Fluid Mechanics of the Aortic Valve

Abstract

The aortic valve consists of three thin (0.1 mm) cusps and three matching sinuses. The cusps are reinforced with collagen strands running from commissure to commissure. Under no load the cusps furl up in the axial direction. When closed, the cusps are stretched out and are supported by the underslung loops of collagen. A photograph of a human aortic root is shown in Figure 1 of chapter 6.1. The left ventricle is marked V the aorta A, a cusp C and its corresponding sinus S. The diameter of the aorta is approximately 25 mm. In measurements of instantaneous velocity in the ascending aortas of dogs, using heated thin-film gauges (1), it was shown that aortic blood-flow was laminar, that there was no central jet and that reversed flow at the end of systole was very small. These measurements showed, however, that velocity was not uniform across the aorta. These results were surprising, because they implied that the valve was open wide at mid-systole, otherwise the flow would have been turbulent (since the peak Reynolds number was 10,000), but that the valve closed, or nearly closed, during systole with flow still going forwards through the valve. Since the thin-film gauge responds to velocity fluctuations of frequencies well in excess of 10kHz, turbulence would certainly have been detected had it existed.