Science Questions

How do air sacs in the lungs help with gas exchange? Louise in Norfolk

Your lungs are not just like balloons. They are filled with millions of tiny balloons called alvioli, which are the air sacs you mentioned. The reason the lungs don't just have one big balloon, but instead have millions of tiny balloons is that each balloon has to have a wall. In the wall of each balloon is tiny capillaries which are very thin walled blood vessels. The blood from your heart gets pumped through your lungs, around the walls of those tiny air sacs first, and then back to the heart before it gets jetted off around the rest of the body. As the blood flows through these tiny blood vessels around the air sacs or alvioli, it exchanges carbon dioxide which is dissolved in the blood, that gets chucked out of the blood because there's more in the blood than there is in the air sac. And oxygen, which there's lots of in the air sac, comes out of the alviolus and into the blood. It forms a compound with the haemoglobin which is the stuff that makes your blood red, and gets carried away. And the average red blood cell takes about 0.3 seconds to pick up all the oxygen it can from the wall of the alviolus but it spends 0.8 seconds actually making that journey. So the red blood cell has nearly three times the time it needs to pick up all the gas it needs. So that's how the gas exchange occurs and why you don't just have one balloon in your chest but millions of tiny balloons. It gives your lungs the surface area of about a tennis court if you were to spread them all out.

January 2007