Hydrodynamics of Aortic Blood Flow.

• The peculiarities of the arterial circulation, including pulsatile flow in tubes with distensible walls and multiple branchings, make it difficult to apply classical hydrodynamic principles to the aortic blood flow without direct experimental verification. An example of the present confusion regarding fundamental concepts is the question of mixing in the aortic root. The generally accepted view is that thorough mixing takes place in this location.' Some of the experimental evidence, however, suggests that complete mixing does not occur regularly in any part of the central circulation.' As a substitute for a direct experimental approach, the Reynolds number has been used to predict the type of flow that may occur in various parts of the aorta. It is doubtful that the Reynolds number can be used with confidence to predict whether aortic blood flow will be laminar or turbulent since even with steady flow in rigid pipes the critical Reynolds number may vary greatly depending on local conditions, while the effects produced by pulsatile flow and distensible tubes are unknown. A few attempts have been made to visualize the distribution of dyes either directly or by cinematography. In view of the difficulties of such visualizations of dyes in the aorta, it is not surprising that the above studies reached different conclusions. The method of sampling simultaneously from multiple arteries following the injections of dyes proximally in the central circulation also is open to question.