Physical Laws in Respiratory Medicine

Published on January 2017 | Categories: Documents | Downloads: 34 | Comments: 0 | Views: 237
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Some Key Physical Laws in Resp Medicine 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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