The human heart is a four-chambered muscular organ located in the center of the chest between the lungs. It is about the size of a fist and weighs around 10 ounces. The heart has three layers - an outer epicardium, middle myocardium made of cardiac muscle, and inner endocardium. It is divided into four chambers - right and left atria which receive blood, and right and left ventricles which pump blood to the lungs and body. The heart has four valves that control blood flow through each chamber.