The respiratory system is an organ system (group of associated organs) that removes carbon dioxide from the blood and replaces it with oxygen.
This is done across a large, moist exchange epithelium surface, composed of the alveoli within the lungs.
During inspiration, fresh air enters the respiratory system through the pared external nares (nostrils; shown here) and/or the oral cavity to the pharynx. Air then travels through the glottis into the trachea, through the bronchi and bronchioles to reach the alveoli. During expiration, air flows in the opposite direction.