In this video, we're going to start off by looking at what the circulatory system is and see why most mammals, like ourselves, have a double circulatory system. Then we'll take a closer look at the heart and cover the names of the different chambers and vessels. Now,the circulatory system, which is also sometimes called the cardiovascular system, is an example of an organ system, which means that it's made up of a group of organs that work together to perform a common function. In this case, those organs are the heart,the blood vessels, and the blood, and the role they're performing is to transport substances around the body. More specifically, the blood is the fluid which carries all of those substances, such as oxygen, nutrients, cells and waste products. Then the blood vessels hold the blood and direct it around the body, and the heart pumps the blood to ensure that it keeps flowing through the vessels. Today, we're going to focus on the heart, but we'll cover the blood and blood vessels in other videos. So, first of all, why do we call it a double circulatory system?Well, basically, it's because it contains two separate circuits. There's this top one called the pulmonary circuit, where blood flows from the heart around to the lungs and then back to the heart again. And then there's the systemic circuit, where the blood flows from the heart around to all of our body tissues, like our muscles, brain, liver, and so on,and then flows back to the heart. So to put all that together, blood's pumped from the heart around the lungs and back to the heart, and then it's pumped around the body and back to the heart. And that process keeps repeating and repeating. If we take a closer look at the heart now, this is an organ, and you can basically think of it as a big muscular pump. Inside the heart are four chambers. These two at the top are called atria, and these two on the bottom are called the ventricles. Then in between the atria and ventricles, on either side are the atrioventricular valves, which ensure that blood only flows in one direction from the atria into the ventricles. Now, whenever you look at an image of the heart like this one, it's as if the heart belongs to another person who's looking at you front on. So when we label the left and right side of the heart, it's actually their left and their right that's important, not ours. This means that the left side of the heart is over hereon our right, and the right side of their heart is over Here on our left. So just remember that the left and right seem to be the wrong way around, but this is why. So, putting that together, the four chambers are the right atrium and the right ventricle, with the right atrioventricular valve between them, and then the left atrium and the left ventricle with the left atrioventricular valve between them.