Why are alveoli so small? How does their structure relate to their function?

Answer 1

The part of your blood that is going to be carrying most of the oxygen is the hemoglobin. So ideally, you want to expose as much hemoglobin as possible to as much oxygen as possible.

The 'problem' is that your hemoglobin is stuck inside red blood cells (it's a problem for oxygen exchange, but a big advantage for other things). This is the primary reason why red blood cells aren't spherical - by being flattened a bit more of them can be near the blood cell membrane where the oxygen will have to come through.

This is also the primary reason why alveoli are so small. Oxygen that can't be forced next to a capillary barely large enough to hold a red blood cell is wasted oxygen. It's better to have a zillion alveoli next to a zillion capillaries than a huge air sac next to a huge vein... in the latter case virtually none of the air and the hemoglobin would be next to each other.

Answer 2

alveoli are small because this gives the lung a much greater surface area. Oxygen diffuses thru the alveolar membrane and into the capillaries and red blood cells. CO2 diffuses from the blood thru the alveolar membrane. the more surface area, the more these gases can move.

Answer 3

The alveoli are the final branchings of the respiratory tree and act as the primary gas exchange units of the lung. The gas-blood barrier between the alveolar space and the pulmonary capillaries is extremely thin, allowing for rapid gas exchange