# Blood flow in capillaries/lumen

Watch
Announcements
#1
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?
0
4 years ago
#2
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.
3
4 years ago
#3
(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.
0
4 years ago
#4
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
0
4 years ago
#5
(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?
0
4 years ago
#6
Isn't it Q= ΔP π r*4/ 8 η l?
0
4 years ago
#7
(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...
0
4 years ago
#8
(Original post by alkyone)
Nah in 3 years of biomed and 1 year of medicine you do...
OooOoooOooo, that seems kinda fun......
0
4 years ago
#9
(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.
0
4 years ago
#10
(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?
0
4 years ago
#11
(Original post by Perfection Ace)
Damn nice bro, what grades did you get in your A Level subjects?
Not great, ABB
0
4 years ago
#12
(Original post by alkyone)
Not great, ABB
Damn, that's still sick o.o, in what subjects? Bio/Phy/Chem?!
0
4 years ago
#13
(Original post by Perfection Ace)
Damn, that's still sick o.o, in what subjects? Bio/Phy/Chem?!
Biology chemistry maths
1
4 years ago
#14
(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
0
4 years ago
#15
(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
0
4 years ago
#16
(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!
1
4 years ago
#17
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
0
#18
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...
0
4 years ago
#19
(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.
0
#20
(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
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### Have you experienced financial difficulties as a student due to Covid-19?

Yes, I have really struggled financially (53)
17.15%
I have experienced some financial difficulties (88)
28.48%
I haven't experienced any financial difficulties and things have stayed the same (117)
37.86%
I have had better financial opportunities as a result of the pandemic (40)
12.94%
3.56%