BP: Neck vs. Calves: Using Formulas to Explain

[justadream]

BP at Neck vs. Calves

TBR page 88 #23

Intuitively (without thinking of formulas), it makes sense that BP at the calves is greater since it experiences all the pressure of the fluid from the upper body.

TBR’s explanation using Bernoulli's formula is somewhat confusing to me.

P + rho*g*h + ½ (rho)v^2 = constant

TBR says that you can take the flow velocity (v) to be somewhat similar between the two situations. If you do this, then the resulting conclusions make sense.

But how can you make that assumption? If the BP at the calves is actually greater, won’t the velocity of the blood in the calves simply compensate for that increased pressure (either go up or down)?

Similarly, if the BP at the neck is lower, won’t the velocity of the blood in the neck simply compensate for the decreased pressure? After all, isn’t velocity is dependent on pressure differences?

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[NextStepTutor_1]

BP at Neck vs. Calves

TBR page 88 #23

Intuitively (without thinking of formulas), it makes sense that BP at the calves is greater since it experiences all the pressure of the fluid from the upper body.

TBR’s explanation using Bernoulli's formula is somewhat confusing to me.

P + rho*g*h + ½ (rho)v^2 = constant

TBR says that you can take the flow velocity (v) to be somewhat similar between the two situations. If you do this, then the resulting conclusions make sense.

But how can you make that assumption? If the BP at the calves is actually greater, won’t the velocity of the blood in the calves simply compensate for that increased pressure (either go up or down)?

Similarly, if the BP at the neck is lower, won’t the velocity of the blood in the neck simply compensate for the decreased pressure? After all, isn’t velocity is dependent on pressure differences?

With regards to your first question - the velocity would be decreasing if the pressure was increasing if all else remained constant. However, this is not the case. Blood is moving down from the upper body to the calves and so "h" in the equation must be decreasing. This will balance out the rise in pressure in order to maintain the equation (and why TBR states that the flow velocity in the neck and in the calves will be somewhat similar).

Again, velocity change would be dependent on pressure change if those are the only two variables, but as this equation shows, they are not.





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