Answer:
The most important are the lungs, more precisely the alveoli.
Explanation:
Alveoli are structures of the lungs where gas exchange occurs. They allow oxygen from the air to pass into blood. Oxygen diffuses through the walls of alveoli into the capillaries that surround it. Oxygen is then bonded with the iron in red blood cells and travels to the heart. The heart then sends the oxygenated blood out to all the cells in the body.
