The human respiratory tract is made up of nostril, larynx, nasal chamber, pharynx, trachea, epiglottis, alveoli, bronchioles, bronchi and lungs. Within the lungs, the oxygen is exchanged for carbon dioxide waste through millions of microscopic sacs known as alveoli. The oxygen inhaled diffuses into the capillaries of the lungs, binds to the haemoglobin and is pumped into the bloodstream. The carbon dioxide from the blood diffuses through the alveoli and is expelled by exhalation.
The alveoli are thin-walled and richly supplied with a network of blood vessels to facilitate the exchange of gases between blood and the air-filled in alveoli. They have a balloon-like structure that provides maximum surface area for exchange of gases.