What causes the gases to move in the lungs during gas exchange?

Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.

In occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli. The walls of the alveoli actually share a membrane with the capillaries in which oxygen and carbon dioxide move freely between the restipory system and the bloodstream