Physiology lecture on respiratory membrane physiology details Diffusion across respiratory membrane
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For the gases to cross the alveoli and enter into the blood and vice versa, the gases should cross the respiratory membrane.
The factors which affect rate of diffusion of gases across any membrane is given by Fick’s law which states that rate of diffusion across respiratory membrane is directly proportional to the area of the membrane, partial pressure gradient and inversely proportional to membrane thickness. Then it is also directly proportional to their solubility in the medium and inversely related to the square root of their molecular weight.
In Fick’s law the two factors of the gas i.e its the molecular weight of oxygen and solubility together form the diffusion coefficient of the gas. The molecular weight of oxygen is less but its blood solubility is much lesser than carbondioxide.. because of this carbondioxide diffuses 20 times faster than oxygen.
So for this respsiratory membrane, the rate at which different gases will cross is expressed by diffusion capacity of the lungs for a given gas. Normally diffusion capacity of lungs for oxygen is 25 ml/min/mm Hg. That means 25 ml of oxygen will cross this membrane in 1 min for every each mmHg partial pressure difference. Obviously diffusion capacity will be much larger for carbondioxide.
Now consider a situation in which respiratory membrane thickens, As Fick’s law states that rate of diffusion is inversely proportional to the thickness of the membrane, so obviously the rate of diffusion of gases will be affected. With increase in the thickness, diffusion time for oxygen will increase say upto 0.5 or even 0.6 sec. This time is adequate for rest, since till 0.8 sec a column of blood is available. But during activity, when the heart rate becomes faster and duration of cardiac cycle decreases, then there will not be adequate time for oxygen to equilibrate and hence it will cause hypoxia. So in this case, perfusion is not the limitation, rather the rate of diffusion becomes the limiting factor. This is known as diffusion-limited.
Since the rate of diffusion carbon dioxide is faster, its diffusion will not be limited. So even with increased respiratory membrane thickness, carbon dioxide could be expelled from the body. That is the reason that even in severe cases of thickening of respiratory membrane like in pulmonary fibrosis hypoxia occurs but carbondioxide retention does not occur.
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Chapters:
00:00 Introduction
00:54 Respiratory membrane layers
01:36 Factors affecting rate of diffusion of gases across respiratory membrane
04:14 Diffusion capacity of respiratory membrane
05:37 Application of diffusion of gases across respiratory membrane
06:05 Diffusion vs perfusion limited
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