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IRoranth
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#1
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during exchange in capillaries, does water (+other substances) move back into the capillaries at the end? Also, is the osmotic pressure throughout the capillary all the same? I'm so confused
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Gaiaphage
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(Original post by IRoranth)
during exchange in capillaries, does water (+other substances) move back into the capillaries at the end? Also, is the osmotic pressure throughout the capillary all the same? I'm so confused
At the arterial end of the capillary, hydrostatic pressure is greater than osmotic pressure so water moves out into the interstitial fluid. As water moves out, this decreases the hydrostatic pressure in the capillary and increases osmotic pressure, so by the venous end the osmotic pressure is greater than hydrostatic pressure and water moves in again

Not all the water moves back though - some is left in the interstitial fluid and this is returned via the lymphatic system
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Nayzar
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Osmotic pressure remains constant; hydrostatic pressure changes
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Gaiaphage
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(Original post by Nayzar)
I can answer part of your question. Things definitely move back into the capillaries at the venous end (eg. CO2), but I cant remember if water also moves back in. Osmotic pressure remains constant; hydrostatic pressure changes
Actually osmotic pressure rises as the water moves out of the capillary
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IRoranth
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(Original post by Gaiaphage)
At the arterial end of the capillary, hydrostatic pressure is greater than osmotic pressure so water moves out into the interstitial fluid. As water moves out, this decreases the hydrostatic pressure in the capillary and increases osmotic pressure, so by the venous end the osmotic pressure is greater than hydrostatic pressure and water moves in again

Not all the water moves back though - some is left in the interstitial fluid and this is returned via the lymphatic system
Thank you so much for a clear explanation - these textbooks waffle too much!
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Nayzar
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(Original post by Gaiaphage)
Actually osmotic pressure rises as the water moves out of the capillary
Sorry boss, I'm following the OCR spec which is a bit dodgy in some places
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IRoranth
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(Original post by Nayzar)
Sorry boss, I'm following the OCR spec which is a bit dodgy in some places
So am I, which is why I was so confused as well? So wait, does it stay constant or not?
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Gaiaphage
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(Original post by Nayzar)
Sorry boss, I'm following the OCR spec which is a bit dodgy in some places
No problem - A level (and GCSE) often simplify things to the point where nothing makes sense which is really frustrating. If you think about it, osmotic pressure is basically the protein concentration in the capillaries, so if water moves out the concentration has to rise
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Nayzar
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(Original post by IRoranth)
So am I, which is why I was so confused as well? So wait, does it stay constant or not?
My biology teacher says that it does stay constant according to OCR so thats probably a safe bet with the mark scheme. But Gaiaphage's comparative wording is better (ie. more than/less than). So I guess if it helps to simplify it then just assume osmotic pressure stays constant but use the comparative phrasing for exams
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IRoranth
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(Original post by Nayzar)
My biology teacher says that it does stay constant according to OCR so thats probably a safe bet with the mark scheme. But Gaiaphage's comparative wording is better (ie. more than/less than). So I guess if it helps to simplify it then just assume osmotic pressure stays constant but use the comparative phasing for exams
Thanks, good luck on Thursday!
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Nayzar
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(Original post by Gaiaphage)
No problem - A level (and GCSE) often simplify things to the point where nothing makes sense which is really frustrating. If you think about it, osmotic pressure is basically the protein concentration in the capillaries, so if water moves out the concentration has to rise
True. At the start I was asking loads of questions about things, but one question lead to another and it just ended up going wayyy beyond the syllabus. That's why I'm just sticking to what's being taught now, even if it makes no sense
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Nayzar
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(Original post by IRoranth)
Thanks, good luck on Thursday!
Lol you too, I'm doing A2 but I appreciate it!
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Gaiaphage
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(Original post by Nayzar)
True. At the start I was asking loads of questions about things, but one question lead to another and it just ended up going wayyy beyond the syllabus. That's why I'm just sticking to what's being taught now, even if it makes no sense
Haha I know the feeling! Keep that attitude for university and you'll go far
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