123chem
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In capillaries, blood rate is slower, but lumen diameter is less. I dont get this, as smaller diameter, surely means faster rate. Can someone explain?
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alkyone
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capillaries are very very thin and high pressure or flow would very likely burst them. They are so small that red blood cells have to deform to go through. The idea is that arterioles will dissipate all the pressure before reaching capillaries, so the flow will be small.

The rule is: flow is directly proportional to pressure difference and to the radius to the power of 4. Meaning that the higher the diameter of a vessel the faster the flow.
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Eloades11
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(Original post by 123chem)
In capillaries, blood rate is slower, but lumen diameter is less. I dont get this, as smaller diameter, surely means faster rate. Can someone explain?
Since all of the nutrient/gas exchange from the blood into the cells happens in the capillaries, they will require a higher surface area/volume ratio in order to meet the osmotic requirements. The smaller diameter of the blood vessel means a higher SA/V ratio, which in turn means more friction between the blood and the capillary walls, which will slow down the rate of blood flow. The rate of blood flow varies inversely with the total cross-sectional area of the blood vessels.
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alkyone
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(Original post by Eloades11)
Since all of the nutrient/gas exchange from the blood into the cells happens in the capillaries, they will require a higher surface area/volume ratio in order to meet the osmotic requirements. The smaller diameter of the blood vessel means a higher SA/V ratio, which in turn means more friction between the blood and the capillary walls, which will slow down the rate of blood flow. The rate of blood flow varies inversely with the total cross-sectional area of the blood vessels.
All this makes sense except your last statement, I am a bit confused as to why the rate of flow is inversely proportional to the cross sectional area? wouldn't that mean that small diameter vessels such as capillaries have a large rate of flow? Thanks
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Perfection Ace
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(Original post by alkyone)
capillaries are very very thin and high pressure or flow would very likely burst them. They are so small that red blood cells have to deform to go through. The idea is that arterioles will dissipate all the pressure before reaching capillaries, so the flow will be small.

The rule is: flow is directly proportional to pressure difference and to the radius to the power of 4. Meaning that the higher the diameter of a vessel the faster the flow.
damn, do you learn that kinda stuff in A Level bio?
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alkyone
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Isn't it Q= ΔP π r*4/ 8 η l?
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alkyone
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(Original post by Perfection Ace)
damn, do you learn that kinda stuff in A Level bio?
Nah in 3 years of biomed and 1 year of medicine you do...
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Perfection Ace
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(Original post by alkyone)
Nah in 3 years of biomed and 1 year of medicine you do...
OooOoooOooo, that seems kinda fun......
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alkyone
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(Original post by Perfection Ace)
OooOoooOooo, that seems kinda fun......
It actually is!

Although I do remember doing an experiment in AS physics about these laws now that I think about it.
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Perfection Ace
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(Original post by alkyone)
It actually is!

Although I do remember doing an experiment in AS physics about these laws now that I think about it.
Damn nice bro, what grades did you get in your A Level subjects?
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alkyone
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(Original post by Perfection Ace)
Damn nice bro, what grades did you get in your A Level subjects?
Not great, ABB
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Perfection Ace
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(Original post by alkyone)
Not great, ABB
Damn, that's still sick o.o, in what subjects? Bio/Phy/Chem?!
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alkyone
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(Original post by Perfection Ace)
Damn, that's still sick o.o, in what subjects? Bio/Phy/Chem?!
Biology chemistry maths
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Perfection Ace
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(Original post by alkyone)
Biology chemistry maths
Damnn, I'm gonna be doing Biology and Chemistry for my A Levels too! Ima hit you up with a follow so you can help me in the future
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alkyone
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(Original post by Perfection Ace)
Damnn, I'm gonna be doing Biology and Chemistry for my A Levels too! Ima hit you up with a follow so you can help me in the future
Sure, I'd be happy to help if I can
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Eloades11
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(Original post by alkyone)
All this makes sense except your last statement, I am a bit confused as to why the rate of flow is inversely proportional to the cross sectional area? wouldn't that mean that small diameter vessels such as capillaries have a large rate of flow? Thanks
I apologise if I caused any confusion. There is a low cross-sectional area in the individual capillaries and a low rate of blood flow. The rate of blood flow is inversely proportional to the cross-section applicable to the arteries and veins which is not applicable here. I shouldn't have mentioned it!
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alkyone
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(Original post by Eloades11)
I apologise if I caused any confusion. There is a low cross-sectional area in the individual capillaries and a low rate of blood flow. The rate of blood flow is inversely proportional to the cross-section applicable to the arteries and veins which is not applicable here. I shouldn't have mentioned it!
My confusion Is purely because of Poiseuille's law that states that Q= (ΔP πr^4)/8ηl, so I thought that this was applicable to all vessels. I'd really appreciate it if you could explain why it is inversely proportional in arteries and veins though!

Not that I'm doing A-levels but will probably need this for my end of year exams. Thanks again
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123chem
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(Original post by Eloades11)
I apologise if I caused any confusion. There is a low cross-sectional area in the individual capillaries and a low rate of blood flow. The rate of blood flow is inversely proportional to the cross-section applicable to the arteries and veins which is not applicable here. I shouldn't have mentioned it!
Shouldn't less area means the blood travels faster? this is what i dont get...
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alkyone
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(Original post by 123chem)
Shouldn't less area means the blood travels faster? this is what i dont get...
Think about you walking through a corridor. A wide corridor allows you to walk fast. A narrower passage that you have to squeeze through though, slows you down. That's exactly what is happening to cells in narrow capillaries, they slow down because the smaller the radius the higher the friction with the vessel and therefore the higher the resistance, so flow rate is low.
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123chem
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(Original post by alkyone)
Think about you walking through a corridor. A wide corridor allows you to walk fast. A narrower passage that you have to squeeze through though, slows you down. That's exactly what is happening to cells in narrow capillaries, they slow down because the smaller the radius the higher the friction with the vessel and therefore the higher the resistance, so flow rate is low.
hmm, i think like this; in a narrow corrider means its more packed, so people will be moving quiker as more people enter the corridor
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