Panda bears are mammals, just like us, and all mammals have circulatory systems that are very similar in their structure and how they function. Mammalian hearts have four chambers: a left and right atrium, and a left and right ventricle. All mammalian blood is contained in blood vessels, which are tubes that contain your blood and ensure it only travels on a specific path around the body. You can think of blood vessels like roads in a city. When you drive you can only travel on the roads, but they extend to all the different locations you may need to go to. Just like a car on a road, blood can only travel around the body inside the blood vessels, but the blood vessels extend to have contact with every cell. Unlike cars on roads, however, blood can only travel through the circulatory system along a specific path. Blood enters the heart through the right atria and travels downwards to the right ventricle. The right ventricle pumps blood through the pulmonary artery to the lungs where it picks up oxygen to be carried to all the cells of the body. The blood returns from the lungs to the left atria via the pulmonary vein, then travels down to the left ventricle, and out of the heart through the aorta. From there it travels to the rest of the body, delivering oxygen to all cells in the body. Not only is this type of circulatory system common to all mammals, but it is shared by all vertebrates (and some invertebrates as well). However, most invertebrates have an open circulatory system. In this type of circulatory system, there are no blood vessels, instead the heart pumps blood into an open cavity, where it washes over the organs and contacts cells surrounding the cavity, delivery oxygen to the animal’s body.