 Hey! Welcome to Physiology Open. The partial pressure of oxygen in alveoli is approximately 100 mmHg while that in veins it is 40 mmHg. Partial pressure of carbon dioxide in alveoli is 40 mmHg while in venous blood it is 45 mmHg. As gases diffuse from area of their higher to lower partial pressures, oxygen diffuses from alveoli to blood while carbon dioxide diffuses from blood to alveoli. Now for the gases to cross the alveoli and enter into the blood and vice versa, they should cross the respiratory membrane. This respiratory membrane has thickness of approximately 0.5 micrometers with many layers. These layers from the alveolar side to the side of the blood are the layer of fluid lining the alveoli, alveolar epithelium, alveolar basement membrane, interstitial tissue, capillary basement membrane and capillary endothelium. So basically the gases should cross all these layers to enter the alveoli from blood or from blood to the alveoli. Obviously it will take some time for the gases to cross these layers. So how much time will the gases take? Will that time be same for all gases? Will it have any clinical implications in rate of diffusion of gases if respiratory membrane thickens? We will try to answer these questions in this video. The factors which affect rate of diffusion of gases across any membrane is given by Fick's law. Fick's law states that rate of diffusion is directly proportional to the area of the membrane, the partial pressure gradient for the gas and inversely proportional to the membrane thickness. Then it is also dependent on the properties of the gases that is it is directly proportional to their solubility in the medium and inversely related to the square root of their molecular weight. So basically the rate of diffusion of gases is dependent on three things partial pressure gradient for the gas then the characteristics of the membrane that is the surface area and the thickness and also the characteristics of the gases. So we can very well infer from this that across the membrane the rate of diffusion of different gases will vary depending on their molecular weight and their solubility in the blood. Solubility affects the rate of diffusion indirectly by affecting the partial pressure. With the same concentration of the two gases the partial pressure developed is much less for a highly soluble gas compared to a gas which is more soluble. This relationship is given by Henry's law which states that partial pressure of a gas in a solution is equal to concentration of the dissolved gas divided by its solubility coefficient. So with these concepts let us consider two gases which are very important for survival that is oxygen and carbon dioxide. In Fick's law the two factors of the gas that is the molecular weight and solubility together form the diffusion coefficient of the gas. Now the molecular weight of oxygen is less but its blood solubility is much lesser than carbon dioxide. Because of this the carbon dioxide diffuses much faster. See the numerator is too less for oxygen right? So carbon dioxide diffuses much faster than oxygen if all of the factors are kept same for both the gases. In fact the diffusion is 20 times faster than oxygen. Now let's come to the properties of the membrane. The properties of the membrane which are important for diffusion are its cross-sectional area and its thickness. Lung consists of approximately 300 million every line which in total have a cross-sectional area of 70 square meters. On an average the thickness of respiratory membrane is 0.5 micrometers. By now you might have understood that different gases will cross the respiratory membrane thickness with different rates. So 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 per minute per millimeter mercury. That means 25 ml of oxygen will cross the membrane in one minute for each millimeter mercury partial pressure difference. Obviously diffusion capacity will be much larger for carbon dioxide In effect with partial pressure difference of 60 millimeter mercury across respiratory membrane that is 100 millimeter mercury in algalye and 40 millimeter mercury in vein so you have to subtract this. So there is a gradient of 60 millimeter mercury. So with this gradient oxygen takes 0.3 seconds to equilibrate. However carbon dioxide diffuses much faster despite a very small partial pressure difference of only 5 millimeter mercury. See partial pressure gradient of oxygen is 12 times more than that of carbon dioxide. 5 into 12 is 60 right? But carbon dioxide diffuses 20 times faster. So still it is having an advantage of 8 times right? So it will take less time for it to reach equilibrium. Well what is the use of knowing all this? See in normal conditions this time that is 0.3 seconds is enough for partial pressure of oxygen to equilibrate and become 100 millimeter mercury in blood and thus hemoglobin gets fully occupied with oxygen. Because a column of blood remains at a spot for about 0.8 second that's the duration of a single heartbeat right? So there was a reserve time of 0.5 seconds. In effect if the blood had moved faster and another column of a blood with lesser partial pressure of oxygen that is 40 millimeter mercury would have arrived near to the alveoli more oxygen would have diffused into the blood due to partial pressure difference. However 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. Perfusion is the limiting factor right? Now what happens when heart rate increases as when your body does more work like when climbing stairs or doing exercise? If we consider maximum achievable heart rate as 200 duration of cardiac cycle at this heart rate will be 60 divided by 200 is equal to 0.33 seconds. So even with this heart rate the time is sufficient for the oxygen to cross the membrane. So if oxygen is able to cross the membrane obviously carbon dioxide will also be able to diffuse in that time. Since carbon dioxide diffuses faster than oxygen. Now consider a situation in which respiratory membrane thickens. Say for example there is pulmonary edema causing collection of fluid in the interstitium and in the alveoli. So obviously gases will have to cross a thicker membrane. As fixed 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 decreased. So basically the diffusion capacity of lungs for gases will decrease. Now we have discussed that oxygen takes 0.3 seconds to equilibrate which is sufficient even during exercise. But with increase in the thickness diffusion time will increase say up to 0.5 seconds. Now see this time is adequate for rest since till 0.8 seconds 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. 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. So in disease conditions the diffusion of oxygen becomes diffusion limited rather than being perfusion limited. What about carbon dioxide? Since rate of carbon dioxide diffusion 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 carbon dioxide retention does not occur. This will cause type 1 respiratory failure where hypoxia occurs but hypercarbia that is increased carbon dioxide levels are not seen. Okay. Thanks for watching the video. If you liked it don't forget to press the share and subscribe button. Thank you.