How does gas exchange take place across the alveolar membrane in lungs?

Think of the alveoli as tiny, super thin-walled balloons inside your lungs. The whole process is basically just a simple game of diffusion. When you take a breath in, the oxygen concentration in these little sacs gets way higher than the amount of oxygen sitting in the blood vessels wrapping around them. Because nature hates an imbalance, the oxygen molecules just naturally slip through the thin membrane from the air sac into your bloodstream.

It works exactly the same way in reverse for carbon dioxide, which is pretty neat when you think about it. The blood coming back from your body is loaded with carbon dioxide, so the concentration there is much higher than what's in the alveoli. The gas just naturally shifts over into the air sac to be breathed out. It’s a constant, smooth exchange that keeps us going without us even noticing it happening every second of the day.