Explain the exchange of gases in humans.

Gas exchange- It is the delivery of oxygen from the lungs to the bloodstream and the elimination of carbon dioxide from the bloodstream to the lungs. 

It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries. 

Walls of the alveoli share a membrane with the capillaries in which oxygen and carbon dioxide move freely between the respiratory system and the bloodstream.