Blood Pressure vs Physics

[labqi]

Hey guys! I am just having a little trouble understanding some concepts when referring to blood pressure and dynamic fluids.

I know that BP = CO x PResistance (thus when Resistance increases, pressure goes up)
Based on A1V1 = A2V2, Area increases as velocity decreases. Furthermore, at the same level, via Bernouilli's equation...Pressure increases as Velocity decreases.

Knowing that, I also know that constricting the blood vessel (Area decreases) leads to an increase in BP.

With all that in consideration (if it is correct), would it be true that increasing area (dilating the BV) increase BP (I know this is not the case) but I can't seem to get the equations and relationships I addressed above to connect.
In essence, Area increases, Pressure Increases, as velocity decreases - for physics.
In vasculature, area decreasing, increases pressure.

I know I am trying to get things to connect, which may be out of scope, but I just want to know where I am making the mistake.

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[DrknoSDN]

imo... The mistake is trying to relate two very different systems. (This response is mostly opinion driven)

Human blood pressure depends on a number of factors that are not present in Bernoulli's equation such as the elastic potential energy capacity of arteries, variation in volume, having non-horizontal flow, and most importantly multiple pathways.
I believe the equations your using from the Bernoulli Eqn requires a continuum where fluid flows from one chamber to the next and differences in chambers properties can predict the properties of the fluid in the chamber. Circulation is not like that because it is highly branched and if you track a blood cell through the circulation it can take many different paths.

The B eqn can't predict when a decrease in area of one parallel path will alter the fluid properties of the other parallel path. This gets more complicated because of how massively branched circulatory flow is, and you also can't simple vaso-dilate everything because only certain vessels have that smooth muscle capability. Also the venous side functions as a very elastic reservoir. So you're basically attaching a stretchable elastic container to a bernoulli pipe making those equations much less useful.

Blood flow/pressure is more like a circuit with resistance. If you dilate blood vessels you would lower the resistance to flow of that parallel path which lowers the resistance of the system which would lower the pressure. Circulatory systems differ too much from simple physics setups to apply Bernoulli logic to predict changes in pressure due to changes in area. Thinking about blood pressure as resistance to flow will give you an intuitive answer.

Lastly, as long as you know how both systems function on their own, you will be fine for the MCAT.

 

[Czarcasm]

https://www.google.com/search?q=stu...ssure+bernoulli+site:forums.studentdoctor.net

 

[DrknoSDN]

https://www.google.com/search?q=student doctor blood pressure bernoulli site:forums.studentdoctor.net&espv=210&es_sm=93&biw=1746&bih=869#q=blood pressure bernoulli site:forums.studentdoctor.net

lol @ Czarcasm. Thank You. I knew that a lot of people try and make the connection but I didn't realize there was that many posts on SDN about it.
Some older replies are much more thorough than mine... Hah

 

[sps27]

Here is another link on the same topic that I think might be helpful......

http://forums.studentdoctor.net/threads/vasoconstriction-and-blood-flow.1029181/#post-14392834

 

[Czarcasm]

lol @ Czarcasm. Thank You. I knew that a lot of people try and make the connection but I didn't realize there was that many posts on SDN about it.
Some older replies are much more thorough than mine... Hah

No probs! I remember having this exact question. Unfortunately, I've neglected this topic for many years (since my initial testing) and so I can't quite explain it myself. I do know there were some great explanations by other SDNer's though