Alveolus

00:01 individual alveolus or alveoli. Let us have a look at the surface of these alveoli.

00:05 On the left-hand side is a section through the lung, the bulk of the lung tissue where the exchange occurs. It is a huge interface. And on the right-hand side is an electron microscope picture showing the details as well. I want you to look at the section on the left-hand side and just look very carefully and point out a couple of features. Find the alveolar capillary. It's a closed sort of tube in a funny sort of a shape lined by endothelial cells.

00:45 So that is the alveolar capillary. That is where the blood is going to come down and exchange across to the air in the alveolus. Have a look then at the lining, the alveolar space. That space is lined by two cells. The alveolar type I cell is a very squamous cell that stretches out and creates the bulk of the exchange surface. So make sure you can see where are the alveolar spaces and where are the capillary spaces. There are two other cells described there also and I will refer to those in a moment. Now this is the gas exchange area but there are also areas where we call the interstitium. That is just small pieces of connective tissue that really is an area where fluid can accumulate. If there's excessive fluid on the surface of the lungs, or on the surface of the alveoli and that fluid can then be taken back from the lung through the lymphatic system. That is one of the jobs of the interstitium. Here is a summary of this air-blood barrier. On the left-hand side, you can see a histological section showing you the very small lumen of the capillary and the surface of the air space lined by type I alveolus cells. In the very edges of the alveolar walls are the type II cells. I am going to mention those in a moment. In the middle, there is a diagram illustrating what you see in the left-hand histological section. I want you and your own time, to look for the capillary, to look for the type I alveolar cell making up the wall of the alveolar septum or the alveolar space. And then on the right hand side, the electron micrograph shows you that the interalveolar septum consists of the type I alveolar cell, very thin squamous epithelium actually right up against the wall of the endothelial cell, the wall of the capillary. So here you have the air space right up against the capillary space. So this interalveolar septum consists of the lining of the air space and the lining of the capillary space shown here and in the middle is a dual or shared basal lamina. But what I want you to know also understand is how thin that interalveolar septum or exchange area is. That large structure you see in the electron micrograph on the right-hand side is the red blood cell and you know the red blood cells are about 7 to 8 microns in diameter.

03:47 So you can use that dimension to estimate how really really thin that exchange area actually is. But I mentioned a couple of other cells that are present in the alveolus. One is the macrophage, an alveolar macrophage. This wanders along the alveolar surface and mops up any debris that happens to found its way down into the lung even though it must have escaped all the other sorts of mechanism we have to try and detect that debris on the way down, but these alveolar macrophages mop up all this debris and then they are moving to the interstitium. And they can stay there for the rest of their lives. If you look at lungs in the autopsy room or the gross anatomy lab, you will see lots of black deposits in the lung. That is the contents of these macrophages having mopped up debris, particularly in a smoker's lung for instance. Sometimes this alveolar macrophage can move up towards the surface and they can move from the lung in that method, but most often stay within the lung itself. And then you have these type II alveolar cells that I mentioned before and you saw before on the diagram. These type II alveolar cells secrete surfactant.

05:10 Surfactant decreases the surface area or the surface tension I should say, between the alveoli.

05:17 So that when these alveoli collapse during expiration, they don’t stick together, so they lower surface tension. Very important cells and these cells can also give rise to type I cells if needed. So they are really a stem cell if you like. The type I alveolar cells are very thin cells forming the exchange surface and they often can be identified because they have this sort of firmy type cytoplasm follow the bodies that secrete or contain this surfactant. Finally, I just want to mention a little bit about the blood supply.