Physiology of Finfish and Shellfish (2+1)

2.1.6. Gas diffusion

In the respiratory organ of animals gas diffuse between the environment and the organism; oxygen enters and carbondioxide leaves the animal. So it is important to know how fast the gases diffuse their rate of diffusion.

The rate of diffusion of a gas is inverse by proportional to the square root of its molecular weight.

$$Diffusion \ rate\ \alpha\ {1 \over sqrt{Molecular \ wt \ of \ gas}}$$

It is a fundamental process in the movement of materials. Those diffusion processes that are of physiological importance take place over short distance, a fraction of millimeter or less. Examples are the diffusion of respiratory gases across the respiratory membranes or the diffusion of ions across the nerve membrane during action potential. Diffusion is limited to short distance transport since diffusion time increases with the square of the diffusion distance.

$$Diffusion \ time \ (t) = {R^2 \over t^2} \ = \ {Square \ root \ of \ average \ diffusion \ distance \over average \ length}$$

$$dQ\ =\ -DA{du \over dx}\ dt$$

Where Q = quantity of substance diffused

A = Area through which diffusion takes place

U = Concentration at the diffusion point x

$$\{ {du \over dx}\ = \ concentration \}$$
t = time

D = Diffusion co efficient

From this equation we can see that the amount of a substance diffusory is proportional to the elapsed time. The next consequence of fick’s equation is if the diffusion distance (x) is increased, the amount of the substance diffusing decreases.

The diffusion over a constant concentration gradient is expressed by fick’s principle: