Air reaches our lungs through the upper airways and lower respiratory tract. The upper airways include the nasal cavity, the mouth cavity, pharynx, larynx and the trachea. In the lower respiratory tract, the trachea divides into the left and right main bronchus in the lungs. The main spread out to become bronchi, bronchioles and finally alveoli at the end.
Oxygen enters the bloodstream through the dense capillary system in the alveoli walls. At the same time, carbon dioxide leaves the blood stream through the same alveoli and exits the body. This is the essence of the gas exchange in the human body.
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