go warm the heart and keep the heart warm. Vice versa, because this venous blood is cold, it's coming, it's going to go to the heart instead of going to the heart and making the heart cold, it absorbs the heat from the arterial blood and keeps the internal organs and heart warm. So that's how countercurrent heat exchange works.