In aortic valve stenosis, the aortic valve opening is narrowed (top row). The narrowing requires increased pressure within the heart to pump blood across a smaller opening. Eventually this reduces the heart's ability to pump blood to the body. This is similar to attaching smaller and smaller nozzles to the end of a garden hose (bottom row). The narrowing from the nozzle slows the forward flow of water and results in pressure buildup within the garden hose.