In which part of the lungs, does the exchange of gases take place in humans?

The exchange of gases between the bloodstream and the atmosphere occurs in humans in the alveoli, which are small air sacs in the lungs. 

The alveoli are surrounded by a dense network of capillaries, the body's smallest blood vessels. 

Gases like oxygen and carbon dioxide can be exchanged efficiently between the lungs and the bloodstream Because the alveoli and capillaries are so close together.

Inhaled oxygen diffuses across moist alveolar walls and into capillaries, where it binds to haemoglobin in red blood cells and is carried to tissues. 

Simultaneously, carbon dioxide, a byproduct of cellular respiration, diffuses from capillaries into alveoli and is exhaled from the body.