The Pressure in Bernoulli Equation

[tym]

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Hi Guys, I have a question regarding the pressure in the Bernoulli equation. Hope someone can help. Thank you!! So here comes the question:

When blood flows through an artery, it exerts pressure on the surrounding arterial wall. Compared to a section of healthy artery of equal size, a narrowed section of diseased artery experiences a pressure on the surrounding arterial wall that is:
A. greater, because the flow velocity decreases.
B. greater, because the flow velocity increases.
C. smaller, because the flow velocity decreases.
D. smaller, because the flow velocity increases.

The answer is D. TBR explains it using the Bernoulli equation. High flow speed leads to low pressure. But I thought the pressure in Bernoulli equation refers to the "static pressure," which is the pressure felt by an object/person suspended in the fluid and moving with it. It is not the pressure felt by the outside (in this case, the artery). Correct me if I am wrong. I got this from my notes from physics class. Maybe I misunderstood the concept.
I also read in some SDN posts that people say don't use Bernoulli equation for blood/artery questions. Is that the rule?

Thanks in advance!👍👍

 

[vpanopoulos]

You should use the Bernoulli equation because it help solve a lot of biological sciences questions with a physics outlook.

The pressure in the Bernoulli equation is the pressure against the walls of the vessel. Because the equation is a constant, you have to make sure that when one variable increases (velocity), another will decrease (pressure), and one will be held constant (height).

The reason that velocity increases if because of Continuity via Poiseuille's law. Q = A1v1 = A2v2.
If you take the diagram that I drew, you can see that A1 is very large compared to A2. That means the velocity through A1 will be small compared to v2, which will be large. Q has to also remain constant, so you need to make up the changes.

Hope this helps!

 

[sps27]

Hi Guys, I have a question regarding the pressure in the Bernoulli equation. Hope someone can help. Thank you!! So here comes the question:

When blood flows through an artery, it exerts pressure on the surrounding arterial wall. Compared to a section of healthy artery of equal size, a narrowed section of diseased artery experiences a pressure on the surrounding arterial wall that is:
A. greater, because the flow velocity decreases.
B. greater, because the flow velocity increases.
C. smaller, because the flow velocity decreases.
D. smaller, because the flow velocity increases.

The answer is D. TBR explains it using the Bernoulli equation. High flow speed leads to low pressure. But I thought the pressure in Bernoulli equation refers to the "static pressure," which is the pressure felt by an object/person suspended in the fluid and moving with it. It is not the pressure felt by the outside (in this case, the artery). Correct me if I am wrong. I got this from my notes from physics class. Maybe I misunderstood the concept.
I also read in some SDN posts that people say don't use Bernoulli equation for blood/artery questions. Is that the rule?

Thanks in advance!👍👍

Bernoulli's eqn refers to the total pressure at any point of fluid flow, which is a sum of static pressure + pressure due to fluid flow + pressure due to elevation or gravitational potential energy. This is one of those questions where you know what they are asking by reading the ans choices. So obviously all they are testing here is the concept that if the flow remains steady, the velocity increases because the dia has gone down, it results in low pressure. Mind you, this only holds true if the flow is steady or uniform. The pressure felt by the walls of the arteries will be the total pressure. To me static pressure for arterial flow means hydrostatic pressure + pressure due to solutes in the plasma or osmotic pressure.

Physics laws are not a slam dunk when it comes to flow in arteries. That is my opinion. For one, the flow is never steady, it changes from time to time, secondly the arterial walls have some flexibility not totally rigid. There are other eqns which talk about resistance to arterial flow which I am not too familiar with but if you search for it on the web you will find them.