The instantaneous blood pressure that is measured within the aorta will be significantly different depending upon how the pressure is measured. As described above, during ventricular ejection, the velocity and hence kinetic energy of the flowing blood is very high. To summarize this concept, blood flowing at higher velocities has a higher ratio of kinetic energy to potential (pressure) energy.Īn interesting, yet practical application of Bernoulli's Principle is found when blood pressure measurements are made from within the ascending aorta. Because of the resistance of the stenosis, and the turbulence the likely occurs, the post-stenosis PE and E will both fall. Once past the narrowed segment, KE will revert back to its pre-stenosis value because the post-stenosis diameter is the same as the pre-stenosis diameter and flow (and therefore, velocity) is conserved. Therefore, an increase in velocity leads to a decrease in lateral pressure, which is the basis for the Venturi effect. The fall in PE represents a decrease in the lateral pressure against the vessel walls. Assuming that the total energy is conserved within the stenosis (E actually decreases because of resistance as shown in the figure), then the 16-fold increase in KE must result in a reciprocal decrease in in the magnitude of PE. Because KE ∝ V 2, the KE increases 16-fold. If the diameter is reduced by one-half in the region of the stenosis, the velocity increases 4-fold. Quantitatively, V ∝ 1/D 2 because flow (F) is the product of mean velocity (V) and vessel cross-sectional area (A) (F = V x A), and A is directly related to diameter (D) (or radius, r) squared (from A = π r 2). In the narrowed region (stenosis), the velocity increases as the diameter decreases. This principle can be illustrated by a blood vessel that is suddenly narrowed then returned to its normal diameter. This is the basis of Bernoulli's Principle.
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