Question About blood pressure

  • #1
red65
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Hello everyone, concerning blood pressure , we know that the pressure exerted by moving fluid has two components: a dynamic, flowing component that represents the kinetic energy of the system, and a lateral component that represents the hydrostatic pressure (potential energy) exerted on the walls of the system.
When we are measuring blood pressure are we measuring hydrostatic pressure only?
 

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  • #2
Merlin3189
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My understanding of BP is that it is measured by stopping the blood flow with a cuff - like a tourniquet. Then as the cuff pressure is reduced, you listen for the resumption of some flow and the resumption of flow for the whole cycle. The first of these would seem to me to represent purely the hydrostatic pressure.
Since both measurements are achieved by measuring the pressure perpendicular to the flow, I suspect neither reflects a kinetic element. The Pitot tube has holes facing the flow and perpendicular to the flow. The first hole gives the total pressure (stagnation pressure) and I assume the other measures the hydrostatic pressure, which is subtracted to give the kinetic pressure.
But as you can see, I don't actually know anything about it, other than having it measured.
 
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  • #3
Godot_
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Late but...

Both.

The pressure is omnidirectional. As mentioned above, the Riva-Rocci-BP-method doesn't measure pressure directly, but by compressing and thus occluding, later obstructing the upper arm arteries. This causes turbulent flow behind the narrowed part is auscultated, either using a stethoscope when you do it manually, or vibration sensors on the automatic gadgets. And when the arterial pressure goes down during the cardial filling phase - diastolic drop - then the flow throught the compressed part of the artery ceases, as does the turbulence.

The elasticity of major arteries smoothes the pulse front, and the blood flow isn't fast enough to exert a kinetic, pressure due to its impulse. Blood is an Newtonian, non-compressible liquid (in first approximation). The ejection phase of the left heart chamber only is roughly 30-40% of the complete cycle time. While the heart expulsion force does contribute, the main regulation mechanism, however, is located in the precapillary sphincter, so in the very end of the arterial system.

Also, the flow speed is in the order of 100cm/sec in the aortaascendens, before the first branchings in the arc, slowing down with diameter (Hagen-Poiseuille Equation applies), while the pulse wave can spread faster. As in addition the total sum of the cross-secctional area of the arterial system increases, flow decreases quite rapidly - the major 2nd-stage arteries branch off before the bifurcation of the illiac arteries, and - thanks to vessel wall elasticity - can even revert in these, being directed towards the heart.

Then, the

Also, while not the standard, BP often enough is measured invasively - and not via a pitot tube, but via a standard plastic cannula. This is done mostly on patients during anaesthesia or some patients in intensive care.

Lastly, while you're sitting or standing upright, the arterial blood pressure in the legs is higher in the legs, and lower in the skull (as is the venous pressure, too). Which makes it so relevant that the cuff is at the heart's height, by placing the cuff there - or holding the measuring gadget in front to your chest. (OK, this would be lege artis, patients don't always observe this during self-measuring...)

If you want to understand that better, I'd advise looking into a medical textbook on physiology, chapter "curculation". Or you might want to take a peek here, "UNIT IV" it is.
 
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  • #4
dlgoff
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  • #5
berkeman
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My understanding of BP is that it is measured by stopping the blood flow with a cuff - like a tourniquet. Then as the cuff pressure is reduced, you listen for the resumption of some flow and the resumption of flow for the whole cycle. The first of these would seem to me to represent purely the hydrostatic pressure.
Since both measurements are achieved by measuring the pressure perpendicular to the flow, I suspect neither reflects a kinetic element.
Not exactly... :wink:

Physiology, Korotkoff Sound
https://www.ncbi.nlm.nih.gov/books/NBK539778/
 
  • #6
pinball1970
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Hello everyone, concerning blood pressure , we know that the pressure exerted by moving fluid has two components: a dynamic, flowing component that represents the kinetic energy of the system, and a lateral component that represents the hydrostatic pressure (potential energy) exerted on the walls of the system.
When we are measuring blood pressure are we measuring hydrostatic pressure only?
You joined Xmas Eve? That's commitment to learning!

Read the refs from the posters all good.
Do not look at these things in isolation, blood pressure measurement has a lot of physiology and anatomy surrounding it. All intricate stuff.
I would also suggest you check out when things go wrong, high blood pressure and low, pathology and treatment.
 

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