<Dd> R = 8 η l π r 4 (\ displaystyle R = (\ frac (8 \ eta l) (\ pi r ^ (4)))) </Dd> <P> While the assumptions of the Hagen--Poiseuille equation are not strictly true of the respiratory tract it serves to show that, because of the fourth power, relatively small changes in the radius of the airways causes large changes in airway resistance . </P> <P> An individual small airway has much greater resistance than a large airway, however there are many more small airways than large ones . Therefore, resistance is greatest at the bronchi of intermediate size, in between the fourth and eighth bifurcation . </P> <P> Where air is flowing in a laminar manner it has less resistance than when it is flowing in a turbulent manner . If flow becomes turbulent, and the pressure difference is increased to maintain flow, this response itself increases resistance . This means that a large increase in pressure difference is required to maintain flow if it becomes turbulent . </P>

Where does the greatest resistance to airflow occur