 5. Welcome to Physiology Open. Why is diffusion capacity also known as transfer factor? Why do we use a gas whose rate of diffusion is diffusion limited for measuring diffusion capacity for lungs? And why is diffusion capacity affected by blood volume in pulmonary capillaries also apart from respiratory membrane parameters? We will answer all these questions in this video along with the clinical conditions which affect the diffusion capacity of the lungs. The main goal of respiratory system is that gases should reach the tissue from atmosphere and vice versa. For this to occur there should be adequate ventilation, adequate diffusion across respiratory membrane, matching of perfusion with ventilation and adequate transport of the gases. Problems in any of these will lead to hypoxia or both hypoxia and hypercapnia. So there are tests to assess various functions of the lung. In this video we will discuss the test for assessing the diffusion across respiratory membrane for a gas. This is done by determining carbon monoxide diffusing capacity. We have detailed the rate of diffusion of gases across respiratory membrane in another video, the link for which is given in the description section below. In brief, the rate of diffusion of a gas across any membrane is given by fixed law, which states that the diffusion rate is directly proportional to partial pressure gradient for the gas. Then it is dependent on characteristics of the membrane where it is directly proportional to the area of the membrane and inversely proportional to membrane thickness and then the characteristics of the gas where it is directly proportional to solubility of the gas and inversely proportional to square root of its molecular weight. If we combine the properties of lung membrane and the properties of gases together, we call it diffusing capacity of lungs for that gas. Diffusing capacity of lungs because it includes properties of respiratory membrane and for a particular gas because it includes properties of that gas that is solubility and molecular weight. Anyways by combining both these properties, we can rewrite the fixed law as rate of diffusion is equal to DL that is diffusing capacity for lung into the partial pressure gradient P1 minus P2. And now since we want to assess how gases diffuse across respiratory membrane, we need to figure out how much this DL is. So we will rewrite this equation as little bit rearrangement here that is DL is equal to rate of diffusion divided by the partial pressure gradient. Now before we progress, one thing you should know is that fixed law talks only about membrane. However physiologically for the uptake of the gases from the alveoli two things should happen. One the gases should cross the respiratory membrane whose rate of diffusion is given by fixed law and second after entering into the blood they should cross the plasma enter into RBC and react with hemoglobin. And see due to their reaction with hemoglobin it will also prevent the build up of the partial pressure of gas in blood. Thus obviously diffusion rate should depend on its reaction with hemoglobin is it not? The faster it reacts lesser build up of partial pressure and hence more the rate of diffusion. Now this second parameter is known as diffusion capacity of blood which is given by a one rate of reaction of the gas with hemoglobin that is how much gas in ML combines with one ML of blood in one minute per millimetre mercury partial pressure of gas and second the volume of capillary blood. So when rate of reaction of the gas with hemoglobin is multiplied by volume of capillary blood it gives how much gas combined with this much volume of blood per minute per millimetre mercury partial pressure that is diffusion capacity of blood. Now in this formula which we have rewritten even though here we are calling it as diffusion capacity of lungs for a gas when we determine this DL using a gas physiologically both these effects are occurring for the uptake of the gas in the body isn't it? So the value which we get represents both the diffusion capacity of membrane as well as the diffusion capacity of blood. So basically DL represents the entire diffusion distance which needs to be covered by a gas physiologically that's why it's also known as transfer factor because it measures the transfer of gas from air in the lungs to RBC in pulmonary capillaries and not only diffusion of gas across respiratory membrane. So let us see now how it is actually measured. This diffusion capacity is measured using a gas whose rate of diffusion is diffusion limited. See there are two types of gases when diffusion is considered. One the gases whose rate of diffusion is diffusion limited and the other where it is perfusion limited. Diffusion limited means that within the time a column of blood is present near the alveoli the gas is not able to equilibrate across alveoli and blood that is more time is required for partial pressure to become equal on both side. This occurs for gases like carbon monoxide. See carbon monoxide as it enters into blood binds so quickly with hemoglobin because of its high affinity with hemoglobin that due to this its partial pressure in blood is almost negligible or we can say zero. So till the column of blood remains there the gas is not able to equilibrate. Thus the diffusion of carbon monoxide is diffusion limited. Perfusion limited means that within the time available the partial pressure of gases becomes equal on both sides that is equilibrium is reached and if column of blood had moved faster more diffusion of gases would have occurred. So perfusion is the limiting factor for the diffusion of gases. This occurs for gases like oxygen and nitrous oxide. So we are saying that for measuring diffusion capacity of lungs for gases a gas whose rate of diffusion is diffusion limited is used. Since for these gases the rate of diffusion is limited by either rate of diffusion across the membrane or rate of its reaction with hemoglobin in blood and not on circulation. This is what we are interested in right. Anyways so to measure diffusion capacity of lungs for carbon monoxide using this formula we need to know rate of diffusion of gas and its partial pressure gradient. For this single breath technique is used. In this the person first exhales maximally and then inhales maximally a dilute mixture of carbon monoxide containing approximately 0.3% carbon monoxide which is diluted with helium. He then holds a breath for 10 seconds to allow for diffusion of gases and then exhales. Then by determining the difference between the amount of gas inhaled and the amount of gas exhaled and the lung volume by helium dilution method we can know the amount of gas which has diffused and also the alveolar partial pressure of carbon monoxide. So we know P1 and as told earlier P2 is 0 that is the partial pressure of carbon monoxide in blood. Well then we put these values in the formula and get the diffusion capacity or transfer factor. Diffusion capacity of lungs for carbon monoxide measured by this method comes to 25 ml per minute per millimeter mercury partial pressure difference. This value increases two to three times in exercise due to capillary recruitment and capillary distention which is a subject of another video perhaps. But when does this diffusion capacity of lungs for gases decreases? Can you predict it from the concepts we have discussed till now? Basically you have to think about those two parameters that is diffusion across membrane and diffusion capacity of blood. So it will decrease in case respiratory membrane thickens like in case of pulmonary fibrosis or if cross sectional area of respiratory membrane decreases as in emphysema. But again you should remember that it also depends on diffusion capacity of blood that is on blood volume and hemoglobin as we have discussed earlier. So if these are less then also diffusion capacity will be less. So you have to interpret the findings with a bit of caution. Well thanks for watching the video if you liked it don't forget to press the share and subscribe button. Thank you.