Question

Why are there so many alveoli in the lungs?

Answer 1

Hint: Your blood gets oxygen from the alveoli and gives it back to the alveoli as it passes through blood vessels (capillaries) in the alveoli walls. This vast surface area is required to process the massive volumes of air required for breathing and oxygen delivery to your lungs.

Complete answer:
To maximise the efficacy of gas exchange in the lungs by increasing the surface area to volume ratio. In our lungs, the gas exchange happens quickly and continuously. Alveoli are tiny sacs at the end of bronchioles. We can see alveoli in human lungs in the image below.

The abundance of Alveoli is due to their small size and high surface area to volume ratio.
This proportion is critical for any organism's existence. A higher surface area to volume ratio indicates that one unit of the volume contains more surface area.

The oxygen gas in the air we breathe in is taken into our bloodstream by diffusion through the blood vessels in the alveoli. This occurs on the alveoli's surface, therefore the larger the surface area, the more oxygen gas is taken up and consumed in our bodies (without taking up so much space in our body because the alveoli are so small).

The 600 million alveoli are the last destination for the air. The lungs expand as these millions of alveoli fill with air. The alveoli allow oxygen from the air to enter your bloodstream. Every minute of the day, all of the body's cells require oxygen. Each alveolus' walls allow oxygen to get through to the small capillaries that surround it. The oxygen enters the bloodstream through tiny capillaries, where it attaches to red blood cells and travels to the heart through layers of blood vessels. The heart then pumps the oxygenated (oxygen-rich) blood to all of the body's cells.

Everything reverses when it's time to exhale (breathe out): now it's the diaphragm's turn to say, "Move it!" Your diaphragm relaxes and rises, expelling air from your lungs. Your rib muscles relax, and your ribs move back in, narrowing the area in your chest.

Your cells have used up all of the oxygen they require, and your blood is transporting carbon dioxide and other wastes out of your body. The wastes enter the alveoli and the blood returns through the capillaries. The air then passes through the bronchioles, out the bronchi, out the trachea, and finally out out your mouth and nose in the reverse order in which it entered.

Note: In the alveoli, the partial pressure of oxygen is high, whereas in the blood of the pulmonary capillaries, it is low. As a result, oxygen diffuses from the alveoli into the bloodstream across the respiratory membrane. Partial pressure differences drive both.