In a study to measure BP, two students Rakesh and Arif conduct a study on a dog. Rakesh measures BP using a mercury sphygmomanometer on the right femoral aery and Arif measures using a pressure transducer and pulse tracing on the left femoral aery. The mean aerial pressure for both Rakesh and Arif is the same i.e.100 mmHg. After 5 minutes of injection of adrenaline, Rakesh measures blood pressure 130 mmHg and Arif as 120 mmHg. The difference of 10 mmHg is explained by:

B i.e. Falsely low values at high pressure in pulse tracing Bernoulli's principle: In a tube or a blood vessel the total energy (kinetic energy offlow+ potential energy) is constant When a vessel is narrowed (e.g. in adrenalin administration), the velocity of flow in the narrowed poion increases & so the kinetic energy (1/2 mv2). As the total energy is constant (Bernoulli's principle), potential energy decreases in narrowed segment. Therefore when the velocity of flow is increased the lateral pressure distending the wall at the constriction is decreased and narrowing tends to maintain itself. Pressure transducer measures aerial pressure directly, so it gives falsely low value after adrenaline injection (at high pressure).