Diffusion of gases through respiratory membrane | Diffusion vs perfusion...

The partial pressure of oxygen in alveoli (pAO2) is approximately100 mmHg while that in veins (pvO2) is  40 mmHg . Partial pressure of carbondioxide in alveoli is 40 mmHg.while in venous blood (pvCO2) is 45 mmHg. As gases diffuse from area of their higher to lower partial pressure, oxygen diffuses from alveoli to blood which carries venous blood from right side of heart to lungs while carbon dioxide diffuses from blood to alveoli. 

 For the gases to cross the alveoli and enter into the blood and vice versa, the gases should cross the respiratory membrane. This respiratory membrane has  layers which  from the alveolar side to the side of blood are: 

1. the layer of fluid lining the alveoli

2. alveolar epithelium 

3. alveolar basement membrane 

4. Interstitium 

5. capillary basement membrane 

6. capillary endothelium 

The factors which affect rate of diffusion of gases across any membrane is given by Fick’s law which states that rate of diffusion 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. 

For this particular 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. Oxygen takes 0.3 sec to equilibrate. In normal conditions,  0.3 sec is enough for partial pressure of oxygen to equilibrate and become 100 mmHg and thus Hb gets fully occupied with oxygen..because a column of blood remains at a spot for about 0.8 sec.   Since circulation doesn’t move, more diffusion does not occur. So basically, circulation is the limiting factor for the diffusion of oxygen. So we call this as diffusion of oxygen being perfusion limited. 

Now what happens when heart rate increases  If we consider maximum achievable heart rate as 200, duration cardiac cycle at this heart rate will be (60/200) = 0.33 sec. So, even with this heart rate, the time is sufficient for the oxygen to cross the membrane. 

 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 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 rate of diffusion carbondioxide is faster, its diffusion will not be limited. So even with increased respiratory membrane thickness, carbondioxide 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.