 Functional residual capacity is the amount of air which is present in the lungs at end of tidal expiration. So, this functional residual capacity cannot be exhaled out of the lungs fully. Yes, some amount can be exhaled out that is part of the expiratory reserve volume. Actually, this is expiratory reserve volume plus residual volume. So, since residual volume cannot be exhaled out of the lungs, functional residual capacity cannot be measured using a spirometer because the spirometer measures only the amount of air which is inhaled from the spirometer and which is exhaled into the spirometer. If the air is remaining in the lungs, it cannot be measured by spirometer. So, there are various methods for measurement of functional residual capacity and these include the helium dilution method, nitrogen washout method and body plethysmography. Now, since functional residual capacity is the amount of air remaining in the lungs at end of tidal expiration, so what we will see that all these methods start at the point of end of tidal expiration. So, this concept you should remember. So, let us see one by one how these methods work. So, in helium dilution method, what we do that is spirometer is filled with a known concentration of helium. So, this concentration of helium is known, so we call it concentration 1 and we know the volume of the spirometer how much it is normally it is 9 liters, so that is volume 1. Now, what we do is we connect the person to the spirometer and we ask the person to start inhaling from the spirometer at end expiration. That means when the volume of the lungs is how much it is equal to functional residual capacity. This is what we have discussed before. So, he starts inhaling at end expiration. Now, he is asked to continuously inhale and exhale into the spirometer. So, what will happen? This helium will go into the lungs and obviously some will come out. So, he is inhaling from the spirometer and exhaling back and when this process continues for few breaths, what happens that helium molecules they become equilibrated in the spirometer and in the lungs. So, the equilibrium will be achieved. So, say suppose final concentration of the helium in the spirometer. So, once the equilibrium is achieved to be again determined the concentration of helium in the spirometer that is C2 and now the volume in which the helium is present is both in the spirometer as well as in the lungs, isn't it? So, how much is the volume now? It is the volume of the spirometer that was V1 plus the volume of the lungs let's take it as VL and since the manoeuvre started at end expiration. So, this is equivalent to FRC. So, now what we do since the amount of total helium is constant it is not being absorbed anywhere it is not getting exhaled out in the atmosphere. So, the total amount of helium is constant. So, we apply that law of mass conservation that is C1 into V1 is equal to C2 into V2 and here V2 is this V1 plus VL. So, this is the diagrammatic representation of that initial V1 C1 and after equilibrium V1 plus VL and new concentration after equilibrium that is in the spirometer as well as in the lungs. So, this red dots are depicting the presence of the helium by using this formula we can determine VL see V1 is already known as I told you C1 is known C2 concentration of helium we have to measure in this spirometer that will be known only VL is not known. So, by using this formula we can determine the VL that is functional residual capacity. So, let us take one example for this say suppose V1 is equal to 9 liters that I told you is the volume of the spirometer and concentration of the helium in the spirometer is say suppose 10 percent then the person starts breathing and after equilibrium say suppose a concentration becomes 8 percent. So, we just take a sample from the spirometer and determine the helium concentration now can we determine the V2 yes we can determine simply apply the formula for law of conservation of mass V1 into C1 is equal to V2 into C2. So, V1 is 9 C1 is 10 right then V2 is not known that is V1 plus the functional residual capacity and C2 is 8 percent. So, this comes to how much divided by 8 is equal to V2. So, this will come to 11.25 liters and now to determine that VL what we have to do remember this V2 was V1 plus VL and since V1 was 9 liters this VL is equal to 2.25 liters. So, here we get the value of FRC using helium dilution method with this let us go on to the next method that is the nitrogen washout method in nitrogen washout method we start from the lungs in C helium dilution method we started from the spirometer spirometer volume and concentration was known in nitrogen washout method we will wash out the already present nitrogen in the lungs. So, when a person is breathing air we know the concentration of nitrogen in air is 80 percent right. So, same will be the concentration of nitrogen in lungs so that is 80 percent and volume at end expiration again it is FRC so V1 which is not known. So, now the person will start inhaling 100 percent oxygen ok. So, he is connected to a spirometer which is having 100 percent oxygen fine and what happens he exhales he is inhaling through this spirometer, but he exhales into another bag and this bag is known as Douglas bag. So, this process is continued where he inhales 100 percent oxygen and exhaled air is going into the Douglas bag. Now remember inhaled air is having only 100 percent oxygen, but exhaled air is having nitrogen also because that nitrogen is present in the lungs. Now if this process is continued for say 6 to 7 minutes then all the nitrogen which is present in the lungs it is going to be replaced by oxygen and this nitrogen will be collected in the Douglas bag. So, what we do after 6 to 7 minutes we determine the concentration of nitrogen in this Douglas bag say that we will name it as C2 and we determine the volume of the air which has come into the Douglas bag. So, in 6 minutes total volume which is exhaled out that we will call as V2. So, again there is no loss of nitrogen whatever nitrogen was present in the lungs has now come into the Douglas bag. Again we will apply that conservation of mass formula that is C1 into V1 is equal to C2 into V2. So, what is going to happen C1 we know it is 80 percent C2 we can determine by taking a sample from this bag we can determine and V2 again we know how much gas is exhaled into the Douglas bag that we can determine. So, now using this formula we can find out how much is V1. So, again let us take one example of this nitrogen washout method. So, C1 we already know that is 80 percent we not do anything for that because the person is inhaling air normally V1 we do not know that is the FRC. C2 we have to determine from sample from the Douglas bag say suppose C2 is 4 percent and the total volume of exhaled air say suppose is 50 liters simple now we can determine V1. So, let us try doing that C1 into V1 is equal to C2 into V2 80 into V1 is not known C2 is a 4 and V2 is 50 liters calculating V1 4 into 50 divided by 80. So, how much it will come it will come to 2.5 liters. So, that is the nitrogen washout method. Now these methods the helium dilution method and nitrogen washout method are not the best methods to determine FRC why because in this if the alveoli are poorly ventilated then these methods will give less value of FRC. For example say suppose helium dilution method I consider and this is the lungs right at FRC say suppose volume is there and here there are some poorly ventilated alveoli. Now here also normal air will be present isn't it but when the person is inhaling from the spirometer what will happen that all this air will equilibrate with the spirometer right but since these are poorly ventilated alveoli it will not equilibrate with the gases in the spirometer it will have that old air only. So, what we determine as a VLA or we can say V2 it includes a V2 includes only the spirometer volume and this much volume okay this this part not this volume. So, you see FRC determinant will be less. So, the best method for determining FRC is body pletismography and let us see what is this body pletismography method. So, this is a schematic diagram showing body pletismograph actually this body pletismograph is a closed cavity where they ask the person to sit and what we do is we measure the airway pressure okay and we measure the pressure in the pletismograph. So, here in the cavity what is the pressure that is measured and the airway pressure is measured which basically represents the pressure in the lungs. Then volume of the pletismograph is measured and again you see at end expiration what is the volume of the lungs that we have to determine. So, these two we are measuring okay volume of the pletismograph will be known like we know the volume of the spirometer similarly we know the volume of the pletismograph and this VL has to be determined. So, in body pletismography what is done that at end expiration the person starts making some efforts to inhale the gas. Now actually there will be no movement of the gas because he is trying to inhale with closed glottis but when we inhale what happens that the lungs tend to expand right the lungs tend to expand okay. So, there is some change in the volume of the lungs and why do they expand because of the change in the pressure in the lungs understanding. So, using Boyle's law we can say P1V1 in the lungs is equal to P2V2 in the lungs where P1 is the airway pressure we are measuring known right. V1 is not known we can call it as VL that is the volume of the lungs at FRC. Then P2 is the pressure of the airways when the person is started making that effort so we will call it PA2 and V2 is that how much lung expansion is occurring so that will be VL plus delta V. So, we don't know this also VL plus delta V also we don't know what we know PA1 and PA2 that is known. But when the person makes this inspiratory effort see the person is sitting in a closed cavity. So, here is the closed cavity any change in volume in the lungs happens at the cost of change in volume in the plethysmograph. So, if there is increase of delta V in the lungs similarly there is decrease of delta V in the plethysmograph. So, we will apply this Boy's law to plethysmograph as well. So, let us see. So, what happens to plethysmograph? PB1 that is the pressure in the plethysmograph multiplied by the volume of the plethysmograph that is the initial one. So, these two are known and then there is PB2 that is when the person starts making the inspiratory efforts then there is decrease in the volume of the plethysmograph but increase in the pressure of the plethysmograph. So, this we can measure we are measuring the pressure in the plethysmograph and this is the VB2 the volume change in the plethysmograph which can be written as VB1 minus delta V because delta V is the change. Now, in this equation if you see we know PB1, we know PB2, we know VB1 only this delta V has to be determined. So, by using this equation we determine delta V. So, first we apply the Boy's law to the plethysmograph obtain delta V from this equation and then we use this delta V in the Boy's law which we applied to the last. So, how it will come? So, this was the equation we were talking airway pressure 1 known VL not known airway pressure 2 again we are measuring it is known and this delta V we have obtained from the equation of the plethysmograph. So, if we use this equation here we can easily determine what is VL that is the functional residual capacity of the lungs. So, this body plethysmograph is the best measure of functional residual capacity. So, that was all about measurement of functional residual capacity and remember if we are able to measure functional residual capacity then we can easily determine residual volume as well. Why? Because functional residual capacity is equal to expiratory reserve volume plus residual volume. So, this expiratory reserve volume we can determine using a spirometer and this we have determined by either of these methods. So, by using this equation we can determine the residual volume as well. So, that was all about the measurement of functional residual capacity and residual volume. 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