The expressed as the diffusion coefficient (Davson, 1959).

The partial pressure of a gas is the product of the total barometric pressure times the concentration of gas in dry air. For every 18,000 ft., the barometric pressure is reduced by half, but the composition of air is essentially unchanged.

To illustrate this effect on animals, the following critical altitudes have been identified for most humans; above 10,000 ft., oxygen must be added to inspired air; at 35,000 ft., 100 per cent oxygen must be inhaled; and at 50,000 ft., little oxygen reaches the alveoli even if 100 per cent oxygen is breathed.

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If gases are to be functionally important to an organism, there must be a mechanism whereby gases can enter and leave the body. Apparently the passage of oxygen across a membrane is accom­plished by diffusion.

The rate of diffusion can be calculated by Fick’s law, which is based on concentration coefficient, amount of surface thickness of the membrane, and time, arid is expressed as the diffusion coefficient (Davson, 1959).

The diffusion coefficient value is not the same for different animal tissues. For example, some representative values of diffusion coefficient of oxygen are: air, 110; water, 0.000034; muscle, 0.000014; chitin, 0.000013; connective tissue, 0.000011.

Not only is the diffusion rate different in various tissues, but the behaviour of each gas also differs. For example, carbon dioxide diffuses through water and animal tissues 25 times faster than oxygen.