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Cardiovascular Physiology Concepts

Richard E. Klabunde, PhD


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The Bernoulli Principle and Energetics of Flowing Blood

Because flowing blood has mass and velocity, it has kinetic energy (KE). This KE is proportionate to the mean velocity squared (V2; from KE = ½ mV2). As the blood flows inside a vessel, pressure is exerted laterally against the walls of the vessel; this pressure represents the potential or pressure energy (PE). The total energy (E) of the blood flowing within the vessel, therefore, is the sum of the kinetic and potential energies (assuming no gravitational effects) as shown below.

E = KE + PE

Since KE ∝ V2), then E ∝ V2 + PE

There are two important concepts that follow from this relationship.

pressure measurement by cathetersAn application of the Bernoulli principle is found when blood pressure measurements are made from within the ascending aorta. As described above, during ventricular ejection, the velocity and hence the kinetic energy of the flowing blood are very high. The instantaneous blood pressure that is measured within the aorta will be significantly different depending on how the pressure is measured. As illustrated to the right, if a catheter has an end-port (E) sensor that is facing the flowing stream of blood, it will measure a pressure that is significantly higher than the pressure measured by a side-port (S) sensor on the same catheter. The reason for the discrepancy in measured pressures is that the end-port measures the total energy of the flowing blood. As the flow stream "hits" the end of the catheter, the kinetic energy (which is high) is converted to potential (or pressure) energy, and added to the potential energy, increasing the pressure energy. The side-port will not be "hit" by the flowing stream, so kinetic energy is not converted to potential energy. The side-port sensor, therefore, only measures the potential energy, which is the lateral pressure acting on the walls of the aorta. The difference between the two types of pressure measurements can range from a few mmHg to over 20 mmHg, depending on the peak velocity of the flowing blood within the aorta.

Revised 12/27/2022

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.