The air then goes through the series of branches in your lungs, through the bronchi and the bronchioles. The air finally ends up in the 600 million alveoli. As these millions of alveoli fill up with air, the lungs get bigger. It's the alveoli that allow oxygen from the air to pass into your blood. All the cells in the body need oxygen every minute of the day.
Oxygen passes through the walls of each alveolus into the tiny capillaries that surround it.
The oxygen enters the blood in the tiny capillaries, hitching a ride on red blood cells and traveling through layers of blood veseels to the heart . Your diaphragm relaxes and moves up, pushing air out of the lungs. Your rib muscles become relaxed, and your ribs move in again, creating a smaller space in your chest. By now your cells have used the oxygen they need, and your blood is carrying carbon dioxide and other wastes that must leave your body. The blood comes back through the capillaries and the wastes enter the alveoli.