Page 1 of 1: physical laws helpful in medicine

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Fick's Law:
• Deals with diffusion of a gas across a membrane (e.g. at the blood-gas interface in the alveoli).
• It states that the rate of diffusion is dependent on the surface area of the membrane and how thin it is.
• The rate is also dependent on the membrane's partial pressure gradient and how soluble the gas is.
• So if the membrane becomes thickened in disease (e.g. fibrosing alveolitis) the rate of diffusion decreases.
• And if the partial pressure gradient increases (e.g. due to oxygen therapy) the rate of diffusion increases.
Laplace's Law:
• States that the pressure within a bubble is equal to twice the surface tension divided by the radius
• This tells us that smaller bubbles must have a greater internal pressure to keep them inflated
• The alveoli are like a succession of tiny bubbles - they are lined with liquid and are in contact with air.
• Because they are so small, the internal pressures should be very high and the alveoli very hard to expand
• In fact, surfactant in the alveolar lining fluid reduces surface tension and makes the 'bubbles' much easier to blow up.
Henry's Law:
• Deals with gases dissolved in liquids.
• It states that the amount of gas dissolved at a given temperature is proportional to the partial pressure of the gas and its solubililty in the liquid.
• The solubility of oxygen in the blood is low.
• To meet the body's demands for oxygen, most is not carried in simple solution.
• CO2 is much more soluble in blood than O2; about 10% can be carried in solution.

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