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Daryl1995v
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Explain the effect that a difference in lumen size between the artery and the vein has on blood flow.
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OfficialChemist
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(Original post by Daryl1995v)
Explain the effect that a difference in lumen size between the artery and the vein has on blood flow.
Arterial walls are actually thicker to cope with the high pressure. It is easier for a thin-walled vessel to rupture than a thick-walled vessel. When under pressure from the heart, blood will exert a great pressure on the walls, so they will have to handle it.

In veins, because there is no pressure, blood has to be able to flow back fairly rapidly to be used again. Because of this, veins have larger lumens to allow blood to pass more quickly when not under pressure.
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Sorcerer
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Arteries have smaller lumen which means that blood flow is faster whilst veins have larger lumen meaning that blood flow is slower.
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frostfly
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(Original post by Sorcerer)
Arteries have smaller lumen which means that blood flow is faster whilst veins have larger lumen meaning that blood flow is slower.
This is incorrect. A larger lumen will allow a higher rate of blood flow.
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Sorcerer
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(Original post by honour)
This is incorrect. A larger lumen will allow a higher rate of blood flow.
How?
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frostfly
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(Original post by Sorcerer)
How?
A larger lumen will - obviously - allow more red blood cells to flow at the same time, whereas if there is limited space, like in a small lumen, fewer red blood cells will be able to pass through at the same time.
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The Sikh Surgeon
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(Original post by honour)
A larger lumen will - obviously - allow more red blood cells to flow at the same time, whereas if there is limited space, like in a small lumen, fewer red blood cells will be able to pass through at the same time.
NO no no... a smaller lumen allows more pressure to build which speeds up the flow of blood (i'm an A2 student so hav covered all this)
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frostfly
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(Original post by Kantth1)
NO no no... a smaller lumen allows more pressure to build which speeds up the flow of blood (i'm an A2 student so hav covered all this)
I see, I'm an AS student. I understand that - but what about in veins, when there is virtually no pressure? Isn't that why these vessels have a much larger lumen than arteries to increase the rate of blood flow?
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Reality Check
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There's some confusion about this on this thread. To understand the effect of 'pipe' diameter on flow rate you need to understand the Bernoulli effect. Try this explanation:

http://galileo.phys.virginia.edu/cla.../Bernoulli.htm
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Sorcerer
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(Original post by Reality Check)
There's some confusion about this on this thread. To understand the effect of 'pipe' diameter on flow rate you need to understand the Bernoulli effect. Try this explanation:

http://galileo.phys.virginia.edu/cla.../Bernoulli.htm
So your conclusion is...
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frostfly
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(Original post by Reality Check)
There's some confusion about this on this thread. To understand the effect of 'pipe' diameter on flow rate you need to understand the Bernoulli effect. Try this explanation:

http://galileo.phys.virginia.edu/cla.../Bernoulli.htm
Wow, information overload! I'm pretty sure the OP has only just started AS Biology.
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VMD100
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Just accidentally rated an incorrect post...
But as blood travels to further extremities of the body in arteries the pressure must be maintained and therefore they are narrower.
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The Sikh Surgeon
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(Original post by honour)
I see, I'm an AS student. I understand that - but what about in veins, when there is virtually no pressure? Isn't that why these vessels have a much larger lumen than arteries to increase the rate of blood flow?
No because by the time blood flows through the capillaries within capillary beds the pressure of the blood drops significantly. A smaller lumen maintains pressure, but when blood gets to the veins there is not much pressure to maintain so the lumen does not have to be small. Instead our body has developed two mechanisms to get the blood back to our ❤️. One is the use of valves which stop the backflow of blood under low pressure. The other is the contraction of muscles. When you walk around your muscles contract which squeeze your veins and push blood upwards (if in your legs) towards your ❤️️.
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frostfly
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(Original post by Kantth1)
No because by the time blood flows through the capillaries within capillary beds the pressure of the blood drops significantly. A smaller lumen maintains pressure, but when blood gets to the veins there is not much pressure to maintain so the lumen does not have to be small. Instead our body has developed two mechanisms to get the blood back to our ❤️. One is the use of valves which stop the backflow of blood under low pressure. The other is the contraction of muscles. When you walk around your muscles contract which squeeze your veins and push blood upwards (if in your legs) towards your ❤️️.
Okay, so the veins has no pressure... that was what I just mentioned. Are you trying to say I'm right or wrong?
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The Sikh Surgeon
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(Original post by honour)
Okay, so the veins has no pressure... that was what I just mentioned. Are you trying to say I'm right or wrong?
'A larger lumen will - obviously - allow more red blood cells to flow at the same time, whereas if there is limited space, like in a small lumen, fewer red blood cells will be able to pass through at the same time'

What you said above suggests that the rate of blood flow is faster in the veins which it isnt. Also you disagreed with Sorcerer even though his/her answer was correct.
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Sorcerer
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(Original post by Kantth1)
'A larger lumen will - obviously - allow more red blood cells to flow at the same time, whereas if there is limited space, like in a small lumen, fewer red blood cells will be able to pass through at the same time'

What you said above suggests that the rate of blood flow is faster in the veins which it isnt. Also you disagreed with Sorcerer even though his/her answer was correct.
I am male but thanks anyway!
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OxFossil
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Arterial blood pressure is significantly higher than venous blood pressure.

The blood must be given a strong impetus as it leaves the heart, to get it all the way round the body . So i) the arteries have to be thick walled and elastic to take the pressure of this powerful heartbeat. 2) the narrowness of the arteries helps maintain the high rate of flow required to get the blood to the extremities.

The blood vessels become tiny capillaries in the skin and organs so that gas and nutrient exchange can take place. But the much increased drag/friction in these vessels slows the rate of flow. Once back in the larger veins, as Kanthh1 says, only ordinary muscle motion helps pump the blood back to the heart, so blood flow tends to be slower than the flow of blood in the arteries.

But if the lumen of the veins was as narrow as the arteries, that would mean that less blood per unit time would be returning to the heart than would be leaving it!

So the lumen of the veins is slightly wider, both because they don't need such a tough, muscly, wall as the arteries, but also so that they can convey a larger volume of this slower moving blood i.e the overall rate of return is equal.
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frostfly
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(Original post by Kantth1)
'A larger lumen will - obviously - allow more red blood cells to flow at the same time, whereas if there is limited space, like in a small lumen, fewer red blood cells will be able to pass through at the same time'

What you said above suggests that the rate of blood flow is faster in the veins which it isnt. Also you disagreed with Sorcerer even though his/her answer was correct.
No, I did not for a moment suggest blood flows faster in veins. I just said that in general, a bigger lumen lets more blood cells pass through at the same time, hence the rate of blood flow will be higher. Now you tell me that's not right.
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The Sikh Surgeon
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(Original post by Sorcerer)
I am male but thanks anyway!
Ooops... should've seen the male sex sign 😬😂
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The Sikh Surgeon
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(Original post by honour)
No, I did not for a moment suggest blood flows faster in veins. I just said that in general, a bigger lumen lets more blood cells pass through at the same time, hence the rate of blood flow will be higher. Now you tell me that's not right.
Yh but in this context it is incorrect. We are talking about the veins and arteries. The rate of blood flow is faster in the arteries due to the high pressure induced by the ❤️.
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