biology question

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

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

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

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

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?

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

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

Thanks, good luck on Thursday!

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