[cougem]

Was having my supervision in human reproduction yesterday at Uni. My supervisor was telling me that the spiral arteries (so called as they're like springs) going to the placenta are spiral so that they slow the blood down. I said why not just lower the bore of the arteries. She said that it was for two reasons; first so that you can adjust your flow rates as the artery changes size (I buy that, that's fine), but secondly because it allows a high surface area (so lots of blood), but as a low velocity. Well that's crap. It's not like that has any effect what so ever, still the same amount of blood going into its capillary bed. Volume gone through is area times speed times time. High bore, low speed, does bugger all.

So I said 'why do you want high surface area and low flow?', and this is the bit that made me post here; she said that it gave time for the oxygen to diffuse out of the capillaries, across the placenta, and into the foetal arteries, as diffusion takes time. Right

Diffusion doesn't take time, it's ESSENTIALLY instantaneous (I know it isn't, but because the oxygen that diffuses out is actually in the plasma (the plasma O2 is in equilibrium with the red blood cell O2, so more O2 goes into the water component of the blood as it leaves across the capillary wall), the oxygen doesn't have far to travel at all. While for 1 molecule it might take a while for it to get across the membrane by chance, and so by having quick flow it might not diffuse, by having quick flow you're restoring the O2 concentration gradient and driving more oxygen to pass across the membrane. Not only that, but even if it did take time for the oxygen to pass across the membrane, once it's inside it, removing the old slightly less well oxygenated blood from the capillary by having a quicker blood flow rate can't do any harm, it can only speed up the diffusion!

It's hard to explain, but when an oxygen molecule is bouncing around at incredibly speeds, blood flow rate really doesn't have much affect on the molecule, only the fluid en masse.

Anyway, I think she's wrong. I expect the spiral is there because it allows the uterus and such to change shape as the baby develops, acting like a spring. Her logic seems fine when you first think about it, but really when you analyse it on a molecular level, it doesn't make sense. It's no different from saying you should have a low flow rate in water cooling so the water has time to absorb the heat; both of them involve molecules (or electrons) bouncing around, and just because you're recycling the source/destination doesn't mean they can't get to their targets.

What do you guys think? Maybe you could put it a bit more eloquently than I.