BP in the veins

[G1SG2]

Okay, so I know that BP decreases as you move from the aorta to the capillaries.

However, somewhere in the fluids chapter in TBR, it says that for veins, BP is greater as you move down the body? For example, the veins in the legs have a higher BP than the veins in the neck because the legs are closer to the ground than the neck? Is the BP in the veins in the legs of a standing human greater because the veins in the legs are subject to more pressure from all the stuff at the top, since they are at the bottom?

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[kobe200LATE]

Yes.

 

[G1SG2]

Yes.

Okay, just making sure, thanks!

 

[kimbebe51]

There are valves in the veins in the leg area that help propel the blood back to the inferior vena cava since the blood pressure is so low in the veins. Also, by walking or any extraneous motion helps increase the blood pressure in the legs because the muscle contractions help squeeze the blood toward the heart. I hope this helps.

 

[phEight]

Wait, are you sure about the part where how you said they are subject to higher pressure because of all the stuff at the top? I thought the higher pressure in the calves was due to the fact it's less off the ground. Looking at Bernoulli's eqn

P + pgh + (1/2)pv^2 = const

one can see if pgh is low (due to h being smaller), the pressure will be greater than if the h value was high. It has nothing to do with all the stuff on top, or so I think...

 

[G1SG2]

Wait, are you sure about the part where how you said they are subject to higher pressure because of all the stuff at the top? I thought the higher pressure in the calves was due to the fact it's less off the ground. Looking at Bernoulli's eqn

P + pgh + (1/2)pv^2 = const

one can see if pgh is low (due to h being smaller), the pressure will be greater than if the h value was high. It has nothing to do with all the stuff on top, or so I think...

I'm not too sure, I thought that would be a good way to think of it but I guess it's not really correct. But yeah I think you're right, the pressure is higher at the bottom because it's closer to the ground.

 

[ThirdEye]

Key word is veins (going back to heart). Veins in the legs have to go against gravity, requiring greater pressure. Veins in the neck flow with gravity.

Also, as the veins travel up (let's say from feet to heart), the pressure decreases because of a longer exposure to resistance from the vessels. Thus, pressure is greatest in the aorta and lowest in the vena cava.

 

[G1SG2]

Key word is veins (going back to heart). Veins in the legs have to go against gravity, requiring greater pressure. Veins in the neck flow with gravity.

Also, as the veins travel up (let's say from feet to heart), the pressure decreases because of a longer exposure to resistance from the vessels. Thus, pressure is greatest in the aorta and lowest in the vena cava.

Well said!

 

[G1SG2]

What about comparing blood pressure in arteries from upper extremities to that of blood pressure in arteries in the lower extremities? Which would be higher (again, in a standing human, of course)? Does the same logic apply (the blood in the arteries of the legs having to go against gravity, and thus having a higher pressure)?

 

[0Complications]

What about comparing blood pressure in arteries from upper extremities to that of blood pressure in arteries in the lower extremities? Which would be higher (again, in a standing human, of course)? Does the same logic apply (the blood in the arteries of the legs having to go against gravity, and thus having a higher pressure)?

If you are talking arm to leg comparison, the leg will have the higher blood pressure as a result of (rho)gh.

I think you guys are overthinking this. This about it this way. If I have a stack of ten textbooks, do you think the second one from the top will experience more pressure than the second one from the bottom? Of course not. The second to the bottom book will have a lot more pressure on it.

When you get these problems translate them into something that makes sense.

 

[G1SG2]

If you are talking arm to leg comparison, the leg will have the higher blood pressure as a result of (rho)gh.

I think you guys are overthinking this. This about it this way. If I have a stack of ten textbooks, do you think the second one from the top will experience more pressure than the second one from the bottom? Of course not. The second to the bottom book will have a lot more pressure on it.

When you get these problems translate them into something that makes sense.

 

[ThirdEye]

That's not a good way to look at it.

Pressure is created by the left ventricle. Pressure is lost as it is exposed to resistance of the blood vessels. So pressure is the highest when it leaves the heart, and lowest when it enters the heart.

I mentioned gravity earlier because we're designed to counter it. Pressure has to be greater at the feet to return it to the heart. But in terms of flowing with gravity, pressure is still decreasing as the blood flows. Pressure is not building up because of the mass above.

 

[ThirdEye]

What about comparing blood pressure in arteries from upper extremities to that of blood pressure in arteries in the lower extremities? Which would be higher (again, in a standing human, of course)? Does the same logic apply (the blood in the arteries of the legs having to go against gravity, and thus having a higher pressure)?

Blood in the arteries of the legs travels down and they don't go against gravity. The arteries that travel to the head do go against gravity.
Pressure is always higher in arteries than in veins. The arteries that go up to the head are probably relatively equal in pressure to the arteries that travel down, with respect to length. Because of gravity, blood can't first go down to the feet, then up to the head, then down to the heart. Because of gravity, it goes straight from the heart (where pressure is greatest), up to the head, then back to the heart. What I'm trying to stress is we are designed this way becasue of gravity, but what really determines the pressure is how long the blood has been exposed to the resistance in the vessels.

I'm not sure if I'm being confusing. Blood flows from point A to point B to point C. In terms of pressure A > B > C

 

[0Complications]

That's not a good way to look at it.

Pressure is created by the left ventricle. Pressure is lost as it is exposed to resistance of the blood vessels. So pressure is the highest when it leaves the heart, and lowest when it enters the heart.

I mentioned gravity earlier because we're designed to counter it. Pressure has to be greater at the feet to return it to the heart. But in terms of flowing with gravity, pressure is still decreasing as the blood flows. Pressure is not building up because of the mass above.

It is building due to the weight and hydrostatic pressure. Yes, BP is greater in the feet than in the arms.

I was addressing a different one than the question you are discussing. You're talking about system blood pressure at the heart, I'm talking about pressure away from the heart, which is influenced by gravity, and thus hydrostatic pressure.

 

[0Complications]

Blood in the arteries of the legs travels down and they don't go against gravity. The arteries that travel to the head do go against gravity.
Pressure is always higher in arteries than in veins. The arteries that go up to the head are probably relatively equal in pressure to the arteries that travel down, with respect to length. Because of gravity, blood can't first go down to the feet, then up to the head, then down to the heart. Because of gravity, it goes straight from the heart (where pressure is greatest), up to the head, then back to the heart. What I'm trying to stress is we are designed this way becasue of gravity, but what really determines the pressure is how long the blood has been exposed to the resistance in the vessels.

I'm not sure if I'm being confusing. Blood flows from point A to point B to point C. In terms of pressure A > B > C

If you're just talking about blood flow from A(heart) B(periphery) C(Heart), then yes. You are missing steps though if you are trying to address the previous question, because blood pressure WILL be greater in the lower legs than in the arms if you are healthy and standing upright. No ifs ands or buts about it.

 

[minutemen11]

so lemme see if i understand this.. arterial pressure: higher at head than at legs? venial pressure: higher at legs than at head? so pressure of inferior vena cavae is higher than pressure of superior vena cavae?

 

[muhali3]

You guys are making it too complicated, all you need to know is venous pressure is lower than arterial pressure. Veins cannot contract like arteries can. If there were not valves in the veins, blood would flow backwards since the ventricular contraction is often not enough to move the blood back to the heart.

The MCAT isn't going to test if venous pressure is higher in the legs or lower in the head. Also, this idea that extra weight on the leg veins is making the pressure higher does not make sense. Even if it did, that pressure would be directed downwards and wouldn't help in bringing the blood to the heart.

 

[capn jazz]

Wait, every time I get a question about blood pressure, the answer is always something related to the BP being higher in the legs due to the column of fluid pushing down on the blood in the legs.

 

[muhali3]

Wait, every time I get a question about blood pressure, the answer is always something related to the BP being higher in the legs due to the column of fluid pushing down on the blood in the legs.

Really? Is this in TBR or on the AAMCs? I heard that TBR can go into too much detail. I understand how arterial BP would be higher in the legs since gravity would aid in the blood going down. With the veins, the blood is trying to go back up, so any pressure created by anything on top would make the blood flow down and away from the heart, which would negate the purpose of veins.

 

[0Complications]

Wait, every time I get a question about blood pressure, the answer is always something related to the BP being higher in the legs due to the column of fluid pushing down on the blood in the legs.

Amen!!! Exactly.

Really? Is this in TBR or on the AAMCs? I heard that TBR can go into too much detail. I understand how arterial BP would be higher in the legs since gravity would aid in the blood going down. With the veins, the blood is trying to go back up, so any pressure created by anything on top would make the blood flow down and away from the heart, which would negate the purpose of veins.

AAMC.

Blood pressure IS higher in the legs than it is in the arms. This is due to gravity and hydrostatic forces.

 

[capn jazz]

Really? Is this in TBR or on the AAMCs? I heard that TBR can go into too much detail. I understand how arterial BP would be higher in the legs since gravity would aid in the blood going down. With the veins, the blood is trying to go back up, so any pressure created by anything on top would make the blood flow down and away from the heart, which would negate the purpose of veins.

Okay, it must be TBR. I just went through all the circulatory system Qs on the AAMCs and in one of the answers they specifically say that the lowest pressure is in the veins.

 

[0Complications]

Okay, it must be TBR. I just went through all the circulatory system Qs on the AAMCs and in one of the answers they specifically say that the lowest pressure is in the veins.

You don't measure BP in the veins you measure it in the arteries. The questions are whether BP would be higher in the arm or legs. The correct answer is the legs. No one is arguing that the veins have a higher blood pressure (at least I'm not arguing that).

My point is that BP is > legs than it is in the arms.

But once again, to measure that you need to measure arterial pressure (Sorry, I thought that went without saying).

 

[capn jazz]

You don't measure BP in the veins you measure it in the arteries. The questions are whether BP would be higher in the arm or legs. The correct answer is the legs. No one is arguing that the veins have a higher blood pressure (at least I'm not arguing that).

My point is that BP is > legs than it is in the arms.

But once again, to measure that you need to measure arterial pressure.

Yea, I'm confusing myself. So the final word is
Leg BP > upper extremity BP
Order of BP through the circulatory system: aorta, arteries, capillaries, veins, atria.

 

[0Complications]

Yea, I'm confusing myself. So the final word is
Leg BP > upper extremity BP
Order of BP through the circulatory system: aorta, arteries, capillaries, veins, atria.

I just realized the title of this thread is BP in the Veins... Doh...

 

[muhali3]

I just realized the title of this thread is BP in the Veins... Doh...

Yeah, I think OP meant arteries hopefully, I don't think we were disagreeing on anything. Thread solved.

 

[G1SG2]

Yeah, I think OP meant arteries hopefully, I don't think we were disagreeing on anything. Thread solved.

Hey, no I meant veins in the beginning. Maybe I should've just said "pressure" instead of blood pressure. So, do we all agree that pressure in the arteries in the legs are greater than the pressure in the arteries in the arm, and the pressure in the veins in the legs are greater than the pressure in veins of the arms?

 

[muhali3]

I agree with the arterial BP being higher in the legs than in the arms. I don't know about the veins, but I really don't think it matters. I think you should just know that veins have a lower pressure than arteries, which is obvious.

However, I do know the "weight on top" theory does not explain how blood from the veins (in the legs) gets to the heart. If there is weight on top, it would move the blood down, and the blood needs to go upwards.

 

[G1SG2]

I agree with the arterial BP being higher in the legs than in the arms. I don't know about the veins, but I really don't think it matters. I think you should just know that veins have a lower pressure than arteries, which is obvious.

However, I do know the "weight on top" theory does not explain how blood from the veins (in the legs) gets to the heart. If there is weight on top, it would move the blood down, and the blood needs to go upwards.

The reason I started this thread to ask about the veins was because I had a question comparing pressure in the veins (it was in TBR fluids section, I believe).

 

[muhali3]

Ok, well I guess you should take TBRs word for it. I have no way of explaining how the pressure becomes higher as you go down, in regards to veins. To me, that only makes sense for arteries. Anyways, I think TBR can be too detailed. The real MCAT isn't going to make you compare venous pressure between one part of the body and another. If it does, it'll probably be in a passage and isn't required knowledge.

 

[G1SG2]

Ok, well I guess you should take TBRs word for it. I have no way of explaining how the pressure becomes higher as you go down, in regards to veins. To me, that only makes sense for arteries. Anyways, I think TBR can be too detailed. The real MCAT isn't going to make you compare venous pressure between one part of the body and another. If it does, it'll probably be in a passage and isn't required knowledge.

Yeah, I agree.

 

[0Complications]

While I think the bp in veins can increase as the fluid pools lower in the body, I don't think it quite works like that in the veins, because you have valves that limit the amount of pressure experienced by the blood in the lower extremities. So as you contract muscles in the legs it forces blood up to the next valve, which is a one way valve.

It is still likely that BP is greater in the legs, just do to the additional fluid, but I would always think arteries first on a BP passage, unless you are told to think otherwise.

 

[Dr Gerrard]

I hope you all don't mind if I make sure I have the correct thinking. I have been trying to learn all MCAT physio this summer, since my bio classes never covered any of it.

Pressure is greatest in the arteries, then the capillaries, and then the veins. Correct?

Hydrostatic Pressure, Ph = Patm + rho*g*h, where h = height of fluid above (essentially to the top of the persons head). Correct?

Blood pressure in the arteries of the legs is greater than in the arteries of the upper extremities, because you have this additionally fluid on top pushing it down. Correct?

Blood pressure in the veins of the legs is greater than in the veins of the upper extremities (although less than in the arteries), because the blood in the lower veins has to go up against the force of gravity, requiring a greater pressure. Correct?


Now just as an aside, since this is not an ideal fluid, velocity is greatest in the arteries, lowest in the capillaries, then greater in the veins. Correct?

Thanks!

 

[muhali3]

To me, it's simple, the further away you get from the initial left ventricular contraction, the lower the pressure is going to be.

Left Ventricle -> Aorta -> Arteries -> Arterioles -> Capillaries -> Venuoles -> Veins -> Inferior/Superior Vena Cava -> Right Atrium

That has worked for me on every practice test I've done, I haven't encountered anything about pressure between veins in the legs and somewhere else.

Blood in the veins of the leg does require more pressure, but to me that doesn't mean that there is more pressure. Where would that pressure be coming from? I would think that any pressure would be coming from muscle contractions, since the leg does have powerful muscles unlike the head. The leg veins don't need more pressure than veins above the heart, all they need is enough pressure for the blood to move up. That amount of pressure could be less than the pressure in above-heart veins.

 

[G1SG2]

To me, it's simple, the further away you get from the initial left ventricular contraction, the lower the pressure is going to be.

Left Ventricle -> Aorta -> Arteries -> Arterioles -> Capillaries -> Venuoles -> Veins -> Inferior/Superior Vena Cava -> Right Atrium

That has worked for me on every practice test I've done, I haven't encountered anything about pressure between veins in the legs and somewhere else.

Blood in the veins of the leg does require more pressure, but to me that doesn't mean that there is more pressure. Where would that pressure be coming from? I would think that any pressure would be coming from muscle contractions, since the leg does have powerful muscles unlike the head. The leg veins don't need more pressure than veins above the heart, all they need is enough pressure for the blood to move up. That amount of pressure could be less than the pressure in above-heart veins.

I think the pressure would be greater than the pressure in above-heart veins. That's why an open wound is elevated above the level of the heart, to reduce pressure and thus bleeding.

 

[muhali3]

If leg veins want to achieve the same rate of blood flow as above-heart veins, then pressure does need to be higher. But even with lower pressure than above-heart veins, there is still enough force to move the blood to the heart. Why must the pressure be higher? It could be lower and blood would still get to the heart, that's all that matters. It doesn't need to be moving as fast as above-heart veins; as long as the blood is getting to the heart, the system is working. It would help for there to be higher pressure, I just don't understand how that would work. If there is higher pressure, I would attribute it to the muscles, squeezing the vessels like a tube of toothpaste.

Anyways, I don't know if my thoughts are right or wrong, I sort of don't care either, since it has never been a problem for me. I would like to know how TBR explained it, did they just say pressure is higher as you go down or did they give a reason. You should take their word for it, not mine, even though it seems unintuitive to me.

 

[G1SG2]

If leg veins want to achieve the same rate of blood flow as above-heart veins, then pressure does need to be higher. But even with lower pressure than above-heart veins, there is still enough force to move the blood to the heart. Why must the pressure be higher? It could be lower and blood would still get to the heart, that's all that matters. It doesn't need to be moving as fast as above-heart veins; as long as the blood is getting to the heart, the system is working. It would help for there to be higher pressure, I just don't understand how that would work. If there is higher pressure, I would attribute it to the muscles, squeezing the vessels like a tube of toothpaste.

Anyways, I don't know if my thoughts are right or wrong, I sort of don't care either, since it has never been a problem for me. I would like to know how TBR explained it, did they just say pressure is higher as you go down or did they give a reason. You should take their word for it, not mine, even though it seems unintuitive to me.

Haha, they sort of didn't really explain it at all, which is why I started the thread because I kinda felt confused. Maybe BerkReviewTech can chime in. I like to think of it as you described in the bold statement above. I also like the gravity explanation, although I myself don't know which one is correct. But I suppose I'll take their word for it for now, in case it shows up on my test.

 

[0Complications]

To me, it's simple, the further away you get from the initial left ventricular contraction, the lower the pressure is going to be.

Left Ventricle -> Aorta -> Arteries -> Arterioles -> Capillaries -> Venuoles -> Veins -> Inferior/Superior Vena Cava -> Right Atrium

That has worked for me on every practice test I've done, I haven't encountered anything about pressure between veins in the legs and somewhere else.

Blood in the veins of the leg does require more pressure, but to me that doesn't mean that there is more pressure. Where would that pressure be coming from? I would think that any pressure would be coming from muscle contractions, since the leg does have powerful muscles unlike the head. The leg veins don't need more pressure than veins above the heart, all they need is enough pressure for the blood to move up. That amount of pressure could be less than the pressure in above-heart veins.

Don't forget the veins are also a reservoir for fluids from the interstitial tissues.

Think about varicose veins, they result from standing for long periods where you end up with pooling blood that causes backflow through the valves in the veins. It's due to an increase in pressure.

But we really don't need to know all this, I don't think at least.