In three dimensions the root$-$mean$-$square diusion

distance is R $=$ p$$x$2=6$Dt$,$ where t is time and D is the diusion constant.

Consider the diusion of oxygen from air to the blood in the lungs. The terminal air

sacs in the lungs, the alveoli, have a radius of about $100$ m$.$ The radius of a capillary

is about $4$ m$.$