The heart has four chambers - two atria and two ventricles. The walls of these chambers are mainly made of special heart muscle. During each heartbeat both of the atria contract first to pump blood into the ventricles. Then both ventricles contract to pump blood out of the heart into the arteries. There are one-way valves between the atria and ventricles, and between the ventricles and the large arteries coming from the heart. The valves make sure that when the atria or ventricles contract, the blood flows in the correct direction.
The aortic valve lies between the left ventricle and the aorta. The aorta is the main artery that takes blood from the heart to the body. The aortic valve has three flaps or 'cusps'. Blood flows through the valve when the left ventricle contracts at the early part of the heartbeat. When the left ventricle relaxes the aortic valve closes and the mitral valve opens to allow more blood into the ventricle ready for the next heartbeat.
Aortic stenosis means that when the aortic valve opens, it does not open fully. It is narrowed ('stenosed') when it is open. Therefore, there is a partial restriction of blood flow from the left ventricle into the aorta. Basically, the more narrowed the valve, the less blood that can get through, the more severe the problem is likely to be. In some cases, aortic stenosis occurs at the same time as aortic regurgitation. (See separate leaflet called 'Aortic regurgitation'.)