The aorta is the main artery carrying blood out of the heart. When blood leaves the heart, it flows through the aortic valve, into the aorta. In aortic stenosis, the aortic valve does not open fully. This decreases blood flow from the heart.
As the aortic valve becomes more narrow, the pressure increases inside the left heart ventricle. This causes the left heart ventricle to become thicker, which decreases blood flow and can lead to chest pain. As the pressure continues to rise, blood may back up into the lungs, and you may feel short of breath. Severe forms of aortic stenosis prevent enough blood from reaching the brain and rest of the body. This can cause light-headedness and fainting.