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mechanics of breathing alveolar ventilation

2. OBJECTIVES. Know the definitions of all the listed lung volumes and capacitiesUnderstand how lung volumes and capacities are measuredUnderstand the concepts of transmural pressure and complianceBe able to predict the impact of changes in lung and chest wall compliances on FRCUnderstand how surface tension impacts on lung compliance and leads to hysteresis in the lung P-V relationshipBe able to explain how surfactant reduces surface tension and reduces the tendency for alveoli to collapse22

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mechanics of breathing alveolar ventilation

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    1. Mechanics of Breathing Alveolar Ventilation SSN Block 4 Shadi (csc43)

    2. 2

    3. 3 TOPICS Lung Volumes and Capacities Alveolar Ventilation Dead Space Minute and Alveolar Ventilation Alveolar Ventilation Equation Alveolar Gas Equation Compliance (Lung, Chest Wall, System) Surface Tension and Surfactant Gas Laws

    4. 4 Slides will be posted on SSN web site Use slide numbers if you like

    5. 5 Lung Volumes and Capacities Capacity = 2 or more volumes

    6. 6 If I’m breathing normally, then I’m breathing at. . . Tidal volume ~ 500 mL Which one is it?

    7. 7 True or False? Inspiratory reserve volume = the maximum volume of air I can breathe in. FALSE! ADDITIONAL volume inspired above tidal volume Which one is it?

    8. 8 Which one is Expiratory Reserve Volume? ADDITIONAL volume of air expired BELOW tidal volume

    9. 9 Residual Volume How much air is left in the lungs after the subject has forcefully and maximally expired CANNOT BE MEASURED BY SPIROMETRY

    10. 10 Lung Capacities Inspiratory Capacity IC = TV + IRV Vital Capacity VC = IRV + TV + ERV Maximum tidal volume Functional Residual Capacity* FRC = ERV + RV* Volume remaining in lungs after normal tidal volume expired EQUILIBRIUM Total Lung Capacity* TLC = VC + RV*

    11. 11 So we take a breath in . . . Air goes into the airways

    12. 12 Dead Space Volume of airways and lungs that does NOT participate in gas exchange Anatomic dead space (FIXED) Volume of conducting airways (~ 1 mL/lb) Alveolar dead space Alveolus not perfused, so no gas exchange Physiologic dead space Anatomic + alveolar Measured by CO2 “dilution” VD = VT × [(PaCO2 – PeCO2)/PaCO2] where VD is physiologic dead space; VT is tidal volume; PaCO2 is arterial PCO2; and PeCO2 is PCO2 of expired air

    13. 13 Ventilation Rate Volume of air moved into and out of lungs per unit time Minute ventilation (mL/min) Tidal volume × Breathing frequency Alveolar ventilation (mL/min) (Tidal volume – physiologic dead space) × Breathing frequency

    14. 14 Alveolar Ventilation Equation If CO2 production is constant, then PCO2 IS DETERMINED BY ALVEOLAR VENTILATION If ventilate more ? get rid of more CO2 ? PCO2 decreases Halve ventilation ? PCO2 doubles (takes a few mins)

    15. 15 Normally, equilibration is achieved between alveolus and capillary PACO2 = PcCO2 Fick’s law: Movement of gas is driven by partial pressure gradient

    16. 16 Regulation of PCO2 is main mechanism of acute regulation of pH CO2 + H2O ? H2CO3 ? H+ + HCO3- Kidneys for long term regulation

    17. 17 What about oxygen? Alveolar Gas Equation Respiratory quotient (R): ratio of CO2 production to O2 consumption R ~ 0.8 PH2O: because air is humidified in trachea PH2O = 47 mm Hg FIO2 = 21%; changes if on ventilator

    18. 18 Equilibration is achieved PAO2 = PcO2 PcO2 is NOT O2 content Dissolved, not bound, O2 is what drives movement Dissolved O2 = PcO2 × 0.003 ml O2 / dL A-a gradient

    19. 19 Compliance How volume changes as a result of pressure change (C = V/P) Describes distensibility of the system Compliance of lungs and chest wall inversely correlated with their elastic properties The greater the amount of elastic tissue, the greater the tendency to “snap back,” and the lower the compliance

    20. 20 Pressure-Volume Curve for Lungs, Chest Wall, and Combined Lung/Chest Wall Slope = compliance Transmural (in – out) For lung alveolar – pleural For chest wall pleural – atm For unit alveolar – atm Lung pressures referred to atm press (zero) Chest wall likes to expand Lung likes to collapse

    21. 21 Pressure-Volume Curve for Lungs, Chest Wall, and Combined Lung/Chest Wall Volume = FRC Equilibrium position Collapsing force = expanding force Volume < FRC Less volume in lung ? collapsing (elastic) force smaller Expanding force on chest wall still greater System wants to expand Volume > FRC More volume in lung ? collapsing force greater Expanding force on chest wall smaller System wants to collapse

    22. 22 At FRC (end-expiration) P alveolar = 0 Ppleura < 0 Pneumothorax Ppleura = 0 Tension pneumothorax (air stuck in) Ppleura rises and keeps rising (? mediastinum shifts to contralateral side)

    23. 23 Emphysema Lung is more compliant ? chest wall stronger ? higher FRC Fibrosis Lung is less compliant ? will be stronger ? lower FRC

    24. 24 Surface tension and Surfactant Surface tension bc of attractive forces btwn liquid molecules lining the alveoli Generates pressure given by law of Laplace P = 2T/r P = collapsing pressure/pressure to keep alveolus open T = surface tension (constant) r = radius of alveolus Smaller alveolus has higher pressure Surfactant molecules break up the attractive forces reduces surface tension (more reduction at lower volumes) ? decreases tendency of smaller alveoli to empty into larger alveoli & decreases pressure required to open a closed alveolus increases lung compliance

    25. 25 Compliance of the Lungs Hysteresis Compliance different for inspiration and expiration Surfactant reduces hysteresis

    26. 26 Gas laws Fick’s law (see above) Dalton’s law: Partial pressure of gas in mixture is equal to pressure gas would exert if it occupied entire volume Henry’s law: Concentration of dissolved gas depends on partial pressure and solubility

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