While oxygen is taken into the body, carbon dioxide leaves the bloodstream in the alveoli of the lungs and is expelled into the atmosphere via the upper respiratory system. In one word gas exchange for humans occur in lungs. Our wind pipe which starts from nasal and oral cavity extends to lungs where it is divided into left and right know as bronchi. The bronchi gets further divided and re-divided many times in lungs. Each time it is divided there is a decrease in its size.