Fick's law of diffusion states that the rate of diffusion across a surface is directly proportional to surface thickness, partial pressures on the two sides and inversely proportional to surface thickness.
It is given as:
`(dV)/dt = A/T xx Dxx(P_1-P_2)`
where, V is the volume of gas, t is time, A is the surface area, T is the thickness, P1 and P2 are partial gas pressures on the two sides and D is the diffusion constant (and is a function of gas solubility).
Gastrovascular Cavity: serves the dual purpose of digestion and nutrient transport and is the only opening for two major animal phyla: Cnidaria (e.g., jellyfish) and Platyhelminthes (e.g., flatworm). Here the body wall, that surrounds the central cavity, is only two cells thick and thick low thickness (small T) allows for high rate of diffusion.
Alimentary canal: or digestive tract, is the pathway that food takes from our mouth to anus. Material exchange (through diffusion) takes place at the small intestine of our body. Digested material is transported in blood vessels through the walls of small intestine by diffusion. Small intestines have very large surface area (A). The inside surface of intestines have several folds from which finger-like structures are projected. This provides them much larger surface area and hence a higher rate of diffusion.
Hope this helps.