Why the hydrostatic pressure of blood DROPS as blood moves away from heart?

Watch
This discussion is closed.
duskflower
Badges: 1
Rep:
?
#1
Report Thread starter 10 years ago
#1
I thought I understood the circulatory system but this question has thrown me. I don't get it! I thought the pressure of the blood INCREASED as it left the heart due to smaller lumen in arteries etc? The only answer I could think of was that its further away from the ventricular contraction in the heart, but it says 'Do not accept' in the mark scheme
Please could someone explain?
0
Mechie
Badges: 15
Rep:
?
#2
Report 10 years ago
#2
Higher level of resistance to blood flow due to the narrower lumen i think.
0
AnonymousPenguin
Badges: 16
Rep:
?
#3
Report 10 years ago
#3
I don't know the exact answer, but it would seem logical to me:
1. The blood does compress at least somewhat and while the heart pushing out the blood increases the pressure it would get lower farther away.
2. Blood plasma can pass into cells/kidneys/other places and that would contribute to the pressure being lower.
3. Arterial walls are less permeable than capillary walls so they could sustain a higher pressure.

Probably the correct answer is a combination of 2 and 3.
0
Mechie
Badges: 15
Rep:
?
#4
Report 10 years ago
#4
(Original post by AnonymousPenguin)
I don't know the exact answer, but it would seem logical to me:
1. The blood does compress at least somewhat and while the heart pushing out the blood increases the pressure it would get lower farther away.
2. Blood plasma can pass into cells/kidneys/other places and that would contribute to the pressure being lower.
3. Arterial walls are less permeable than capillary walls so they could sustain a higher pressure.

Probably the correct answer is a combination of 2 and 3.
I know I'm right. Look at my post
0
Mechie
Badges: 15
Rep:
?
#5
Report 10 years ago
#5
And OP, why would a narrower lumen in any way INCREASE the pressure?
0
AnonymousPenguin
Badges: 16
Rep:
?
#6
Report 10 years ago
#6
(Original post by davidmarsh01)
I know I'm right. Look at my post
Ok it makes sense but I don't think it would get full points as it doesn't logically connect the higher resistance to a lower pressure.
0
raceforthefishman
Badges: 2
Rep:
?
#7
Report 10 years ago
#7
Also, because the blood spreads from just one artery into many many capilaries, its effectively flowing through a much greater surface area (you know what I mean :p:) so the pressure drops...
0
duskflower
Badges: 1
Rep:
?
#8
Report Thread starter 10 years ago
#8
Thanks for all your answers.. not as complicated as it seems. Still think it's quite contradictory because we've been told in a previous chapter that there's high blood pressure in the arteries due to the small lumen. But I guess hydrostatic pressure is different because like raceforthefishman said, the blood can leave the arteries.. didn't think of it like that.

(Original post by davidmarsh01)
And OP, why would a narrower lumen in any way INCREASE the pressure?
Because there is a set volume of blood flowing through a smaller space?..There's a higher pressure than there is in the veins anyway right? Is my logic wrong?
0
raceforthefishman
Badges: 2
Rep:
?
#9
Report 10 years ago
#9
(Original post by duskflower)
Thanks for all your answers.. not as complicated as it seems. Still think it's quite contradictory because we've been told in a previous chapter that there's high blood pressure in the arteries due to the small lumen. But I guess hydrostatic pressure is different because like raceforthefishman said, the blood can leave the arteries.. didn't think of it like that.



Because there is a set volume of blood flowing through a smaller space?..There's a higher pressure than there is in the veins anyway right? Is my logic wrong?
Like I mentioned before (same idea) if you have a narrower lumen, the pressure will be greater...volume thing...

analogy time

If you have a hose pipe...normal water running out of it is fine, if you put your thumb over the whole effectively making it narrower...you can tell the pressure has increased...
0
AnonymousPenguin
Badges: 16
Rep:
?
#10
Report 10 years ago
#10
(Original post by raceforthefishman)
Like I mentioned before (same idea) if you have a narrower lumen, the pressure will be greater...volume thing...

analogy time

If you have a hose pipe...normal water running out of it is fine, if you put your thumb over the whole effectively making it narrower...you can tell the pressure has increased...
This isn't necessarily true. If you have a set volume of liquid traveling through a pipe with a smaller diameter the liquid will travel faster, but the pressure will be lower. Something to do with Bernulli equations, which I used to know just before my Physics exam.
0
duskflower
Badges: 1
Rep:
?
#11
Report Thread starter 10 years ago
#11
(Original post by raceforthefishman)
Like I mentioned before (same idea) if you have a narrower lumen, the pressure will be greater...volume thing...

analogy time

If you have a hose pipe...normal water running out of it is fine, if you put your thumb over the whole effectively making it narrower...you can tell the pressure has increased...
Exactly, but the question is why the hydrostatic pressure DROPS as it leaves the heart. That's why I'm so confused
x
0
Pheonixx
Badges: 15
Rep:
?
#12
Report 10 years ago
#12
looks like I'll have to step in.

because there is more space, simple. there would have to be a lower pressure outside the heart otherwise blood would stay inside the heart would it not? Thats the main and only reason.


your number 1 and 2 is wrong and your number 3 isn't relevant.
0
raceforthefishman
Badges: 2
Rep:
?
#13
Report 10 years ago
#13
(Original post by AnonymousPenguin)
This isn't necessarily true. If you have a set volume of liquid traveling through a pipe with a smaller diameter the liquid will travel faster, but the pressure will be lower. Something to do with Bernulli equations, which I used to know just before my Physics exam.
Ye, I knew it wouldn't be as simple as that really....but ye...shh :p:
Its that kind of idea

I think when we did this we had some example with straws as well :lolwut:
0
raceforthefishman
Badges: 2
Rep:
?
#14
Report 10 years ago
#14
(Original post by duskflower)
Exactly, but the question is why the hydrostatic pressure DROPS as it leaves the heart. That's why I'm so confused
x
Its the first thing I said...larger volume etc, because of lots of capillaries etc

oh ok

just dug out the AS text book (appreciate that) I'm getting a little muddled between hydrostatic pressure and blood stuff...

So I suppose if the hydrostatic pressure is influenced by the blood pressure, and the blood pressure decreases because its getting spread out....then there will be less hydrostatic pressure
0
kashim91
Badges: 15
Rep:
?
#15
Report 10 years ago
#15
(Original post by raceforthefishman)
Its the first thing I said...larger volume etc, because of lots of capillaries etc

oh ok

just dug out the AS text book (appreciate that) I'm getting a little muddled between hydrostatic pressure and blood stuff...

So I suppose if the hydrostatic pressure is influenced by the blood pressure, and the blood pressure decreases because its getting spread out....then there will be less hydrostatic pressure
Blood pressure also decreases due to the frictional force coming from the arteriole walls. I suppose that explains why the hydrostatic pressure is higher at the arteriole end of the capillary( smaller lumen, more resistance), and lower at the venous end(larger lumen, less resistance).
0
grovichik
Badges: 9
Rep:
?
#16
Report 10 years ago
#16
Hey OP! I think you might be interpretting the question a bit differently to what it might mean.

You are correct in thinking that the pressure immediately outside the valve increases when blood is pumped into it from the ventricle. The sequence of events: valve is shut, ventricle contracts, valve opens because pressure inside is greater than outside, blood moves out down pressure gradient, pressure is greater in artery than ventricle so valves closes (these are "pocket" valves and closed by back pressure), blood flows away from the heart in the artery, heart contracts and this repeats.

What I think the question is asking is why the pressure far from the heart is smaller than the pressure close to the heart. I think this is to do with friction. Also it could be because arteries contract but smaller vessels like capillaries don't have any muscles so can't contract. What does the mark scheme say?

Hope this helps. :-D
1
Lit2010
Badges: 17
Rep:
?
#17
Report 10 years ago
#17
Is it anything to do with plasma fluid and ultrafiltration and all that stuff? I find it confusing, myself, but as the hydrostatic pressure of the blood is higher than that of the tissue fluid, substances are forced out of the blood in the capillaries and into the surrounding tissue fluid. This results in the hydrostatic pressure in the arteries and capillaries dropping, until at the venular end of the capillary, substances from the tissue fluid are forced back in as the hydrostatic pressure of the blood in capillaries is less than that of the tissue fluid.

I'm not 100% on this, though.
0
grovichik
Badges: 9
Rep:
?
#18
Report 10 years ago
#18
(Original post by Lit2010)
Is it anything to do with plasma fluid and ultrafiltration and all that stuff? I find it confusing, myself, but as the hydrostatic pressure of the blood is higher than that of the tissue fluid, substances are forced out of the blood in the capillaries and into the surrounding tissue fluid. This results in the hydrostatic pressure in the arteries and capillaries dropping, until at the venular end of the capillary, substances from the tissue fluid are forced back in as the hydrostatic pressure of the blood in capillaries is less than that of the tissue fluid.

I'm not 100% on this, though.
Nope I don't think this question relates to tissue fluid formation; this comes later in the cycle of heart>arteries>arterioles>capill aries (capillaries are where all the tissue fluid jazz happens). By the time blood reachs capillaries there would have already been lots of friction decreasing blood pressure.

You are very nearly 100% right with your explanation of tissue fluid; although osmotic pressure is very important too, not just hydrostatic pressure :-)

Hope this helps
0
Lit2010
Badges: 17
Rep:
?
#19
Report 10 years ago
#19
(Original post by grovichik)
Nope I don't think this question relates to tissue fluid formation; this comes later in the cycle of heart>arteries>arterioles>capill aries (capillaries are where all the tissue fluid jazz happens). By the time blood reachs capillaries there would have already been lots of friction decreasing blood pressure.

You are very nearly 100% right with your explanation of tissue fluid; although osmotic pressure is very important too, not just hydrostatic pressure :-)

Hope this helps
Thank you That's helped a lot
0
grovichik
Badges: 9
Rep:
?
#20
Report 10 years ago
#20
(Original post by Lit2010)
Thank you That's helped a lot
No probz...good luck :-D
0
X
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

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

Personalise

Should there be a new university admissions system that ditches predicted grades?

No, I think predicted grades should still be used to make offers (712)
33.94%
Yes, I like the idea of applying to uni after I received my grades (PQA) (897)
42.76%
Yes, I like the idea of receiving offers only after I receive my grades (PQO) (396)
18.88%
I think there is a better option than the ones suggested (let us know in the thread!) (93)
4.43%

Watched Threads

View All