Aortic stenosis is abnormal narrowing of the aortic valve. A number of conditions cause disease resulting in narrowing of the aortic valve. When the degree of narrowing becomes significant enough to impede the flow of blood from the left ventricle to the arteries, heart problems develop. The oxygenated blood then returns to the left atrium and passes through the mitral valve into the left ventricle. 2  Blood is pumped by the left ventricle across the aortic valve into the aorta and the arteries of the body.