# Physics of blood vessels

by BramhaBull
Tags: blood, physics, vessels
 P: 2 I am taking Physics and Anatomy simultaneously and was confused by something that was taught and seemed contradictory. In Bernoulli's equation (for fluids) I learned that as the cross sectional area of a pipe or tube decreases, the velocity of the fluid increases. And I also learned that the pressure that a fluid exerts on the walls of the tube is inversely proportional to the velocity (PV = constant). In Anatomy, when the blood pressure of a patient was high, he/she was given drugs which dilated their blood vessels. Dilating the blood vessel (increasing cross sectional area), according to Bernoulli's equation, would decrease the velocity of the blood. And if the velocity falls, shouldn't pressure INCREASE due to them being inversely proportional??
 Sci Advisor HW Helper P: 4,300 I don't think the V in the cited equation, PV = constant is the velocity, but rather the volume (I'm thinking about the ideal gas law). Then dilating the blood vessel would increase the volume and therefore reduce the pressure. Try turning on your garden hose. Does it make sense that the more you open the water, the less pressure is on it?
P: 59
 Quote by BramhaBull Dilating the blood vessel (increasing cross sectional area), according to Bernoulli's equation, would decrease the velocity of the blood. And if the velocity falls, shouldn't pressure INCREASE due to them being inversely proportional??
Dilating the blood vessels would increase the cross sectional area and the volumetric capacity of the blood vessels. Like if you increase the volume, and keeping everything constant, the pressure will decrease. Correct me if I'm wrong that for closed or sealed systems like the circulatory system, the volume will affect wall pressures more than Bernoulli effect.

It also had the added benefit of reducing the pumping pressure for the heart due to higher volume of flow.