The airway is the upper part of the respiratory tract, which allows the passage of air into the lungs.  It is comprised of the mouth, nose, pharynx and larynx.  When we inhale, negative pressures in the thoracic cavity causes air (and therefore oxygen) to enter through the upper airway via the pharynx into the trachea to reach the alveoli. Subsequent gas exchange occurs between the alveoli and the blood stream providing oxygenation of the blood via the blood gas membrane.