So confused and frustrated...

[beponychick]

Ok, I am going crazy trying to figure this out...

Does pressure increase as cross sectional area decreases or is it the other way around? I was under the impression that as Area increases, velocity decreases and pressure increases. But this does not make sense when you think about cappilaries. In capillaries, the cross sectional area is highes and pressure is lowest. Which one is it? Im going craaaazzy!!!

Thanks everyone and good luck!

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[shuzee]

Ok, I am going crazy trying to figure this out...

Does pressure increase as cross sectional area decreases or is it the other way around? I was under the impression that as Area increases, velocity decreases and pressure increases. But this does not make sense when you think about cappilaries. In capillaries, the cross sectional area is highes and pressure is lowest. Which one is it? Im going craaaazzy!!!

Thanks everyone and good luck!

i think pressure decreases as cross sectional area decreases and the velocity increases. The relation for velocity can be made by the flow rate equation

Q= AV
so area and velocity are inversely related and area and pressure are proportional to each other.

just remember as AREA decreases Pressure decreases too!!!!

good luck!!!

 

[beponychick]

i think pressure decreases as cross sectional area decreases and the velocity increases. The relation for velocity can be made by the flow rate equation

Q= AV
so area and velocity are inversely related and area and pressure are proportional to each other.

just remember as AREA decreases Pressure decreases too!!!!

good luck!!!

That is what I thought also. However, in capillaries, the cross sectional area is the highest, and the pressure is the lowest (the pressure is the highest closest the heart). So is this just a special case?

 

[ComicBookHero20]

Ok, I am going crazy trying to figure this out...

Does pressure increase as cross sectional area decreases or is it the other way around? I was under the impression that as Area increases, velocity decreases and pressure increases. But this does not make sense when you think about cappilaries. In capillaries, the cross sectional area is highes and pressure is lowest. Which one is it? Im going craaaazzy!!!

Thanks everyone and good luck!

If cross sectional area is highest in capillaries, then by P=F/A, pressure has to be low...and from Av=Av, its velocity has to be low as well

 

[beponychick]

If cross sectional area is highest in capillaries, then by P=F/A, pressure has to be low...and from Av=Av, its velocity has to be low as well

So you are saying that: area is inversely proportional to pressure? For some reason, I was so confident that area was directly proportional pressure. I have been answering questions correctly on exams with that assumption. Now I dont know anymore!

 

[OBGYN2010]

Pressure is always equal to Force/Area...so as the pressure increases, the area decreases, they are inversely proportional to each other.

 

[manfood.com]

weird thing, the pressure is the lowest in the venules, I believe, or is it the arterioles, it's one of those, not the capillaries.

 

[RayhanS1282]

Ok, I am going crazy trying to figure this out...

Does pressure increase as cross sectional area decreases or is it the other way around? I was under the impression that as Area increases, velocity decreases and pressure increases. But this does not make sense when you think about cappilaries. In capillaries, the cross sectional area is highes and pressure is lowest. Which one is it? Im going craaaazzy!!!

Thanks everyone and good luck!

Pressure is inversely proportional to area, as area increases...the pressure decreases. And as area increases velocity decreases.


A = 1/v

P = F/A


Now, applying this to the circulatory system...since capillaries have the highest cross-sectional area...they have the smallest velocity. AND since, the capillaries have the highest cross-sectional area, they have the lowest pressure. I think that it would make more sense if you looked at each concept separately...like first compare velocity to area and then compare area to pressure. I used to be confused about that too.

 

[manfood.com]

I'm pretty sure taht the pressure is not the lowest in the capillaries, according to taht equation they are, but they aren't in real life. I'm pretty sure it's the venules. anyways, what they are saying above is correct. two more days......

 

[Apparition]

Does this have anything to do with hydrostatic pressure growing lower?

 

[Turkeyman]

you guys have officially confused the heck out of me

Can't we use the bernoulli equation to do this, for flowing pipes?

So...pressure = density(rho) * g * h

Bernoilli Eqn for a flowing fluid:
Some Constant = P * (rho*g*h) * (1/2 * rho * V^2)

P = the potential difference across the sytem, as in the circulatory system

(rho*g*h) = The elevation head of flow...or as we know it, pressure itself that we're talking about. This decreases as the next term(velocity) increases.

(1/2 * rho * v^2) = The velocity head of flow...or as we know it, the velocity itself. This increases as the above term(pressure) decreases, because it's gotta maintain that constant on the other side of the equation.

So we're saying here that the elevation head(pressure) and the velocity head are inversely proportional.

Now for the circulatory system, there is a base pressure difference(kinda like a voltage difference) across the entire system. It's as manfood said...I believe as well that the pressure in the capillaries is simply somewhere in between that in the aorta, and that in the venules, because it's a continuum. That takes care of pressure, don't relate it to the next paragraph.

Now we look at the continuity equation, A1v1 = A2v2 and see that since the capillaries have the largest cross sectional area, the blood velocity in them is lowest.

I probably got all that wrong and fed you guys a bunch of BS but please correct me ;O

Shriiiikeeeeeeee *does summoning dance thing*

edit: You can also use the bernoilli equation to show why a plane can fly, because air has to flow in equal amounts of time across the wing to maintain the flow rate. Since the path above the wing is greater than the path below the wing
_______________
/______________ \ <---airplane wing

The velocity has to be faster above the wing(meaning lower pressure), and since there's lower pressure above the wing, we get lift

 

[tsazmand]

isnt the pressure lower in the veins than capillaries....?

 

[tsazmand]

I'm pretty sure taht the pressure is not the lowest in the capillaries, according to taht equation they are, but they aren't in real life. I'm pretty sure it's the venules. anyways, what they are saying above is correct. two more days......

yeah that shoudl be right

 

[diosa428]

Pressure is lower in the veins than the capillaries because pressure decreases the further you are from the heart. This relationship has nothing to do with P = F/A or Q = vA, it's just something you need to learn. In all other cases, use the equations.

 

[tigress]

I'm confused. In a pipe, isn't the pressure lowest where the cross-sectional area is smallest and there is a greater velocity?

 

[Lolo08]

I agree with you, Tigress. Now I'm really confused..

 

[BaylorGuy]

pipes...you need 2 equations 1) A1V1=A2V2 and 2) Bernoulli's equation (i didnt feel like typingit out)

As cross-sectional area increases (I.E. gets larger), velocity decreases....as cross-sectional area decreases (I.E. gets smaller), velocity increases. The best example of this is a giant cylindrical tank filled with water that is open at the top and has a spigot at the bottom. The spigot has a much much smaller cross-sectional area than the top of the cylindrical tank...thus when the spigot is open, velocity is greater at the spigot end of the tank than at the open top of the tank.

Secondly, as velocity increases, pressure decreases....according to Mr. Bernoulli, all of the terms in his equation are equal to a constant (as previously stated by TurkeyMan). Thus as the velocity increases, pressure decreases....velocity decreases, pressure increases. The best example to remember this is an airplane wing....the only way an airplane can rise is if the pressure above the wing is less than the pressure below the wing (high pressure to low pressure, thus net positive force pushing up on the bottom of the wing, thus lift), and the only way this can occur is if the velocity above the wing is greater than the velocity below the wing.

I dont know if there is an equation that combines cross-sectional area, pressure, and velocity, thus it is hard to tell what the maxims are for such ideal situations (also considering that you have to use the two above equations in addition to F=P/A).

 

[ComicBookHero20]

Secondly, as velocity increases, pressure decreases....according to Mr. Bernoulli, all of the terms in his equation are equal to a constant (as previously stated by TurkeyMan). Thus as the velocity increases, pressure decreases....velocity decreases, pressure increases. The best example to remember this is an airplane wing....the only way an airplane can rise is if the pressure above the wing is less than the pressure below the wing (high pressure to low pressure, thus net positive force pushing up on the bottom of the wing, thus lift), and the only way this can occur is if the velocity above the wing is greater than the velocity below the wing.

This only is applicable at a constant height...as far as i know for the circulatory system, there isn't a constant height so it doesn't apply

 

[BaylorGuy]

You are correct, when dealing with height and pressure, it is necessary to use Mr. Bernoulli's equation. P=rho*g*h.

I think part of the reason why things are not ideal in the circulatory system is that its a closed system...perhaps I'm wrong but it makes sense that it would differ.

 

[tigress]

Thanks guys. That's pretty much what I thought but then this thread confused me. I think I'll just stick to keeping physics and bio separate for now, at least when it comes to pressure/velocity/area in the circulatory system

 

[firebird69guy]

Pressure is low in the capillaries because you have just passed the primary resistance vessels, the arterioles.

Think about it.. it has nothing to do with flow here. We are passing the arterioles that have HUGE resistance (especially under smooth muscle constriction). Under extreme conditions, the flow will just dribble out into a huge area of capillary (large surface area), and pressure will still be low.


It's kinda like the glomerular filtration that occurs if the afferent artery is constricted. You get low pressure into Bowman's capsule.

 

[Turkeyman]

tigress I'd just like to say I LOVE your avatar

 

[TicAL]

Yeah, I'll just reiterate what some of the above posters have already said. The reason the pressure is the lowest in the capillaries, despite it's high surface area, is because the pressure used to drive the blood through the capillaries is derived from the left ventricle contraction, and has nothing to do with the size/shape of the capillaries themselves. The pressure may slightly increase as it enters the capillaries from the arteriole, but the increase is so small relative to the decreasing pressure from the left ventricle contraction that it is negligible.

 

[cheapdate]

Ok, I am going crazy trying to figure this out...

Does pressure increase as cross sectional area decreases or is it the other way around? I was under the impression that as Area increases, velocity decreases and pressure increases. But this does not make sense when you think about cappilaries. In capillaries, the cross sectional area is highes and pressure is lowest. Which one is it? Im going craaaazzy!!!

Thanks everyone and good luck!

Forget the equation, just remember that when you water the lawn, what happens when you squeeze on the hose? If flow rate is constant (volume/second) then as you squeeze the hose, there's less water getting through, so it has to flow at a higher velocity to get the same flow rate. naturally, you'll also feel a pressure increase. This is the same thing as all those equations above, without actually having know the equation. Hope this help.

 

[HistoRocks]

If you trace the flow of blood from the aorta, you can see why pressure is low in the capillaries: left ventricle ---> aorta ---> elastic arteries ---> muscular arteries ---> arterioles ---> capillaries ---> venules ---> veins. The pressure becomes lower the further away you move from the heart. The diameter of a capillary also happens to be quite narrow. The reason, of course, has to do with the function of a capillary, which is to facilitate diffusion of gases. Think of an alveolus, which is surrounded by lung capillaries: C02 will diffuse out, and oxygen will diffuse in. The tissue type corresponding to diffusion is simple squamous epithelium. Just think of the scales of a fish, which are flat and squashed (e.g. simple squamous). Same tissue type. Hope this kinda helps.

Check out the picture:

http://www.med.uiuc.edu/histo/small/atlas/image/w51b/40a15.htm

Its from a dog, but same idea. You can see how thin the simple squamous epithelium is.

 

[tigress]

Forget the equation, just remember that when you water the lawn, what happens when you squeeze on the hose? If flow rate is constant (volume/second) then as you squeeze the hose, there's less water getting through, so it has to flow at a higher velocity to get the same flow rate. naturally, you'll also feel a pressure increase. This is the same thing as all those equations above, without actually having know the equation. Hope this help.

Actually this is sort of dangerous, because common sense doesn't work. Pressure is actually LOWER when the velocity is higher (and the cross-sectional area is smaller).

 

[tigress]

tigress I'd just like to say I LOVE your avatar

thanks he's my late kitty Mickey (Michelangelo)...he was a great cat

 

[Tinker Creek]

Thanks guys. That's pretty much what I thought but then this thread confused me. I think I'll just stick to keeping physics and bio separate for now, at least when it comes to pressure/velocity/area in the circulatory system

There was a physics passage in kaplan where the passage had to do with the circulatory system and the questions were based on fluids equations, pressure, etc. It was hell.