The diaphragm , the muscle that contracts and relaxes in breathing, separates the thoracic cavity from the abdominal cavity. On inspiration, air enters the body through the nose and the mouth. Less warming, filtering, and humidification occur when air is inspired through the mouth. When air is inspired, the walls of the esophagus are collapsed, preventing air from entering the stomach. The larynx, which also contain the vocal cords , is lined with mucus that further warms and humidifies the air.
Air continues continues down the trachea, which branches into the right and left bronchi.
The main-stem bronchi divide into smaller bronchi, then into even smaller tubes called bronchioles . The bronchial structures contain hair-like, epithelial projections, called cilia , that beat rythmically to sweep debris out of the lungs toward the pharynx for expulsion. It is at the alveoli that gas exchange occurs. Tiny blood vessels, capillaries , surround each of the alveoli. On inspiration, the concentration of dissolved oxygen is greater in the alveoli than in the capillaries.