The alveoli are the site of gas exchange in the lungs and they're located here at the very end of the bronchioles. They have a structure that's something like this. We often describe it as having a sort of cauliflower shape, but basically they're just tiny little air sacs at the end of the bronchioles. If we see it here represented in more of a diagram format, we can see what's going on. First of all...
Oxygen and carbon dioxide are constantly being supplied and removed through breathing. Let's look a little bit closer at the lining here. Notice that it's only one cell thick.
Oxygen can diffuse across the cell membrane of the cell that lines the alveolus. From there, it can obviously diffuse across to outside of that cell. The same thing applies for the lining of the blood capillary until eventually it diffuses into the red blood cell where it can bind with haemoglobin to form a cell.
oxyhemoglobin. Similarly carbon dioxide can also diffuse across these cell membranes and reach the alveolar sac to then exit the body. Now then oxygen as we've already said is constantly being supplied through breathing but it's also being taken away from this area because of the blood flow. What this means is that we are constantly maintaining a concentration gradient which is what allows this transfer of gas, this gas exchange to continue.
There are some important features of the gas exchange surface that you need to know. First of all it's got a large surface area which is provided by this particular shape of the alveoli and also by the fact that there are lots and lots and lots of them inside of the lungs. It's very thin which provides a short diffusion pathway. Notice that the lining is only one cell thick.
It's also got a moist lining. The inside of each alveolar sac is moist, which allows gases to dissolve, which then allows them to diffuse across the lining. It's got a very good blood supply. Each one of the alveoli has lots and lots of blood capillary surrounding it. If we have a little look at what lung tissue looks like under a microscope, We can see this sort of zigzag, crisscross sort of mesh of little almost streams.
This is the capillary network. And we can tell that because if we look at one of these individual little red splotches here, those are individual red blood cells. Finally, there is very good ventilation. And that basically just means there's a really good supply of air, which is supplied through breathing.
A little bit of exam help on this. You might get asked to describe the features of the lungs that make gas exchange efficient. Well, the exchange surface has a large surface area and that's provided by the large number of alveoli and the surface is only one cell thick. Remember that that provides a very short diffusion pathway and it is supplied with blood by a very dense capillary network. So there you're easily going to get those three marks.