Now this shows you the relationship between pressure and volume in the lung. This is a normal lung, and on this axis, you see pressure: what you have to pay. And this is volume: what you get in return. So in a nice healthy lung, you put in a little pressure, and you get a nice volume, and then you have expiration. So that's what a nice healthy lung looks like. This line is called the compliance line, the slope of it tells you the numerical value compliance.