What is blood pressure?

[zogoto]

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EK Bio says that blood pressure is that hydrostatic pressure of the blood (pressure pushing outwards). Does that mean that if the speed of the blood through a capillary increases, then the hydrostatic pressure decreases (Bernoulli's Equation), so then fluid will enter from the interstitium, etc.?

But, it seems that if vasoconstriction occurs, we would say that the capillary now has increased blood pressure, not decreased (for example angiotensin causes vasoconstriction, so if you have high blood pressure, you take ACE inhibitors to relieve the vasoconstriction and drop the blood pressure). But vasoconstriction causes the blood in a capillary to flow faster, DECREASING the blood pressure, right?

I'm confused 😕

 

[not respect]

yeah this has always confused me too, one thing I know is that vasoconstriction actually SLOWS the blood velocity, so it makes sense that the blood pressure increases, but I can't really explain it further than that 😛 if anyone knows I'm interested too!

 

[MLT2MT2DO]

I'm confused as to why you think vasoconstriction causes the blood to flow faster. The main variable in how fast blood flows is the diameter...the larger the vessels diameter the faster the blood will flow. So vasoconstriction actually slows blood flow and INCREASES blood pressure.

 

[spyderracing32]

Vasoconstriction = inc. resistance = dec. flow rate = inc. BP

Increasing the volume of fluid does essentially the same thing which is why the renin-angiotensin-aldosterone system works so well to increase your BP. Low BP stimulates secretion of renin which stimulates secretion of angiotensin. Angiotensin constricts blood vessels and triggers the release of aldosterone which makes you retain water, and thus increasing your blood volume. Inc. blood volume + Dec. vessel diameter = blood flow resistance double whammy.

 

[not respect]

that makes a lot of sense, but I previously thought of it as an ideal fluid flow so that's where the confusion came in, but I guess it isnt.

 

[zogoto]

Vasoconstriction = inc. resistance = dec. flow rate = inc. BP

Increasing the volume of fluid does essentially the same thing which is why the renin-angiotensin-aldosterone system works so well to increase your BP. Low BP stimulates secretion of renin which stimulates secretion of angiotensin. Angiotensin constricts blood vessels and triggers the release of aldosterone which makes you retain water, and thus increasing your blood volume. Inc. blood volume + Dec. vessel diameter = blood flow resistance double whammy.

So how do you balance the effect resistance with the effect of decreased area? By the continuity equation, if the area is halved, the velocity should double. So even if the resistance is 50% more, isn't the overall speed higher? It has to be because the blood behind it is still coming, and pushes it through right? Then that means that the hydrostatic pressure is less.

But I just want to clarify one thing for sure: the definition of hydrostatic pressure is definitely right, right? I feel like I've had to use definitions of blood pressure before in passages that had higher velocity in capillaries related to higher blood pressure, because I guess if you put something in the capillary or something, then it would be pushed out with higher pressure.

And is the arterial pressure higher in the legs or the arms? Does the pgh term of Bernoulli's play here? The legs are farther away so the blood would be slower, resulting in higher hydrostatic pressure? But couldn't it gain some hydrostatic pressure by falling a distance h?

 

[nsx]

The legs are farther away so the blood would be slower

Then there's that gravity thing...

Having a hard time following your logic.

 

[nsx]

Ok I think I finally understood this.

So how do you balance the effect resistance with the effect of decreased area? By the continuity equation, if the area is halved, the velocity should double. So even if the resistance is 50% more, isn't the overall speed higher? It has to be because the blood behind it is still coming, and pushes it through right?

No, blood doesn't flow like that. If it did, people would bleed to death pretty quickly. Think about the body's first defense against bleeding. Blood can't move as fast through constricted vessels.

But I just want to clarify one thing for sure: the definition of hydrostatic pressure is definitely right, right? I feel like I've had to use definitions of blood pressure before in passages that had higher velocity in capillaries related to higher blood pressure, because I guess if you put something in the capillary or something, then it would be pushed out with higher pressure.

Same principle. Blood can't flow as fast through a smaller passageway.

And is the arterial pressure higher in the legs or the arms? Does the pgh term of Bernoulli's play here? The legs are farther away so the blood would be slower, resulting in higher hydrostatic pressure? But couldn't it gain some hydrostatic pressure by falling a distance h?

Falling????? It could also be changed by something as simple as elevating a leg.

It's cool that you're thinking critically, but you need to cover all the bases

 

[spyderracing32]

And is the arterial pressure higher in the legs or the arms? Does the pgh term of Bernoulli's play here? The legs are farther away so the blood would be slower, resulting in higher hydrostatic pressure? But couldn't it gain some hydrostatic pressure by falling a distance h?

Your blood isn't going to fall much if at all. There are one-way valves in the veins that keeps gravity from doing its work. If you've ever seen anyone with bad diabetes and swollen legs it is because of a comorbid cardiopathy that stops these valves from working and then fluid does pool in the feet/legs.

 

[Oceaner]

Bernuolli's principle
http://en.wikipedia.org/wiki/Bernoulli's_principle
which applies to ideal fluids, says that as a fluid is moving from an area to another, energy in conserved (as velocity of a fluid increases, pressure decreases, and vice versa): so if you can imagine a pipe that gets smaller in size in it's second section:

duct 1
cross sectional area: smaller
fluid pressure: lower
fluid velocity: higher

duct 2
cross sectional area: larger
fluid pressure: higher
fluid velocity: lower


In an ideal fluid
-The fluid is always fluid -- in the case of blood, once you go to the capillaries, the red blood cells are passing through one by one, acting essentially as a stream of solid particles
-fluid flow is inviscid -Energy is conserved -- you lose kinetic energy (dissipated as heat) when blood flows

(http://forums.studentdoctor.net/archive/index.php/t-480138.html)

The heart is a pump working in a closed circuit and blood is not an ideal fluid, there is energy loss at each step of a way, for this reason as we go from:

artery
cross sectional area: smaller
fluid pressure: highest
fluid velocity: highest

capillary
cross sectional area: larger
fluid pressure: lower
fluid velocity: lower

vein
cross sectional area: smaller
fluid pressure: higher
fluid velocity: higher


to understand the local factors you need to understand blood flow

-Change in pressure = pressure difference along two points in a vessel
-for blood flow = the pressure difference is considered to be the difference between Pa (arterial) and Pv (venuous)


F (flow) = delta P (Pa-Pv) / R (resistance)

Of course, one big thing to notice is that there is a variation between P in arteries and P in venuoles otherwise the system would be 'static' and not 'flow'. So when blood is distributed into different vessels, the pressure pushing from behind is the same (Pa-Pv), the length of the vessels are the same, any change in flow is pretty much determined by the lcoal resistance

Resistance to blood flow within a vascular network is determined by the
1) size of individual vessels (length and diameter),
2) the organization of the vascular network (series and parallel arrangements),
3) physical characteristics of the blood (viscosity, laminar flow versus turbulent flow),
4) extravascular mechanical forces acting upon the vasculature.
http://www.cvphysiology.com/Hemodynamics/H002.htm

Taking into consideration the simple diameter, obviously something that's wider (less R) is going to allow for greater flow, and something that is less wide (greater R) reduces the flow.

VASOCONSTRICTION:
- narrowing of the blood vessels
- smaller area
- less flow
- leads to high blood pressure

VASODILATION:
- widening of the blood vessels
- larger area
- more flow
- leads to low blood pressure

this is elegantly displayed by Poiseuille's Law
"relates the rate at which blood flows through a small blood vessel (Q) with the difference in blood pressure at the two ends (P), the radius (a) and the length (L) of the artery, and the viscosity 👎 of the blood."
http://math.arizona.edu/~maw1999/blood/poiseuille/

The law describes a "slow viscous incompressible flow through a constant circular cross-section"

Q = (k * P * r ^ 4)/(n * l)

where "Q is flow, P is the pressure difference, r is the radius, n is viscosity, l is length, and k is just a constant."

essentially relating Q (F), P, r, so while higher pressure leads to greater flow rate and so on and so forth, radius by far has the most important role (a good site for this is http://hyperphysics.phy-astr.gsu.edu/Hbase/ppois2.html)

why is there a pressure change? to put it simply because of homeostasis. The blood system is a circuit with a finite amount of blood, and if something is obstructing its way, the blood is going to build up behind in order to allow for the same 'flow' to occur. In the same way if the system is too dilated, the pressure difference is going to drop to bring the system back to its ideal condition.


phew... that took a long time.


feel free to contact me if you have any other Qs (and if anyone catches anything wrong here please let me know)

 

[medicant]

I think this confusion stems from a confusion between Ideal and real situations. Bernoulli's equation only applies in very specific situations. Notably, you can only use it when the fluid is not-viscous and when flow is smooth. Blood is most certainly viscous, and the flow is not laminar (smooth) when atherosclerosis and clots are present. The equation relating flow rate to the change in pressure and resistance is a more general equation describing incompressible flow. Most differences have to do with resistance that increases significantly with decreased size, mostly do to changes in viscosity. Finally, lets not forget that vessels are elastic for the most part, and this affects the continuity equation that many people have been quoting.

 

[medicant]

Bernuolli's principle
http://en.wikipedia.org/wiki/Bernoulli's_principle
which applies to ideal fluids, says that as a fluid is moving from an area to another, energy in conserved (as velocity of a fluid increases, pressure decreases, and vice versa): so if you can imagine a pipe that gets smaller in size in it's second section:

duct 1
cross sectional area: smaller
fluid pressure: lower
fluid velocity: higher

duct 2
cross sectional area: larger
fluid pressure: higher
fluid velocity: lower


In an ideal fluid
-The fluid is always fluid -- in the case of blood, once you go to the capillaries, the red blood cells are passing through one by one, acting essentially as a stream of solid particles
-fluid flow is inviscid -Energy is conserved -- you lose kinetic energy (dissipated as heat) when blood flows

(http://forums.studentdoctor.net/archive/index.php/t-480138.html)

The heart is a pump working in a closed circuit and blood is not an ideal fluid, there is energy loss at each step of a way, for this reason as we go from:

artery
cross sectional area: smaller
fluid pressure: highest
fluid velocity: highest

capillary
cross sectional area: larger
fluid pressure: lower
fluid velocity: lower

vein
cross sectional area: smaller
fluid pressure: higher
fluid velocity: higher


to understand the local factors you need to understand blood flow

-Change in pressure = pressure difference along two points in a vessel
-for blood flow = the pressure difference is considered to be the difference between Pa (arterial) and Pv (venuous)


F (flow) = delta P (Pa-Pv) / R (resistance)

Of course, one big thing to notice is that there is a variation between P in arteries and P in venuoles otherwise the system would be 'static' and not 'flow'. So when blood is distributed into different vessels, the pressure pushing from behind is the same (Pa-Pv), the length of the vessels are the same, any change in flow is pretty much determined by the lcoal resistance

Resistance to blood flow within a vascular network is determined by the
1) size of individual vessels (length and diameter),
2) the organization of the vascular network (series and parallel arrangements),
3) physical characteristics of the blood (viscosity, laminar flow versus turbulent flow),
4) extravascular mechanical forces acting upon the vasculature.
http://www.cvphysiology.com/Hemodynamics/H002.htm

Taking into consideration the simple diameter, obviously something that's wider (less R) is going to allow for greater flow, and something that is less wide (greater R) reduces the flow.

VASOCONSTRICTION:
- narrowing of the blood vessels
- smaller area
- less flow
- leads to high blood pressure

VASODILATION:
- widening of the blood vessels
- larger area
- more flow
- leads to low blood pressure

this is elegantly displayed by Poiseuille's Law
"relates the rate at which blood flows through a small blood vessel (Q) with the difference in blood pressure at the two ends (P), the radius (a) and the length (L) of the artery, and the viscosity 👎 of the blood."
http://math.arizona.edu/~maw1999/blood/poiseuille/

The law describes a "slow viscous incompressible flow through a constant circular cross-section"

Q = (k * P * r ^ 4)/(n * l)

where "Q is flow, P is the pressure difference, r is the radius, n is viscosity, l is length, and k is just a constant."

essentially relating Q (F), P, r, so while higher pressure leads to greater flow rate and so on and so forth, radius by far has the most important role (a good site for this is http://hyperphysics.phy-astr.gsu.edu/Hbase/ppois2.html)

why is there a pressure change? to put it simply because of homeostasis. The blood system is a circuit with a finite amount of blood, and if something is obstructing its way, the blood is going to build up behind in order to allow for the same 'flow' to occur. In the same way if the system is too dilated, the pressure difference is going to drop to bring the system back to its ideal condition.


phew... that took a long time.


feel free to contact me if you have any other Qs (and if anyone catches anything wrong here please let me know)

I disagree with your statement that it is somehow intuitively obvious that a decreased radius should lower the 'flow'. First of all, the continuity equation, which is as close to intuition as you get in fluid dynamics, would actually predict that while 'flow rate' would stay the same, 'velocity' would increase to compensate. I think you have also been confusing flow and fluid velocity, which are not equivalent.

 

[Oceaner]

I disagree with your statement that it is somehow intuitively obvious that a decreased radius should lower the 'flow'. First of all, the continuity equation, which is as close to intuition as you get in fluid dynamics, would actually predict that while 'flow rate' would stay the same, 'velocity' would increase to compensate. I think you have also been confusing flow and fluid velocity, which are not equivalent.

hemodynamics is entirely different from idea fluid dynamics for the reasons I stated above

Flow = (k * P * r ^ 4)/(n * l)

smaller radius = less flow 👍

 

[zogoto]

The heart is a pump working in a closed circuit and blood is not an ideal fluid, there is energy loss at each step of a way, for this reason as we go from:

artery
cross sectional area: smaller
fluid pressure: highest
fluid velocity: highest

capillary
cross sectional area: larger
fluid pressure: lower
fluid velocity: lower

vein
cross sectional area: smaller
fluid pressure: higher
fluid velocity: higher

Ok so if the fluid is going fastest in the artery, then shouldn't the pressure be the least? Sorry I'm still confused. 😕

 

[zogoto]

Also, on one of the AAMC tests, it asks if the arterial pressure is lower or higher in the legs compared to the arms. The answer is that it is is higher, because of the gravity thing. But the legs are farther from the heart so the blood will be moving slowly. Isn't that a more important reason (slow moving blood = high pressure).

 

[Oceaner]

Ok so if the fluid is going fastest in the artery, then shouldn't the pressure be the least? Sorry I'm still confused. 😕

no because the vascular system is not an ideal system...

 

[Oceaner]

Also, on one of the AAMC tests, it asks if the arterial pressure is lower or higher in the legs compared to the arms. The answer is that it is is higher, because of the gravity thing. But the legs are farther from the heart so the blood will be moving slowly. Isn't that a more important reason (slow moving blood = high pressure).

No the would be the same in all the arteries if the system was horizontal, such as when you are lying down.

But when in an upright position because of gravity we have a pressure gradient throughout the body (and a blood conc gradient throughout the lungs for anyone who is curious). It's about 80mmHG from top to bottom, with the blood pressure in the legs generally 10%-20% higher than those of arms.

When there is increased pressure in the blood, the fluid is going to filter out, as well the blood is going to just lie around in the veins and not go up as it should and because of this for example when people stand up for very long period their feet start to swell.

For this reason the veins are structurally different than the arteries. Whereas the arteries pressure is produced by the force of the heart, the blood in veins moves largely by muscle pumps and respiratory pumps, otherwise the blood would become stagnant and never return to the heart. So basically even the slightest movements helps pump blood up towards the heart.

 

[nsx]

no because the vascular system is not an ideal system...

Zing

 

[Excelsius]

...
In an ideal fluid
-The fluid is always fluid -- in the case of blood, once you go to the capillaries, the red blood cells are passing through one by one, acting essentially as a stream of solid particles
-fluid flow is inviscid -Energy is conserved -- you lose kinetic energy (dissipated as heat) when blood flows

That's probably all you need to know. The blood is not an ideal fluid. Even gases are not ideal and in the real world ideal gas law is not always used (van der Waals is used instead).

 

[zogoto]

Ok I think I know what is confusing me. The EK Bio book shows a graph where BP decreases from arteries to capillaries to veins. But if I go by what is said here, the blood pressure should be highest in the capillaries since the blood is going the slowest right?

 

[wrathofgod64]

basically this mantra is right. Memorize it.

artery
cross sectional area: smaller
fluid pressure: highest
fluid velocity: highest

capillary
cross sectional area: larger
fluid pressure: lower
fluid velocity: lower

vein
cross sectional area: smaller
fluid pressure: lowest
fluid velocity: higher

Fluids always flow from high pressure to low pressure. So pressure decreases from the aorta to the capillaries. The blood velocity is slowest in the capillaries to allow more time for diffusion and exchange of nutrients and wastes. Seems like velocity increases from capillaries to veins, but pressure decreases even more, until it's almost 0 by the time it reaches the right atrium from the venae cavae.

 

[Excelsius]

I don't think anyone said that here. EK is right. Ok, think of it this way - the largest artery in your body is the aorta. It has the thickest walls. The reason it has those walls is to support the enormous blood pressure. Now, would you say that the tiny capillaries would have much more pressure or much less pressure than the aorta? It is the latter. Again, you would be right if we were talking about an ideal fluid in an ideal system. However, in the body, as the blood reaches the capillaries, the energy of the flow is lost , whether due to the elasticity of the blood vessels or the constant leakage of fluid from the blood and vice versa (e.g., lymphatic).

 

[zogoto]

Wait when you say fluid pressure is that the sum of the pressure going forward (the dynamic pressure, given by 1/2pv^2) and the hydrostatic pressure? Or is it just the hydrostatic pressure? Fluids don't flow just in the way of decreasing hydrostatic pressure, right? I'm not sure about that though.

 

[zogoto]

basically this mantra is right. Memorize it.

artery
cross sectional area: smaller
fluid pressure: highest
fluid velocity: highest

capillary
cross sectional area: larger
fluid pressure: lower
fluid velocity: lower

vein
cross sectional area: smaller
fluid pressure: higher
fluid velocity: higher

Fluids always flow from high pressure to low pressure. So pressure decreases from the aorta to the capillaries. The blood velocity is slowest in the capillaries to allow more time for diffusion and exchange of nutrients and wastes. The velocity and pressure then increase in the veins because of decreased cross-sectional area and the need to overcome gravity.

EK's graph says that veins have lower pressure than capillaries.....

 

[wrathofgod64]

ur right, i messed up and editted my post

 

[Oceaner]

Thanks for catching that. (I was thinking of 'increase' in context of something else, at bottom)

You're right regarding vein pressure, and it being higher in the capillaries (makes sense too right, given that blood pools in the veins in the case of the example I brought up), and I imagine flow would be physiologically problematic if both sides of the capillaries had higher pressure than inside (I think we'd have severe edema). The loss in pressure is again due to the fact that blood becomes 'particulate' in capillaries (instead of fluid) and that's why veins need their own pumps because very little of the pressure from the heart remains.

here's an image which might be helpful btw in context of previous discussions:
http://www.rci.rutgers.edu/%7Euzwiak/AnatPhys/Blood_Vessels_files/image018.jpg
and here's a good site: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Circulation2.html describing pressure in simple terms

oh yeah one thing I should add, note that the BP from from the beginning of 1cm of the artery is for all intents and purposes the same as the end of that cm (ignoring the effects of gravity, on an ideal horizontal system), while in capillaries, due to exchange a significant drop in pressure occurs (hydrostatic and osmotic pressure), if anyone doesn't understand how/why let me know and I'll explain.

**one question from me: does anyone know how the muscle pumps affect the pressure in the veins, they increase the pressure right... so that the blood flows back to the heart? do review books not consider this? they wouldn't ask something like this on the MCAT? (so is it same to says the venous pressure decreases -loss of force from heart- than increase -introduction of new pumps-)

 

[Excelsius]

EK's graph says that veins have lower pressure than capillaries.....

Didn't catch that. Another way to think: Fluid goes from high pressure to low pressure. So imagine what would happen if veins had higher pressure than capillaries:

1. If the pressure of the veins was the same as the pressure in capillaries then the blood would not flow. The pressure would be balanced.
2. If the pressure in veins was higher than in capillaries, then the blood would flow in the reverse direction. Still, this would not happen because veins have valves that prevent any reverse flow. Thus, we'd end up with no circulation again.


I hadn't actively thought about all these little details, but I can see how explaining these things helps one be more conscious about the systems and understand them better. If you take physics, it helps a lot in terms of breaking down systems and analyzing them part-by-part.

 

[Dirt]

You guys seem to be missing the key point here. All blood going out goes through the aorta, which is far larger than a capillary. The thing is, when talking about pressures and areas, you have to consider all the capillaries together, so, the cross-sectional area at the level of the

capillaries >>>>>>>>>>>>>>>>> aorta.

Therefore pressure is much lower and velocity is much lower.

 

[Excelsius]

Thanks. That's another great way to think about this. Very insightful physically.

 

[Oceaner]

You guys seem to be missing the key point here. All blood going out goes through the aorta, which is far larger than a capillary. The thing is, when talking about pressures and areas, you have to consider all the capillaries together, so, the cross-sectional area at the level of the

capillaries >>>>>>>>>>>>>>>>> aorta.

Therefore pressure is much lower and velocity is much lower.

I don't think anyone missed that 😉 it's just the normal laws didn't agree. if it wasn't hemodynamics and for example engineered tubes that wouldn't work so well.

 

[Vihsadas]

I don't think anyone missed that 😉 it's just the normal laws didn't agree. if it wasn't hemodynamics and for example engineered tubes that wouldn't work so well.

That's exactly right. According to bernoulli's principle which applies only to ideal fluid flow and therefore NOT the blood flow: larger Cross-sectional area = higher gravitational pressure, smaller crossectional area = lower pressure. It's counterintuitive, but that's the truth. Well anyway, here are my thoughts on the subject:

http://forums.studentdoctor.net/showpost.php?p=6695606&postcount=3
http://forums.studentdoctor.net/showpost.php?p=6883225&postcount=3

 

[zogoto]

Vihsadas, based on that second post you linked, how does vasoconstriction increase blood pressure? If you constrict, then you said you increase velocity. This would usually decrease blood pressure, right? But what about the effects of the extra viscosity (more surface area to volume ratio now)? Wouldn't this cause the pressure to drop also, just like when going from arteries to capillaries (now it's just taking it to a more extreme level)?

 

[TheBoondocks]

Vihsadas, based on that second post you linked, how does vasoconstriction increase blood pressure? If you constrict, then you said you increase velocity. This would usually decrease blood pressure, right? But what about the effects of the extra viscosity (more surface area to volume ratio now)? Wouldn't this cause the pressure to drop also, just like when going from arteries to capillaries (now it's just taking it to a more extreme level)?

The reason this is the case is because blood pressure refers to hydrostatic pressure. Hydrostatic pressure is the weight of unmoving substance. Hydrostatic pressure is proportional to blood volume. This is different from dynamic pressure which accounts the movement. When you constrict the blood vessels, you are LOWERING the flow. Consequently, if there is less flow, it means that their is less blood and HYDROSTATIC pressure is reduced. This is why when blood pressure is LOW, the body increases water reabsorbtion to increase the VOLUME of blood to increase pressure. Again, you need to remember that this blood isn't the typical liquid.

remember this, that BP ∝ CO/r^4. R is the radius. When the blood vessel is constricted, the radius becomes smaller, which assuming constant Cardiac Output, increases blood pressure alone. However, the sympathetic nervous system generally increases CO as well. This is why vasodilation, decreases blood pressure. This is a great thread, I think it highlights the importance of a physiology class. I haven't taken one but will this fall. If you can, I suggest getting a book because EK and Kaplan aren't in depth enough. Oceaner, Vihsadas, and the other guys have a physics background or some upperlevel physiology background so they gave great explanations. It seems this is what you want and you won't get it from most review books.

 

[zogoto]

When you constrict the blood vessels, you are LOWERING the flow. Consequently, if there is less flow, it means that their is less blood and HYDROSTATIC pressure is reduced.

This implies vasoconstriction reduces blood pressure...

 

[TheBoondocks]

This implies vasoconstriction reduces blood pressure...

I was tired when I wrote that part my bad. Constriction DECREASES flow, but it INCREASES resistance. So the heart must pump harder for blood to flow through the reduced area. Consequently, CO is increased so the end result is an increase in pressure.