The hysteresis in the pressure-volume plot seen during tidal breathing results from airflow resistance. Turbulent flow is caused at higher flow rates in the airways, and the driving pressure necessary to overcome resistance during turbulent flow is dependent on the velocity of flow squared. The autonomic nervous system innervates the airway smooth muscle. Parasympathetic innervation is predominant and is carried in the vagus nerve. Sympathetic innervation to the airway smooth muscle is scant, although the airway smooth muscles have a large number of adrenergic receptors in their membranes.
Acetylcholine causes contraction of airway smooth muscle, and sympathetic agonists cause relaxation.
The work of breathing is normally small. In disease, however, the work required to breathe increases substantially and may limit a patient's ability to exchange gas adequately.