systolic BP Q

Question I’ve been pondering, from BRS Physio:

(See the question here ).

At which point is systolic blood pressure the highest?

A. Aorta

B. Central vein

C. Pulmonary a.

D. right atrium

E. renal artery

F. renal vein

Per the answers:

In systemic circulation, systolic pressure is actually slightly higher in the downstream ateries (e.g. renal artery) than in the aorta because of the reflection of pressure waves at branch points.

Might anyone be able to clarify this for me, please?

Thanks much!

You would expect the answer to be (A) Aorta. After all, the pressure is generated in the left ventricle, so it makes sense that it would be at its highest immediately after generation.

However, the pressure pulse is a finite length (several inches). As this pulse continues down the descending aorta and encounters branch points (e.g. the renal arteries, the iliac branch), the leading edge of this pressure pulse will reflect off of the transverse walls. This will generate a pressure pulse traveling in the opposite direction. When added to the remaining original pressure pulse, this counterpulse produces even higher pressure than the original.

Think of it this way: If you shout into a long tube, where is the sound the loudest? Probably right at the beginning of the tube where you’re shouting – unless the tube has a kink or sharp bend in it, where the sound wave might reflect back on itself and create a sort of standing wave. This will then have the amplitude of (original wave) + (reflected wave), which if the original wave amplitude has not decayed very much might well be louder than the original.

Remember the rule that pressure is a function of 1/(diameter of the vessel)^4. Therefore the larger the diameter the smaller the pressure and the smaller the diameter the larger the pressure.

Cool, thanks for the clarification, spoxjox and gabe I think I get it, with the bouncing waves and all.