The pulmonary loop brings oxygen-poor blood to the lungs. There, gases cross by simple diffusion, down their concentration gradients.
Key idea
In the alveoli, tiny air sacs at the ends of the airways, oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the air. Huge surface area, very thin walls and steady blood flow and breathing keep the gradients steep and the exchange fast.
Why it works
Diffusion is fastest when the distance is short, the area is large and the difference in concentration is big. The lungs maximize all three. An adult's lungs hold about 300 million alveoli with a combined surface of about 70 square meters, and the wall between air and blood is only about two cells thick. Breathing keeps bringing fresh, oxygen-rich air, and blood flow keeps carrying oxygen away, so the gradient never runs down.
Worked example: Which way?
Air in an alveolus has a high level of oxygen and a low level of carbon dioxide. Blood arriving in the lung capillary has low oxygen and high carbon dioxide. Which way does each gas move?
- Each gas diffuses from where it is higher to where it is lower.
- Oxygen: higher in the air, so into the blood.
- Carbon dioxide: higher in the blood, so into the air.
Answer: Oxygen into the blood; carbon dioxide into the alveolus, to be breathed out.
Worked example: Why it keeps going
If blood stopped flowing past an alveolus, what would happen to oxygen diffusion there?
- The blood would fill with oxygen until it matched the air.
- With no difference left, net diffusion stops.
Answer: It would stop, because the gradient would disappear.
At the other end of the trip, in the tissues, the gradients run the opposite way: cells use oxygen and make carbon dioxide, so oxygen diffuses out of the blood into the cells, and carbon dioxide diffuses into the blood.
Common mistake
It is natural to picture the lungs pumping oxygen into the blood. No pump moves the gases across. Gas exchange is passive diffusion down concentration gradients. What breathing and blood flow do is keep those gradients steep.