Air flow occurs only when there is a difference between pressures. Air will flow from a region of high pressure to one of low pressure-- the bigger the difference, the faster the flow. Thus air flows in during inspiration because the alveolar pressure is less than the pressure at the mouth; air flows out during expiration because alveolar pressure exceeds the pressure at the mouth such that to double the flow rate one must quadruple the driving pressure.
When air flows at higher velocities, especially through an airway with irregular walls, flow is generally disorganized, even chaotic, and tends to form eddies. This is called turbulent flow, and is found mainly in the largest airways, like the trachea.
A relatively large driving pressure is required to sustain turbulent flow. Driving pressure during turbulent flow is in fact proportional to the square of the flow rate such that to double the flow rate one must quadruple the driving pressure.
When flow is low velocity and through narrow tubes, it tends to be more orderly and streamlined and to flow in a straight line. This type of flow is called laminar flow. Unlike turbulent flow, laminar flow is directly proportional to the driving pressure, such that to double the flow rate, one need only double the driving pressure.
Laminar flow can be described by Poiseuille's Law :
During quiet breathing, laminar flow exists from the medium-sized bronchi down to the level of the bronchioles. During exercise, when the air flow is more rapid, laminar flow may be confined to the smallest airways.
Transitional flow, which has some of the characteristics of both laminar and turbulent flow, is found between the two along the rest of the bronchial tree.