 Hello friends, welcome to physiology open. Today we will talk about Laplace law and its application in alveolus in physiology. So, for a thin walled sphere, Laplace law is 2t is equal to p into r, where t is the wall tension and p is the pressure in the sphere. So, what is this tension and pressure? See any sphere is if it is filled with some contents, its contents will exert a pressure on its wall. So, here this pressure which is acting on the walls is the p and tension which is also defined as force per unit length. It is a force acting tangentially on the wall something like this. So, one thing you should remember that the vertical component of this tension that if we take a pictorial of this tension which is acting on the wall, it should always counterbalance this pressure of the contents on the walls. If it does not happen what will happen that the pressure which is acting it will rupture the walls. So, in physiological conditions this tension is counterbalanced by the pressure. Now, in certain physiological conditions we see this equation as this like how much pressure of the contents is causing how much tension on the walls. So, we can write it something like this, but in other physiological conditions we see this equation is like this that is tension on the walls is generating the pressure inside the sphere. So, in case of alveolus we see the equation that how much tension is generating the pressure inside the alveoli. So, let us take an example of two alveoli to better understand this. So, here we will take one big alveoli and one smaller alveoli that is with a smaller radius. Now, see that all the alveoli are connected to each other ok. So, these alveoli are lined by a lining of fluid and remember that the air these alveoli are air filled ok. So, basically this fluid is in contact with the air in the alveoli. Now, whenever fluid comes in contact with it generates a surface tension. So, here this fluid lining its contact with the air is generating a tension on the walls of the alveoli something like this that told you that it is a tangential force and because of this tension a pressure develops in the alveoli. Now, if we say that the tension is T in both the alveoli according to the equation which we have written before that is 2T is equal to P into R. Can you tell me that how much pressure will be generated in these alveoli will it be same or it will be different? See in the bigger alveoli since R is more R is larger its radius is larger same tension will create less pressure. So, say suppose in the larger alveoli we say pressure is P1 while in the smaller alveoli same tension you see radius is lesser same tension is going to generate a more pressure. So, in this case pressure in this alveoli is going to be P2 and P2 will be greater than P1 because of the radius. So, basically radius is determining how much tension is generating how much pressure ok. So, if P2 is greater than P1 and by the virtue of all the gases and liquids being flowing from higher pressures to lower pressures you can very well understand that the air from this smaller alveoli will empty into the larger alveoli. So, basically this alveoli is going to collapse because of the higher pressure being generated because of the tension ok. And this particular alveoli may be connected to some bigger alveoli let us draw like this. So, you might have understood that according to this equation in this larger alveoli pressure will be lesser. So, in this case P1 will be greater than P say suppose P is the pressure here. So, again this the air in this will empty into the larger alveoli. So, because of this we can say that all the smaller alveoli will subsequently enter into larger and larger alveoli thus forming a big alveoli single alveoli. But does lung consist of only single alveoli? No. So, that means this law as such is not manifesting here there is something which is preventing this and that thing is surfactant. So, surfactant is something which reduces the surface tension in the alveoli. So, you may say that obviously it will reduce the surface tension in both the alveoli correct. But the effect of the surfactant in the smaller alveoli is more. So, it reduces the tension in the larger alveoli, but it reduces the tension in a smaller alveoli much much more. We can understand it this way that say suppose 10 molecules of surfactant are there just arbitrary numbers we are taking in the larger alveoli they will be dispersed something like this right. So, they will be dispersed far apart, but same 10 molecules in the smaller alveoli will be dispersed which is closer. So, their effect on the smaller alveoli will be more. So, it is an easy way of understanding that how surfactant is reducing the surface tension more in case of smaller alveoli. So, this surface tension which we had spoken will be much lesser in the smaller alveoli. So, obviously the pressure generated will also be not that much right and smaller the alveoli more will be the effect of the tension and hence more will be the effect on the pressure generated. So, this phenomena is not happening in alveoli. So, that is how surfactant prevents collapse of a smaller alveoli into the larger alveoli. Okay friends thanks for watching the video that is all for Laplace law in alveoli. If you like the video do like and share the video and don't forget to subscribe to the channel Physiology Open. Thank you.