Background information respiration | Physiology | Definitions | Airflow

Relationship between air flow and driving pressure

Air flow is inversely proportional to airway resistance. It 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 smaller than the pressure at the mouth; air flows out during expiration because alveolar pressure exceeds the pressure at the mouth.

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, or at airway ramification.

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 slower 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 only needs to double the driving pressure.

During quiet breathing, laminar flow exists from the medium-sized bronchi down to the level of the bronchioli. 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.


Klinke R, Silbernagel S. Lehrbuch der Physiologie. 2001. Thieme: Stuttgart

Shier D, Butler J, Lewis R. Hole’s human anatomy and physiology. 2004. McGraw Hill: New York

Laminar and turbulent flow

This animated model shows the airflow from the upper into the lower respiratory tract describing areas of turbulent or laminar flow. It is possible to stop and start the movie using the operating panel at the bottom of the screen window. Additionally the slider can be shifted to move to a certain section.

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