Blood Flow

[MedPR]

Advertisement - Members don't see this ad

When blood flows thru an artery, it exerts pressure on the surrounding arterial wall. Compared to a section of healthy artery of equal size, a narrowed section of diseased artery experiences a:

A. greature pressure on the surrounding arterial wall, because the flow velocity decreases
B. greater pressure on the surrounding arterial wall, because the flow velocity increases
C. smaller pressure on the surrounding arterial wall, because the flow velocity decreases
D. smaller pressure no the surroundnig arterial wall, beacuse the flow velocity increases.

Why isn't B correct?

 

[mcloaf]

Bernoulli's equation states that as flow velocity increases, pressure will decrease (assuming the height at which the fluid is flowing stays the same).

 

[MedPR]

Bernoulli's equation states that as flow velocity increases, pressure will decrease (assuming the height at which the fluid is flowing stays the same).

But don't people with clogged or blocked arteries have hypertension? And doesn't vasodilation decrease blood pressure?

 

[mcloaf]

But don't people with clogged or blocked arteries have hypertension? And doesn't vasodilation decrease blood pressure?

You're applying systemic logic to a question about a localized situation. If you clog an artery fluid will build up ahead of it, increasing pressure in the preceding circulatory system, but within the narrowed portion of the artery, pressure will decrease. For the purposes of this question, you're treating the narrowed artery as a piece of pipe, ignoring what's going on with the rest of circulatory system since those corollary effects are outside the scope of what they're asking.

 

[MedPR]

You're applying systemic logic to a question about a localized situation. If you clog an artery fluid will build up ahead of it, increasing pressure in the preceding circulatory system, but within the narrowed portion of the artery, pressure will decrease. For the purposes of this question, you're treating the narrowed artery as a piece of pipe, ignoring what's going on with the rest of circulatory system since those corollary effects are outside the scope of what they're asking.

Oh I see, that makes sense. Thanks.

 

[MedChad]

I would have thought the same thing medpr!

 

[MedChad]

Actually, I don't see why its still not b due to Poiseuilles law that has a positive, linear, proportional relationship between flow rate and change in pressure. Thoughts??

 

[MedChad]

Answer B would also be supported if we were to use Laplaces law that Tension in the wall = pressure times radius right?

 

[Pisiform]

There is indeed a pressure difference as you said, but its important to realize that for cardiac output to remain constant, a low pressure in the clogged artery means a high pressure in the systemic system (heart) because now heart has to pump blood more forcefully than before to make sure Cardiac Output (volumetric flow rate) remain constant. If they were asking for systemic blood pressure, then you could have said pressure has increases. But they are asking for pressure in the clogged artery.

 

[SaintJude]

Actually, why does Poiseulle's law not apply here? So laminar flow rate of an incompressible fluid along a pipe is proportional to the fourth power of the pipe's radius. So I thought a decreased radius mean a decrease in velocity within the clogged artery. But the velocity increases in the clogged artery?!

And presume I understand why velocity increases in the clogged artery. Does that mean that the average flow velocity in systemic circulation system is lower than that in the narrowed/clogged section of the artery?

Can anyone please suggest additional reading material on this?!...Kaplan won't cut it for this topic.

 

[milski]

Actually, why does Poiseulle's law not apply here? So laminar flow rate of an incompressible fluid along a pipe is proportional to the fourth power of the pipe's radius. So I thought a decreased radius mean a decrease in velocity within the clogged artery. But the velocity increases in the clogged artery?!

And presume I understand why velocity increases in the clogged artery. Does that mean that the average flow velocity in systemic circulation system is lower than that in the narrowed/clogged section of the artery?

Can anyone please suggest additional reading material on this?!...Kaplan won't cut it for this topic.

The key here is that they are asking you to compare the pressure on the walls of the narrow section to the pressure on the walls of a healthy section of the aorta. The mass flow through both sections has to be the same since it's a continuous flow of blood which means that the blood will move faster through that section and the pressure on the walls will be lower than the pressure on the walls of a healthy section.

What Poiseulle's law give you is the total pressure drop that you'll get for a certain flow rate and section of the tube. If you decrease the radius of the tube, the pressure at which you have to pump will be higher but that is not what is being asked in the question.

Here is an example with some made up numbers:

In a healthy artery with no constrictions the pressure is 100 mmHg over all of its length.
In a non-health artery the pressure is 120 mmHg in the healthy sections, 105 mmHg in the congested sections.

The question is asking you to compare the 105 mmHg to 120 mmHg.

 

[MedPR]

So atherosclerosis results in systemic hypertension, so when you measure someone's BP with a cuff and stethoscope, it will be elevated.

But if you were to measure the pressure within any atherosclerotic artery, the pressure would be lower than normal?

 

[milski]

So atherosclerosis results in systemic hypertension, so when you measure someone's BP with a cuff and stethoscope, it will be elevated.

But if you were to measure the pressure within any atherosclerotic artery, the pressure would be lower than normal?

Not lower than normal, just lower than the healthy parts. So if your left hand was healthy but your right had artherosclerosis, you'll measure higher pressure in the left (healthy) hand than in the right one. Both will be higher than normal.

 

[MedPR]

Not lower than normal, just lower than the healthy parts. So if your left hand was healthy but your right had artherosclerosis, you'll measure higher pressure in the left (healthy) hand than in the right one. Both will be higher than normal.

Ok thanks. I understand now.

 

[Maxine450]

When do you use Bernoulli's as opposed to P = pgh as opposed to Poiseille as opposed to A1v1=A2v2. The amount of fluid pressure equations are confusing.

 

[milski]

When do you use Bernoulli's as opposed to P = pgh as opposed to Poiseille as opposed to A1v1=A2v2. The amount of fluid pressure equations are confusing.

Poiseille gives you the pressure differential between the ends of a tube with certain length, radius and specified flow. It lets you solve problems like: here is a tube, you have to move so much fluid over it, how hard do you need to push at one end. Or if you move the fluid that way, how much pressure are you going to lose from one end to the other.

The equation is for a cylindrical tube. If you have a tube with different cros-sections, you'll have to split it in parts and add the pressure differentials for each sub-section.

A1v1=A2v2 - if you have a steady state flow in a tube with crossection A1 moving at speed v1 and the tube changes crossection to A2, how fast is the fluid going to move in that part of the tube.

Bernoulli - same as the second problem, but now you know the pressure in the first part of the tube (with A1 crossection), you can calculate the pressure in the winder/narrower part.

 

[SLC]

So atherosclerosis results in systemic hypertension, so when you measure someone's BP with a cuff and stethoscope, it will be elevated.

But if you were to measure the pressure within any atherosclerotic artery, the pressure would be lower than normal?

This is not the case at all. Atherosclerosis does not cause hypertension, the two are only related in the fact that the predisposing factors for both conditions are similar.

I will say that hypertension puts people who have atherosclerosis at greater risk of complications. Not to get too off topic but when an atheroma develops and matures, the top layer of tissue (called the fibrous cap) is very thin and brittle. An increase in local pressure from systemic hypertension may be all that's needed to tear it open cause a clot and cause any number of scenarios to play out depending on the size, and location of the clot formed. Some potential issues:
1. A worsening of the occlusion if the clot is small and stays attached to the lumen of the artery
2. An embolus that travels downstream and causes an infarction wherever it settles (could be brain and cause stroke, could be lung and cause PE, could be lower limb and cause localized ischemia, could be heart and cause MI etc.)
3. Total occlusion and the clot stays put resulting in local infarction over a larger area than the scenarios above.

But no, do not be mistaken into thinking that the mere presence of atherosclerosis causes hypertension. If anything it's the other way around, hypertension and turbulent blood-flow setting people up for atheroma development. You'll learn all this in your CP block at med school (I'm in the final stretch of mine at the moment).

And to your second point, the pressure downstream from the atherosclerosis would technically be lower, but unless the occlusion was severe I doubt you'd notice it much. Still, some people have diminished pulse pressure right vs. left arm and vice-versa (or could be ankles but you get the idea) and atherosclerosis is a frequent contributer to that condition. There are tests like the ABI that help clinicians evaluate this type of thing, but that's not MCAT material.

Hope that helps!