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Question about Pressure

...JR

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J
Joined
Nov 21, 2005
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82
So i'm studying for my vert. phys. test and we're going over circulation. Our lecture notes say that an increase in blood resistance decreases blood flow (which i can understand) and it also increases blood pressure. This last part is what i have a problem with, because earlier in the lecture, it was stated that the greatest resistance occurs in the arterioles due to their diameter, but the arterioles also have the greatest drop in pressure. This contradicts what was said earlier about a greater resistance having a greater pressure, so could anyone who knows about this clear it up? Thank you.

edit: n/m i think the increase in R and P occurs as a result of fatty plaques from atherosclerosis so i can see how the decrease in vessel area would increase the pressure, but does an increase in blood pressure cause an increase in heart rate?

edit2: So my new question is, in terms of blood, does a greater resistance equal a greater blood pressure? Or is the relationship inversely proportional? I would think since the smaller the diamter (A) equals a greater resistance, that it would also equal a greater pressure, but i want to be sure.
 
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question 2: According to a chart looking at the velocity of blood flow in accordance with the total area of the vascular bed, it makes clear that the blood velocity is slowest in the capillaries and largest in the cross sectional area. I don't quite understand this, is it saying that the smaller the area (capillaries) the greater the velocity? Since the smaller the area (diameter), the greater the resistance, this doesn't make any sense. Any help?

p.s.: i would ask my teacher my questions and i have, but he is not as helpful as some might think.
 
Typically when you talk about pressure drop, you are referring to difference of the pressure before and after that tubing. So greater resistance, the greater the pressure drop. As such the smaller tubules, you get the greater pressure drop.

Think of pressure as a voltage and a pressure drop as a voltage potential.

if you have resistor and apply a voltage potential across it, you get a current (a flow). If you get a bigger resistor, you need to apply a greater voltage to get the same current.
 
...JR said:
question 2: According to a chart looking at the velocity of blood flow in accordance with the total area of the vascular bed, it makes clear that the blood velocity is slowest in the capillaries and largest in the cross sectional area. I don't quite understand this, is it saying that the smaller the area (capillaries) the greater the velocity? Since the smaller the area (diameter), the greater the resistance, this doesn't make any sense. Any help?

p.s.: i would ask my teacher my questions and i have, but he is not as helpful as some might think.
Click to expand...
afraid of answering your homework, but think of this
how many cappilaries are there compared to aterioles compared to arteries compared to the aorta?
 
Hello JR

The pressure drop along a "pipe" (vessel) will depend on the flowrate and the resistance at that flowrate.


For a particular, vessel, the a small flowrate means a small pressure drop, a large flowrate means a large pressure drop. Enough pressure at the start is needed to overcome the resistance at that flowrate.


For two vesels with the same flowrate, the smaller diameter, rougher vessel will need a higher pressure at the start because it has more resistance at the same flowrate.

Regards
 
joobz said:
afraid of answering your homework, but think of this
how many cappilaries are there compared to aterioles compared to arteries compared to the aorta?
Click to expand...


thanks for all your help and no this is not homework, but just reviewing for my exam on friday. And so now that you put it in that way it would seem that all of the capillaries put together will have a greater vascular bed area and thus a greater velocity and now it makes sense. thanks again.
 
...JR said:
question 2: According to a chart looking at the velocity of blood flow in accordance with the total area of the vascular bed, it makes clear that the blood velocity is slowest in the capillaries and largest in the cross sectional area. I don't quite understand this, is it saying that the smaller the area (capillaries) the greater the velocity? Since the smaller the area (diameter), the greater the resistance, this doesn't make any sense. Any help?

p.s.: i would ask my teacher my questions and i have, but he is not as helpful as some might think.
Click to expand...

Or think of putting your thumb over the end of a garden hose.
 
...JR said:
thanks for all your help and no this is not homework, but just reviewing for my exam on friday. And so now that you put it in that way it would seem that all of the capillaries put together will have a greater vascular bed area and thus a greater velocity and now it makes sense. thanks again.
Click to expand...
almost

Ignoring vessel permeability, total flow (volumetric flow) will be the same (it has to for conservation of mass reasons)
Volume flow divided by the crosssectional area of the pipe system gives you the velocity.

Here, I do believe that the sum of the crosssectional area of all capillaries is much greater than the crossectional area of the aorta. As such, the velocity in the capillaries is much slower than the aorta.

Now hears a question for you, The residence time of blood in the lungs is tuned to allow optimal transfer of gases. How then, under high cariac output (during exercise) how does the lungs maintain a near constant blood residence time.
 
...JR said:
So i'm studying for my vert. phys. test and we're going over circulation. Our lecture notes say that an increase in blood resistance decreases blood flow (which i can understand) and it also increases blood pressure. This last part is what i have a problem with, because earlier in the lecture, it was stated that the greatest resistance occurs in the arterioles due to their diameter, but the arterioles also have the greatest drop in pressure. This contradicts what was said earlier about a greater resistance having a greater pressure, so could anyone who knows about this clear it up? Thank you.

edit: n/m i think the increase in R and P occurs as a result of fatty plaques from atherosclerosis so i can see how the decrease in vessel area would increase the pressure, but does an increase in blood pressure cause an increase in heart rate?

edit2: So my new question is, in terms of blood, does a greater resistance equal a greater blood pressure? Or is the relationship inversely proportional? I would think since the smaller the diamter (A) equals a greater resistance, that it would also equal a greater pressure, but i want to be sure.
Click to expand...

Dogpile,Google or wiki Bernoullis' Principle.
 
joobz said:
almost

Ignoring vessel permeability, total flow (volumetric flow) will be the same (it has to for conservation of mass reasons)
Volume flow divided by the crosssectional area of the pipe system gives you the velocity.

Here, I do believe that the sum of the crosssectional area of all capillaries is much greater than the crossectional area of the aorta. As such, the velocity in the capillaries is much slower than the aorta.

Now hears a question for you, The residence time of blood in the lungs is tuned to allow optimal transfer of gases. How then, under high cariac output (during exercise) how does the lungs maintain a near constant blood residence time.
Click to expand...

the velocity equation REALLY helped. And for your question: For the heart to maintain a near constant blood residence time, I would say that the velocity needs to be increased so that there is constantly blood to deliver O2 and remove CO2. Although i'm sure i'm missing the point of your question seeing as how the answer i just gave seems a bit too obvious.
 
...JR said:
the velocity equation REALLY helped. And for your question: For the heart to maintain a near constant blood residence time, I would say that the velocity needs to be increased so that there is constantly blood to deliver O2 and remove CO2. Although i'm sure i'm missing the point of your question seeing as how the answer i just gave seems a bit too obvious.
Click to expand...
glad to help
and my question iwas poorly worded.

When the heart rate increases, the blood's Volumetric flow rate increases.
But if you increase the flow rate and nothing else changes, the velocity of blood within the lungs has to decrease. This decrease in time would mean that there would be less time for blood to exchange gas with the air. However, the lungs have a bunch of capillaries that aren't used under rest. When the blood flow increases, these capillaries open up thereby increaseing the available crosssectional area, and keep the velocity of the blood through the lungs constant. A few other things go on as well, but that one I always thought was cool.
 
joobz said:
glad to help
and my question iwas poorly worded.

When the heart rate increases, the blood's Volumetric flow rate increases.
But if you increase the flow rate and nothing else changes, the velocity of blood within the lungs has to decrease. This decrease in time would mean that there would be less time for blood to exchange gas with the air. However, the lungs have a bunch of capillaries that aren't used under rest. When the blood flow increases, these capillaries open up thereby increaseing the available crosssectional area, and keep the velocity of the blood through the lungs constant. A few other things go on as well, but that one I always thought was cool.
Click to expand...

the more i learn about my body and those of other animals, the more i am impressed with nature and how it uses the same parts in different manners sometimes with different organisms.
 
Maximum fluid pressure in any closed circulating system (at constant temperature) is the pump outlet pressure.
Vessel wall friction, fluid rheology and gravity cause pressure losses along the system all the way back to the pump. These losses also vary with flow rate, changing sharply if flow becomes turbulent.
The output pump pressure must be high enough to overcome all the pressure losses at the required flow rate, including any due to thixotropic gellation when the pump starts. (This should not be a problem with blood, as the pump does not normally stop.) If the pump is not rated to move fluid at the required rate while overcoming these pressure losses, you need to reduce the losses (bigger pipes, less viscous fluid), or get a bigger pump.

I'm talking about oil well drilling here, but the physics is much the same. The beauty of the blood system is that the blood vessel size adapts via feedback mechanisms to maintain optimum flow rates without overloading the pump. (Normally). Would that we could do the same on a drilling rig.
 
Well, there are positive and negative displacement pumps, centrifugal pumps... the idea of the blood vessels themselves acting as pump enhancers by varying their elasticity is one people like me would love to see applied in industry.
 
...JR said:
the more i learn about my body and those of other animals, the more i am impressed with nature and how it uses the same parts in different manners sometimes with different organisms.
Click to expand...

And I want my parts back, dammit!:p
 
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