The left side of the heart pumps blood to the body. The heart is divided into two separate pumping systems, the right side and the left side. The right side of the heart receives blood that is low in oxygen because most has been used up by the brain and body. It pumps this to your lungs, where it picks up a fresh supply of oxygen. The blood then returns to the left side of the heart, ready to be pumped back out to the brain and the rest of your body. The left side of your heart receives oxygen-rich blood from your lungs and pumps it through your arteries to the rest of your body. The heart is made up of four chambers, two on the left side and two on the right. The two small upper chambers are the atria, and the two larger lower chambers are the ventricles. These left and right sides of the heart are separated by a wall of muscle called the septum.