 Floor volume curve is used to study the flow rates of air as we exhale the air forcefully and maximally from the total lung volume up to the residual volume. So basically first we ask the subject to fill his lungs maximally to total lung capacity and then we ask them to exhale forcefully and maximally and determine the flow rates right. So you see this aspect is showing the expiration flow rates and then flow rate is determined when he is filling his lungs from residual volume to total lung capacity. So this is the inspiration phase and here again also we are determining the flow rates. Now if you see this graph here the person has started exhaling from the total lung capacity and you see what is happening that the flow rate is continuously increasing up to a maximum limit. So even when the person is putting maximum pressure he is trying to exhale maximally. Now the flow rate has reached a certain limit it cannot increase any further even if we ask them to put more and more effort okay and after this the flow rates start decreasing. So why is this happening that there is a particular maximum flow rate? Well to understand this we need to understand the concept of dynamic flow limitation. It is important because in diseases like COPD this maximum flow rate will not be reached despite whatever force the person applies okay. So let us see what is this dynamic flow limitation. See for any gas or liquid to move from point A to point B there should be some pressure difference between these two points right and the pressure at point A should be more than that of the point B right then the flow will occur from A to B. So in expiration what happens that the flow has to occur from alveoli towards the atmosphere so obviously alveolar pressure has to be more than that of the atmospheric pressure. Now in tidal expiration the alveolar pressure is plus 1 millimeter mercury that of the atmosphere. So we know that the atmospheric pressure is 760 millimeter mercury so the alveolar pressure is how much? It is 761 millimeter mercury. Now as we exhale and the air moves from the alveoli passing from the various branches of the tracheobronchial tree towards the mouth and then to the atmosphere there is continual drop of pressure along this tracheobronchial tree. So here if the pressure is 761 millimeter mercury then just outside alveoli it will be 760.9 here it will be 760.8 okay. So here I am talking inside inside these airways the pressure is decreasing as it is moving towards the mouth and this drop in pressure is due to frictional resistance to flow because the molecules are colliding with each other as they are flowing always there is some friction. So that is frictional resistance to flow and it is known as R A W. A W is airway and R is the resistance and its value is approximately less than 2 centimeters of water per liter per second fine. Now let's see what happens in forceful expiration. See in forceful expiration air flow rate increases and whenever the air flow rate increases pressure drop along the airways is much much more what pressure drop we saw in case of tidal expiration right it was very minute as it is change going from alveoli to the mouth but here the pressure drop is much more and this pressure drop is known as a dynamic component of resistance and it is occurring due to the increase in the velocity of flow. So let us see how and why it is occurring. See our tracheobronchial tree is basically branching. So where branches are more cross-sectional area is more. So if we compare these central airways compared to these branches here the cross-sectional area is more isn't it? Now flow we write as volume upon time okay. So this flow is constant throughout the airways. So just take one example say suppose 250 ml of air is flowing per second right so that is the volume upon time that is the constant. Now depending on which portion of the tracheobronchial tree this flow is occurring the velocity is different along the tracheobronchial tree. Why is it so? See velocity is equal to distance upon time right. Now this velocity we can also write as flow upon area. Now you will ask how come this has come? See it is simple flow we said is volume upon time right and this is the area so we can write area here okay. Now just divide this is volume area so this will cancel and one dimension will be left that is distance. So we are getting same thing velocity is equal to distance upon time. So let us put it as flow upon area right. So if flow is constant wherever area is more velocity will be less right. So in case of branches the velocity of flow is less understanding and as the air is moving through the central airways where the area is much less the cross-sectional area is much less the velocity is much higher. So what we are telling is that here the velocity is less and here the velocity is much more. Now from where will the energy for this increase in velocity should come when it will come from the pressure difference okay. So we were talking about a forced expiration now in forced expiration here in the alveoli pressure is much more it is approximately 30 millimeter more than that of the atmosphere. So what we saw in tidal expiration it is 761 here it will become 790 millimeter mercury and the atmospheric pressure we said is 760 millimeter mercury. So you see the drop in pressure is going to happen much more in case of forceful expiration because this is brought about by increase in the velocity. So the energy is coming from the pressure right. So this will cause the drop in airways much more than that of the tidal expiration. So what is the point of all this? Basically we are trying to say that despite the maximum effort which the person is putting there is a maximum limit to increase in the peak flow rate. Now let us see how it will occur. See what happens that there is something known as trans airway pressure which keeps the airways open. Now this trans airway pressure is equal to airway pressure that is the pressure inside the airways minus the plural pressure okay. So there is pressure outside also and there is pressure inside also fine. So when there is movement of the air the pressure is dropping so here it is maybe 785 millimeter mercury then here it is a 780 millimeter mercury so it is continuously dropping. So here this trans airway pressure is keeping the airways open right but you see further it is dropping. So in this portion you see trans airway pressure is less right because airway pressure has dropped. So trans airway pressure has decreased right then it will have dropped further and where the velocity is increasing further it would have dropped and there may be a point where this airway pressure will become less than that of the plural pressure. So what will happen? That there will be an outside pressure which will be more than that of the pressure inside the airways and this is going to cause the narrowing of the airways fine. So you see as the velocity increases there is more drop in airway pressure right airway pressure is decreasing and this is going to lead to the narrowing of the airways. Are you getting the point? So narrowing of the airways now is going to decrease this velocity because it is the diameter of the airways which also determines the velocity okay. So you see how a balance is coming that the effort by the individual is causing increase in velocity which is causing decrease in airway pressure which will cause the narrowing of the airways and hence this is going to cause the decrease in velocity. So you see a point is reached where a balance is occurring and despite the maximal effort the velocity or the peak flow rate is not going to increase any further and that is what we saw in that diagram here that there is a maximum peak flow rate which is reached fine. Now with this concept let's try to understand this flow volume curves more. So here you see with maximal effort it reaches a maximum limit and then flow rate is decreasing but why the flow rate is decreasing and why it is not constant throughout like this till residual volume okay it is not reaching further maximum that we have understood but why it is not remaining the same why the flow rate is continuously decreasing. Well you see what is happening that as the lung volumes is decreasing the recoil of the lungs is decreasing and if the recoil of the lung decreases what will happen the alveolar pressure is going to be less and if pressure is going to be less then the driving force is going to be less and hence the flow rate is going to be less. So that is why with decrease in the lung volume the flow rate decreases because of decrease in the recoil of the lungs and finally at residual volume no air flows out because this is the point that the pressure in the alveoli is so less that the pressure in the airways also becomes very less and finally this leads to the compression of the airways since we said that plural pressure becomes more okay so this causes the compression of the airways at such low lung volumes there is a compression of the airways causing the trapping of the air that is what that is residual volume. So at residual volume no flow because of the dynamic compression of the airways. So if I just summarize this you see what we are saying is that maximum expiratory flow rate at any particular lung volume right. So at this lung volume this is the maximum flow rate this lung volume this is the maximum flow rate and this is the total maximum expiratory flow rate which can be reached right. So what are the factors on which this maximum expiratory flow rate at various lung volume depends well obviously first it depends on the lung volume itself as lung volume decreases maximum expiratory flow rate is decreasing because of decrease in recoil then it depends on the airways airway cross section how distensible it is if there are any diseases which cause the narrowing of the airways then this maximum expiratory flow rate is going to decrease then it also depends on the frictional loss right. So for example there is gas which is very high density you are annealing a gas with high pollutants then what will happen then also this frictional resistance is going to be more and maximum expiratory flow rate is going to be less. Okay I hope you have understood this concept so let us see certain diseases which can affect this flow volume curve. Now there is this red graph shown with the logic which we have understood till now can we say that what kind of graph or what kind of disease this is representing so first we will observe what has happened see first thing is obviously the peak expiratory flow rate has decreased right what about the lung volume the lung volume is more right you see total lung capacity is more and residual volume is more here so it is operating at higher lung volume so despite more lung volume the expiratory flow rates are less right so lung volume is not the problem in this case then what can be the problem problem can be the airway problem right so yes this is depicting a disease with the airway problem and that is the obstructive lung diseases okay fine now let's go to the second graph here in this graph what is happening see the lung volumes are less here right residual volume is less total lung capacity is also less but you see at a particular lung volume so at this particular lung volume say suppose this particular lung volume in the main graph the maximum expiratory flow rate was this but in this particular graph for a particular lung volume expiratory flow rate is more isn't it similarly you see here here there is no flow but in this still flow is occurring so in this particular graph expiratory flow rate is more why is it so well in this the lungs are collapsing more recoil of the lungs is more and this is basically a restrictive lung disease if their lungs are not expanding rather they are collapsing more and as we have said that increase in the elastic recoil will increase the alveolar pressure and this is going to keep the airways distended so in this case because of the more elastic recoil of the lungs airway pressure is more and for a particular lung volume you see the flow rate is more so this is a case of restrictive lung diseases where lung volumes are reduced but flow rate for a particular lung volume is more so hopefully you have understood this concept of flow volume curve the concept of dynamic limitation to airflow as well thanks for watching the video if you liked it do share the video with your friends and don't forget to subscribe to the channel physiology open thank you