Airway resistance - wikipedia Airway resistance In respiratory physiology, airway resistance is the resistance of the respiratory tract to airflow during inhalation and expiration. Airway resistance can be measured using body plethysmography. Contents 1 Definition 2 Determinants of airway resistance 2.1 Hagen -- Poiseuille equation 2.2 Laminar flow versus turbulent flow 3 Changes in airway resistance 4 Derived parameters 4.1 Airway conductance ( G ) 4.2 Specific airway resistance ( sR ) 4.3 Specific airway conductance ( sG ) 5 See also 6 References 7 External links Definition Analogously to Ohm 's Law : R A W = Δ P V  ̇ ( displaystyle R_ ( AW ) = ( \frac ( ( Delta ) P ) ( dot ( V ) ) ) ) Where : Δ P = P A T M − P A ( displaystyle ( Delta P ) = P_ ( ATM ) - P_ ( A ) ) So : R A W = P A T M − P A V  ̇ ( displaystyle R_ ( AW ) = ( \frac ( P_ ( mathrm ( ATM ) ) - P_ ( mathrm ( A ) ) ) ( dot ( V ) ) ) ) Where : R A W ( displaystyle R_ ( AW ) ) = Airway Resistance Δ P ( displaystyle ( Delta ) P ) = Pressure Difference driving airflow P A T M ( displaystyle P_ ( ATM ) ) = Atmospheric Pressure P A ( displaystyle P_ ( A ) ) = Alveolar Pressure V  ̇ ( displaystyle ( dot ( V ) ) ) = Volumetric Airflow ( not minute ventilation which, confusingly, may be represented by the same symbol ) N.B. P and V  ̇ ( displaystyle ( dot ( V ) ) ) change constantly during the respiratory cycle. Determinants of airway resistance There are several important determinants of airway resistance including : The diameter of the airways Whether airflow is laminar or turbulent Hagen -- Poiseuille equation In fluid dynamics, the Hagen -- Poiseuille equation is a physical law that gives the pressure drop in a fluid flowing through a long cylindrical pipe. The assumptions of the equation are that the flow is laminar viscous and incompressible and the flow is through a constant circular cross-section that is substantially longer than its diameter. The equation is also known as the Hagen -- Poiseuille law, Poiseuille law and Poiseuille equation. Δ P = 8 η l V  ̇ π r 4 ( displaystyle ( Delta P ) = ( \frac ( 8 eta l ( dot ( V ) ) ) ( pi r ^ ( 4 ) ) ) ) Where : Δ P ( displaystyle Delta P ) = Pressure difference between the ends of the pipe l ( displaystyle l ) = Length of pipe η ( displaystyle eta ) = the dynamic viscosity V  ̇ ( displaystyle ( dot ( V ) ) ) = the volumetric flow rate ( Q is usually used in fluid dynamics, however in respiratory physiology it denotes cardiac output ) r ( displaystyle r ) = the radius of the pipe Dividing both sides by V  ̇ ( displaystyle ( dot ( V ) ) ) and given the above definition shows : - R = 8 η l π r 4 ( displaystyle R = ( \frac ( 8 eta l ) ( pi r ^ ( 4 ) ) ) ) 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. 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. Laminar flow versus turbulent flow 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. Whether flow is laminar or turbulent is complicated, however generally flow within a pipe will be laminar as long as the Reynolds number is less than 2300. R e = ρ v d μ ( displaystyle Re = ( ( \rho ( mathrm ( v ) ) d ) over mu ) ) where : R e ( displaystyle Re ) is the Reynolds number d ( displaystyle d ) is the diameter of the pipe. v ( displaystyle ( mathbf ( mathrm ( v ) ) ) ) is the mean velocity. μ ( displaystyle ( mu ) ) is the dynamic viscosity. ρ ( displaystyle ( \rho ) , ) is the density. This shows that larger airways are more prone to turbulent flow than smaller airways. In cases of upper airway obstruction the development of turbulent flow is a very important mechanism of increased airway resistance, this can be treated by administering Heliox which is much less dense than air and consequently more conductive to laminar flow. Changes in airway resistance Airway resistance is not constant. As shown above airway resistance is markedly affected by changes in the diameter of the airways, therefore diseases affecting the respiratory tract can increase airway resistance. Airway resistance can change over time, for example, in asthma during an attack the airways constrict causing an increase in airway resistance. Airway resistance can also vary between inspiration and expiration, for example, in emphysema there is destruction of the elastic tissue of the lungs which help hold the small airways open, therefore during expiration, particularly forced expiration, these airways may collapse causing increased airway resistance. Derived parameters Airway conductance ( G ) This is simply the mathematical inverse of airway resistance. G A W = 1 R A W ( displaystyle G_ ( AW ) = ( \frac ( 1 ) ( R_ ( AW ) ) ) ) Specific airway resistance ( sr ) s R A W = R A W V ( displaystyle sR_ ( AW ) = ( R_ ( AW ) ) ( V ) ) Where V is the lung volume at which R was measured. Also called volumic airway resistance. Due to the elastic nature of the tissue that supports the small airways airway resistance changes with lung volume. It is not practically possible to measure airway resistance at a set absolute lung volume, therefore specific airway resistance attempts to correct for differences in lung volume at which different measurements of airway resistance were made. Specific airway resistance is often measured at FRC, in which case : s R A W = R A W × F R C ( displaystyle sR_ ( AW ) = ( R_ ( AW ) ) \times ( FRC ) ) Specific airway conductance ( sg ) s G A W = G A W V = 1 R A W V = 1 s R A W ( displaystyle sG_ ( AW ) = ( \frac ( G_ ( AW ) ) ( V ) ) = ( \frac ( 1 ) ( R_ ( AW ) V ) ) = ( \frac ( 1 ) ( sR_ ( AW ) ) ) ) Where V is the lung volume at which G was measured. Also called volumic airway conductance. Similarly to specific airway resistance, specific airway conductance attempts to correct for differences in lung volume. Specific airway conductance is often measured at FRC, in which case : s G A W = G A W F R C ( displaystyle sG_ ( AW ) = ( \frac ( G_ ( AW ) ) ( FRC ) ) ) See also Turbulent flow Laminar flow Reynolds number Upper airway resistance syndrome ( UARS ) References Jump up ^ Physiology : 4 / 4ch2 / s4ch2_51 - Essentials of Human Physiology Jump up ^ `` Reynolds Number ''. ^ Jump up to : `` US EPA Glossary of Terms ''. Jump up ^ Kirkby, J. ; et al. ( 2010 ). `` Reference equations for specific airway resistance in children : the Asthma UK initiative '' ( PDF ). European Respiratory Journal. 36 ( 3 ) : 622 -- 629. doi : 10.1183 / 09031936.00135909. Retrieved 9 April 2012. External links Overview at med.jhmi.edu Calculator at medstudents.com.br Respiratory physiology Respiration breath inhalation exhalation respiratory rate respirometer pulmonary surfactant compliance elastic recoil hysteresivity airway resistance bronchial hyperresponsiveness constriction dilatation mechanical ventilation Control pons pneumotaxic center apneustic center medulla dorsal respiratory group ventral respiratory group chemoreceptors central peripheral pulmonary stretch receptors Hering -- Breuer reflex Lung volumes VC FRC Vt dead space CC PEF calculations respiratory minute volume FEV1 / FVC ratio Lung function tests spirometry body plethysmography peak flow meter nitrogen washout Circulation pulmonary circulation hypoxic pulmonary vasoconstriction pulmonary shunt Interactions ventilation ( V ) Perfusion ( Q ) Ventilation / perfusion ratio V / Q scan zones of the lung gas exchange pulmonary gas pressures alveolar gas equation alveolar -- arterial gradient hemoglobin oxygen -- haemoglobin dissociation curve ( Oxygen saturation 2, 3 - BPG Bohr effect Haldane effect ) carbonic anhydrase ( chloride shift ) oxyhemoglobin respiratory quotient arterial blood gas diffusion capacity ( DLCO ) Insufficiency high altitude oxygen toxicity hypoxia Retrieved from `` https://en.wikipedia.org/w/index.php?title=Airway_resistance&oldid=822883658 '' Categories : Respiratory therapy Respiratory physiology Mathematics in medicine Talk Contents About Wikipedia Deutsch Español Edit links This page was last edited on 29 January 2018, at 01 : 41. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: where does the greatest resistance to airflow occur
Answer: at the bronchi of intermediate size, in between the fourth and eighth bifurcation.