Narrowing of aortic valve in aortic valve stenosis

In aortic valve stenosis, the aortic valve opening is narrowed, as seen in the top image. The heart must work harder to pump blood across the smaller opening. This increases pressure within the heart. Eventually the strain reduces the heart's ability to pump blood to the body. This is like placing smaller and smaller nozzles on the end of a garden hose, as shown on the bottom image. The narrower the nozzle is, the slower the flow of water. This results in pressure buildup within the garden hose.