I really need help with this. It's driving me crazy. The kidney countercurrent multiplier is a freaking joke compared to this. This thread since it already contains so much relevancy. I've gotten to a point where I'm so confused that I can't even put in words what exactly I don't understand here. Been reading Berne & Levy and Guyton, and relistening to recorded seminars and lectures given by my professors, and I simply can't get a grasp of this. Why is the concept of transpulmonary pressure needed and what is it anyway? Especially since everyone claims it's equal to elastic recoil pressure. Also, why is this elastic recoil described as pressure? Isn't it more like potential energy? Back to the first questions the op asked. Please ignore the pathophysiology and pneumothoraxes. Let's deal with these basic concepts first. What determines the elasticity of the lungs? Is it the parenchymal tissue between the alveoli plus the surface tension on the walls of the alveoli? I can't put all these pressures and parameters together. It's so frustrating. What the hell is transpulmonary pressure? What exactly does that mean - the difference between alveolar pressure and intrapleural pressure? And how does that equal elastic recoil pressure? I can understand how elastic recoil pressure is equal to transpulmonary pressure if elastic recoil pressure is caused by the elasticity of the connective tissue in the interalveolar septa, and the elastic recoil pressure is in fact this tissue applying force on the alveoli due to passive recoil. Then elastic recoil pressure (which now is indeed pressure) would be equal to alveolar pressure MINUS the intrapleural pressure which is pulling the lungs in the opposite direction (thereby decreasing the force applied on the alveoli), BUT ONLY IF THE INTRAPLEURAL PRESSURE IS NEGATIVE. If it's positive (and it's positive in forced expiration), it further "boosts" the otherwise passive elastic recoil pressure. Two problems (given that all this is true): 1. Alveolar pressure is not solely defined by this elastic recoil pressure (as I've defined it above) and intrapleural pressure, but also by other factors - for example the surface tension. I don't know if these are negligible (surfactant to counter this tension) but I think alveolar pressure is not really outlined in such a simple, straightforward manner (i.e. being a result of the pushing/pulling effect of the elastin fibers between alveoli). 2. Why the hell do we need to invent a term of transpulmonary pressure and whatever concept it represents, when it's absolutely the same thing as elastic recoil pressure, which can actually be visualized and more intuitively understood? Thank you for bearing with me, it's very late here and I don't know what the hell I've just written here so please forgive me if there are illegible or nonsensical parts. I can't think clear right now. Btw, I have a ton of other questions so I'll be posting again soon. I really hope someone will reply and try to help. I've registered and posted on this forum for several reasons. I've been reading certain posts here for a while now because my searches would frequently lead me to this forum, and I've found that people here are very intelligent, knowledgeable, helpful and amiable. I've found this thread when searching for answers to this particular topic and it struck me as by far the most pertinent. So that's it! Thank you all in advance! I can now put a smiley despite the agony respiratory physiology is putting me through.