As well as being divided into halves, each side of the heart is divided into an upper and a lower section. The upper section is called an atrium. The atrium collects and stores the blood. The lower section is called the ventricle. The ventricle gives the powerful contraction that pumps the blood out of the heart.
Blood flows from the atrium into the ventricle through a valve.
Valves are the doorways of the heart. They separate the atria from the ventricles and the ventricles from the blood vessels with strong seals. For example when the mitral valve opens it lets blood flow from the left atrium into the left ventricle. In a healthy heart the mitral valve then closes again tightly before the left ventricle beats, thereby preventing blood from flowing back into the left atrium instead of out through the aorta. Because the heart is a muscle, it needs a steady source of blood to nourish it.