NORMAL RESPIRATION

Effective respiration requires frequent movement of a sufficient amount of air in and out of the alveoli. For a sufficient amount of air to move in and out of the alveoli, several conditions must occur: (1) the diaphragmatic contractions must be strong and timely; (2) the chest wall must not collapse; (3) the lung visceral pleura must remain fixed to the somatic rib cage pleura; (4) the airway must remain patent; and (5) the alveoli must remain open.
A strong and timely diaphragmatic contraction depends on the integrity of the phrenic-diaphragmatic unit.The chest wall does not collapse because of the structural integrity of the rib cage and the effective and timely contraction of the intercostal muscles. Lung visceral and somatic pleura stay together because the negative interpleural tension is sufficiently strong to oppose the physical forces that tend to separate them during expiration. The upper airway is kept patent by the structural integrity of the rigid airway and the effective and timely contractions of the upper respiratory muscles. The alveoli are kept open by constant interalveolar tension.