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RESPIRATORY PHYSIOLOGY

Mechanits of Respiration: Airway Flo»

Laminar flow occurs mainly in smali pcripheral airways where ratę ot airflow through any airway is Iow. Driving pressure is proporlional to gas viscosity

Turbulent flow octurs at high flow rates in trachea and larger airways. Driving pressure is proportional to square of flow and is dependent on gas density

Iransitional flow occurs in larger airways, particularly at branches and at sites of narrowing. Oriving pressure is proporlional to both gas density and gas viscosity

Resistance -1

Poiscuille's law. Resistance to laminar flow is mverseły proportional lo tubę radius to the 4th power and directly proportional to length ol lube. Wlien radius is halved, resistance is increased 16-lold. If drivmg pressure 15 constant. flow will fali lo one sixteenth. Dnubling length only doubles icsisiance. II driving pressure is constant. flow will fali to one hall

Resistance ~2    Resistance ~4

Figurł 5.11 Airway Flow

Airflow through ihe large airways of the lung is turbulent and thero-e heard with a stethoscope (i.e., brcath sounds). Laminar pnly in the smali airways. Tfłe major factor that deter-mines resistance to airflow is the diameter of the airway, bccause resistance varies as the fourth power of the radius.