[abrogard]

In Dr Karl Kruszelnicki's book "Short Back & Science", page 134, he states that:

"In each 24 hour day about 50,000 litres of water crosses the many, many membranes in your body...."

Astounding.

What are these water molecules doing as they cross the membranes?  What function, what process, what?

[Borek]

No idea where he got this number from (I am not saying the estimate is wrong, I just have never seen it) but my bet is, many of those molecules just diffuse back and forth without serving any particular process. Then, concentration gradients are what drives many transport processes, and concentration gradients are always accompanied by osmosis. So I see nothing surprising here.

[Enthalpy]

This figure could even account the random movements of individual water molecules.