Blood pressure and vascular distance from the heart

[Lys0some]

Hello everybody. I have seen many people asking questions on this forum about the reason to why blood pressure drops as you get farther from the heart.

 

But honestly, not a single answer satisfied me. My major is physiology, so it's quite funny that I still don't know this "trivial" detail. What is the physics behind the drop in blood pressure?

 

Because I was not satisfied with any of the answers, such as,

 

- Capillaries have smaller diameters (because diameter shouldn't make a difference locally: decreasing arteriolar diameter increases mean arterial pressure, not arteriolar pressure, because the blood in the arteries faces higher resistance ahead of it so the arteries experience higher hydrostatic pressure and because the overall vascular volume in the body decreases, increasing blood pressure)

- The blood "losing energy" as it travels across the long resistance journey (because that just doesn't make sense)

- The arteries are closest to the pump (that's a cool answer, but doesn't explain anything)

- Velocity drops in capillaries (cool, but shouldn't that increase pressure according to Bernoulli's principle? Besides, blood travels faster in veins)

- Capillaries have higher resistance (well the total cross sectional area of capillaries is actually greater than arteries and greater than veins and resistance adds in parallel)

 

So, I decided to come up with my own explanation and I hope someone corrects me if I am wrong.

 

The only way I could make sense out of this is by this reason, imagine a long tube that gets wider in the middle, then back to normal diameter at the end, and a pump at one end of it. When the pump pumps the fluid through, the initial part of the tube is facing a large volume of fluid in front of it that is spread along a long distance, as you proceed through the tube, there is progressively less fluid in front of you. So if we consider the pressure a result of a force at one of the end of the tube and another at the other end, then the weaker the force (impedance) in front of you, the less pressure you would experience. As you have less fluid in front of you, you have less mass to push against a resistant tube, so your pressure drops. The further you move, not only you have less volume of fluid to push, but also less resistance in series, since whatever is left of the tube is getting shorter and shorter. The only issue with this explanation, is that the circulatory system is a closed loop. But again, during diastole, the right atrium pressure is pretty much almost zero (3-8 mmHg), so we can almost assume the tube is open at one end letting with a waterfall of blood pouring in the heart.

 

So can anyone please point out issues in my explanation?

 

Thanks