Blood Pressure & Flow (Examkrackers Error?)

[To be MD]

Hello SDN,


On p. 121 of the Systems book for Examkrackers, they give us the following equations relating pressure and flow in the cardiovascular system: Q = ΔP / R (i.e. Flow = Pressure Difference / Resistance)

They they go on to say in the same paragraph and on the side of the page that "this equation demonstrates the Inversely Proportional Relationship between pressure and resistance..."

Check my math: if Q = P/R .... then QR = P .... then P and R must be directly proportional

Differently, if Q is a constant (let's say it's "2"), and ΔP = 2 and R = 1 .... then, if ΔP goes up by 2 to 4, R must also increase (to 2) to keep Q constant (i.e. 2 = 4 / 2) .... that means to me, again, that they are directly proportional to one another.


Does EK Systems have this wrong? Even in their Verbal Reasoning book (p. 17-18), they let you know that two variables are directly proportional if "one is in the numerator and the other is in the denominator when they are on the same side of the equation." That, to me, applies here, and thus.... P and R are directly proportional

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[Astra]

The equation is correct. ΔP = Q x R just like ΔV = IR.

You are right about them being directly proportional

 

[To be MD]

The equation is correct. ΔP = Q x R just like ΔV = IR.

You are right about them being directly proportional

Suh-weet! Thank you for your input.

I also ended up watching the Khan cardiology videos (https://www.khanacademy.org/science...-it-all-together-pressure-flow-and-resistance) if anyone's interested on clarification of this topic. They agreed with us.

They do a much better job. In fact, EK's entire section here is so F'ed because of their incorrect wording. Strongly recommend, SDNers, to avoid p. 121-122 and learn the process from Khan.

 

[aldol16]

They they go on to say in the same paragraph and on the side of the page that "this equation demonstrates the Inversely Proportional Relationship between pressure and resistance..."

Depends on what pressure you're talking about. If you're talking about the pressure differential, then that equation shows that pressure differential and resistance are directly proportional. If you're talking about final pressure, then again, final pressure and resistance are directly proportional. However, if you're talking about initial pressure, then initial pressure and resistance are indeed inversely proportional. As you increase initial pressure, the numerator goes down and you have to compensate by decreasing resistance.

 

[To be MD]

Depends on what pressure you're talking about. If you're talking about the pressure differential, then that equation shows that pressure differential and resistance are directly proportional. If you're talking about final pressure, then again, final pressure and resistance are directly proportional. However, if you're talking about initial pressure, then initial pressure and resistance are indeed inversely proportional. As you increase initial pressure, the numerator goes down and you have to compensate by decreasing resistance.

I guess they don't specify which is which, and that ruins a lot of the section.