At the 1999 World Symposium on Heart Valve Disease, there was some discussion regarding the mechanism by which the aorta dilates prior to opening of the aortic valve. We propose that this dilation and other seemingly 'active' processes can be explained totally by passive fluid dynamics. Because there is a mechanical interaction between the aortic valve cusps and the aortic root, hydrostatic load on the aortic valve cusps is transferred to the aortic root, causing an inward pull on the commissures. This inward pull reduces the extent to which the aorta would otherwise dilate due to the internal pressure. During systole, as the pressure in the ventricle increases and the aortic valve begins to be unloaded, the inward pull on the commissures is reduced and the aorta is able to dilate fully. This observed dilation therefore occurs prior to the aortic valve opens.