Smooth muscle cells in the walls of the bronchioles adjust their diameter and help to control the flow of air into the alveoli of the lungs. The muscles of the thoracic cavity, including the diaphragm and intercostal muscles, change the volume of the thoracic cavity to force air into and out of the lungs.
The diaphragm contracts and pushes itself into the abdominal cavity, increasing the volume of the thoracic cavity and expanding the lungs to draw in fresh atmospheric air.
When it relaxes, the diaphragm returns to its original position and forces air out of the lungs during exhalation. The intercostal muscles also aid in respiration by lifting the ribs during deep inhalation to increase the size of the thoracic cavity and by pushing the ribs together during deep exhalation to decrease the thoracic cavity.