Human circulatory system
Blood from the lungs is pumped by the heart through arteries to all parts of the body. This is known as circulation. Your heart lies in your chest. It is about the size of your fist. It is divided into four chambers. Blood consist of different types of cells floating in liquid plasma. These cells included white blood cells and red blood cells. It contains iron compounds called haemoglobin, which gives red colour to red blood cells and allow red blood cells to pick up oxygen from lungs.
The right atrium receives blood from all over the body. This side of the heart pumps blood to the lungs where oxygen enters it. The left atrium receives blood from the lungs and pumps around the rest of the body. This side of the heart is bigger because it has to pump harder to get the blood all the way around the body.