 Hello all, welcome to Physiology Open. Before we start, just want to let you know that I have put up a question in the comment section below. Try to think about it and answer it after watching the video and I will give an instant feedback to your responses. Okay, now let's discuss lung compliance. Compliance in simple terms means stretchability. If an external force tries to pull or stretch any elastic substance, it stretches or expands. If we ask the question that how much compliant that elastic substance is, we should know how much force caused how much change in length and then we express it as change in length per unit applied force. Now let's take an example of two different springs. If we apply same force to pull both of them, suppose that blue spring pulls this much length, while the red one stretches more, maybe this much length. Can you tell which of these is more compliant spring? Well, obviously this red one isn't it? Since change in length per unit change in force is more for this red one. So we call that this red spring is more compliant than the blue spring. Similar kind of concept applies to our thoracic cage and lungs. Both are elastic structures and can expand. But here compliance is expressed as change in their volume per unit change in pressure across their walls. So basically force created by contraction of inspiratory muscles pulls the thoracic cage and this at the same time causes change in pressure across the thoracic cage. Similarly, the change in plural pressure due to thoracic cage expansion creates pressure difference across lung causing its expansion and hence increase in its volume. Now whose compliance are we talking about here? Is it lung compliance, thoracic cage compliance or is it compliance of both thoracic cage and lung combined? Think about this, that since lungs are housed inside the thoracic cage, shouldn't we determine the compliance of both together as a unit? Well, both thoracic cage and lung function are important for normal functioning of the respiratory system. For example, say Y amount of change in pressure is required to cause X amount of change in volume. Now see thoracic cage is a stiff, our inspiratory muscles need to work more to cause same expansion of thoracic cage and hence cause change in Y pressure across the lungs. So even if lungs are normal, work will increase. Similarly, thoracic cage is normal and lungs are stiff. Then the Y pressure across lung wall will cause lesser volume change than X in lungs. They may be X minus 10. So to bring X amount of air, more pressure change is required. So again inspiratory muscles will have to work more and cause more expansion of thoracic cage so as to generate more pressure across the lung walls. So in first case for the generation of the same pressure more force is required while in the second case the same pressure is not enough for lung expansion and hence thoracic cage should expand more to cause more pressure change. So basically individual compliance will tell us about where the problem is and combined compliance of lungs and thoracic cage tells us about how lungs and thoracic cage are operating as a unit. From now on we will focus on lung compliance only. Okay till now we have discussed that to determine how much the lung is compliant we need to know the pressure difference across its walls and how much lung volume change is occurring with that much pressure difference. But here by pressure difference across its walls means which pressure? There is atmospheric pressure outside thoracic cage then plural pressure between the plural membranes and within the alveoli there is alveolar pressure. So which pressure should we take? See for any unit we should take the difference between the pressure in the space immediately adjacent to it. So for lung compliance we will take difference between alveolar and plural pressure. They are the ones which are immediately adjacent to it. This difference is also known as transpulmonary pressure. Can you guess difference between which pressure should we take for determining compliance of thoracic cage? Yes for thoracic cage we will take difference between plural pressure and atmospheric pressure. Okay so how much is the change in normal transpulmonary pressure during tidal inspiration and tidal expiration? At equilibrium that is at the end of tidal expiration when no air is flowing from atmosphere to the alveoli alveolar pressure is 0 that is equal to the atmospheric pressure while plural pressure is minus 5 centimeters water. So transpulmonary pressure at this point is 0 minus minus 5 so this is equal to plus 5 centimeters water. Now plural pressure during inspiration is minus 8 centimeters water while alveolar pressure is minus 1 centimeters of water. So transpulmonary pressure will be minus 1 minus minus 8 so it will be plus 7 centimeters water. Okay now let's calculate it for expiration. Plural pressure during expiration is minus 5 centimeters water while alveolar pressure is plus 1 centimeters of water. So transpulmonary pressure will be plus 1 minus minus 5 so it will be plus 6 centimeters of water. Considering tidal volume as 500 ml for a volume change of 500 ml during inspiration the change in transpulmonary pressure is from plus 5 to plus 7 that means 2 centimeters water while during expiration this change is from plus 6 to plus 5 only that is 1 centimeters water. So for similar change in volume during inspiration and expiration pressure changes are different. That means lung compliance is different during inspiration and expiration. This pressure volume relationship of lung during inspiration and expiration is expressed using pressure volume curves. So now let's look at the pressure volume curve of the lungs. In this graph X represents transpulmonary pressure in centimeters water while Y represents volume change in ml. Notice two things here. First one that the relationship between pressure and volume change is not linear rather slope is less or it's kind of flat at the base and at the top. Second thing the relationship is different during inspiration and expiration. See the graph lines are different. Okay first let's see why it's flat at base and top. Flatter path means that for a similar change in pressure volume change is less. See I will take similar change in X axis for the transpulmonary pressure. Mark it on the graph maybe on inspiration line and then see how much volume is at base pressure point. See at the top volume change is only this much while it is more in the central part of the graph for the similar pressure difference. Similar is the case with low lung volumes here at the base. So that means lungs are stiffer at very high and very low lung volumes. You can think of it as a balloon at the beginning of its inflation it's difficult to inflate isn't it? Then everything goes on smoothly in the middle part of balloon inflation and when it's completely filled then also more force is required for causing minor change in its volume. Okay now let's come to other phenomena that why the graphs are different during inspiration and expiration. See in this graph we will take two points of volume say from here to here and now we will extend it to the transpulmonary pressure for both inspiration and expiration lines. See the pressure values during inspiration and expiration graphs? It is much lesser for expiration graph isn't it? That simply means that pressure required to inflate the lungs is much greater compared to the pressure required to deflate the lung. This phenomena of lung behaving differently during inflation and deflation is known as hysteresis. But why is this suffering? Why more pressures are required for inflating the lungs than deflating? Well see again and again we are seeing that lungs are elastic structures that is they can expand but remember that they also have a tendency to recoil back to their neutral position when it's stretched and as soon as that external force is removed which is causing the stretch they come back to their resting position because of this recoil. This is because major component of lung connective tissue is elastin and there is collagen also. Elastin provides elasticity and collagen gives tensile strength to lungs. That means due to inherent makeup of the lung tissue that is due to elastin this elastic tissue is resisting expansion but is promoting recoil. So during inspiration there is an opposing force that recoil is there and during expiration there is promoting force. So obviously pressure change required will be different during inspiration and expiration since one more factor is coming inside right? So can you think what will happen to lung compliance if this elastic tissue is destroyed? This occurs in emphysema or in case of aging lungs or what will happen if there is too much collagen as occurs in fibrosis. See in emphysema recoil is decreased causing increase in lung compliance. Too much fibrosis on the other hand makes it a step affecting its expansion. There is another factor which affects lung compliance during inspiration and expiration. It's the surface tension. See all alveoli are lined by a thin layer of fluid which is secreted by their lining epikellium only. As fluid has a natural cohesive property fluid molecules pull together tending to collapse the alveoli thus opposing the expansion of the lungs. That means left to itself there will be two forces opposing lung expansion one is elasticity and second the surface tension of fluid lining the alveoli. It will be too difficult for the lungs to expand isn't it? So this surface tension is counteracted by surfactant secreted by a type 2 alveoli cells which decrease the quiesce pose between the molecules and hence decrease the surface tension. So can you tell what will happen to lung compliance if surfactant is not produced? Yes, lung expansion will be affected too much and there will be extreme tendency to recoil due to surface tension. Alright now try to answer the question given in the comment section below to check whether you have understood the concept or not. And thanks for watching the video if you liked it do share the video and yes don't forget to press the subscribe and like button thank you.