GasExchangePreClass_Eiting2023.pptx

Gas Exchange Thomas P Eiting, PhD RESP I—Fall 2023 Image credit: Meckes/Ottawa Learning Objectives Guyton Ch. 40 Describe the physical determinants of diffusion (Fick’s Equation). Describe the concept of diffusing capacity. Describe the diffusion of oxygen from alveoli into the blood. Distingui...

Gas Exchange Thomas P Eiting, PhD RESP I—Fall 2023 Image credit: Meckes/Ottawa Learning Objectives Guyton Ch. 40 Describe the physical determinants of diffusion (Fick’s Equation). Describe the concept of diffusing capacity. Describe the diffusion of oxygen from alveoli into the blood. Distinguish between perfusion limitation and diffusion limitation of gas transfer in the lung. Explain the regional differences in the matching of ventilation and perfusion of the normal upright lung and the consequences of these differences. Predict the consequences All ofimages mismatched ventilation and perfusion. from Guyton text unless noted Diffusion • Molecules are free to move among each other by diffusion, owing to their kinetic energy • Net diffusion will occur from area of high concentration to area of low concentration Diffusion—Fick’s Law Rate of transfer through a sheet of tissue is proportional to tissue area and the difference in partial pressure of gas, and inversely proportional to thickness D is the diffusion coefficient, which is proportional to solubility / square root of molecular weightz 1.0 Oxygen: Carbon dioxide: 20.3 Carbon monoxide: 0.81 Nitrogen: 0.53 Helium: 0.95 Berne and Levy, Fig. 24.2 Diffusion Across the Respiratory Membrane • Thickness of 0.2-0.6 µm • Area of about 70 m2 • Volume of blood at any instant is about 100 mL Partial Pressure Henry’s Law—partial pressure of gasses dissolved in a fluid Concentratio Partial n Solubili pressure = ty Solubility coefficients Oxygen: 0.024 Carbon dioxide: 0.57 Carbon monoxide: 0.018 Nitrogen: 0.012 Helium: 0.008 In water The pressure of a gas acting on the surfaces of respiratory passages; directly proportional to the concentration of the gas molecules What are the partial pressures of nitrogen and oxygen in normal air at sea level? CO2 about 20x higher than O2 N2 O2 So, for a given concentration, how does the PP of CO2 relate to that of O2 dissolved in water/tissue? Diffusion and Perfusion Limitation Uptake of gases by the pulmonary capillaries depends on perfusion and diffusion Look at nitrous oxide in the graph to the left. • What do you notice? West, Fig. 3.2 • Would you say nitrous oxide is perfusion limited, or diffusion limited, in this case? Exchange of Gases between Alveoli and Blood Composition of Alveolar Air Concentration of gases in alveolar air is different from atmospheric air: 1. Alveolar air only partially replaced with each breath 2. O2 constantly being absorbed into pulmonary blood from alveoli 3. CO2 constantly diffusing from pulmonary blood into alveoli 4. Atmospheric air is humidified before it reaches alveoli Gas Atmospheric Humidified Air Air Alveolar Air Expired Air N 2 597 (78.62) 563.4 (74.09) 569 (74.9) 566 (74.5) O 2 159 (20.84) 149.3 (19.67) 104 (13.6) 120 (15.7) 0.3 (0.04) 0.3 (0.04) 40 (5.3) 27 (3.6) H 2O 3.7 (0.50) 47 (6.20) 47 (6.2) 47 (6.2) Total 760 (100) 760 (100) 760 (100) 760 (100) CO 2 Removal of Excess Gas What is the FRC of the lungs (approx.)? How much atmospheric air reaches the alveoli? So, how many breaths are required at minimum for complete exchange? Oxygen Concentration and Partial Pressure in Alveoli CO2 Concentration and Partial Pressure in Alveoli Partial Pressures of Expired Air Think about ventilation: Which of this air was first to enter? Last? Which is first to leave? Last? Ventilation-Perfusion and Alveolar Gas Concentration What determines PO2 and PCO2 in alveoli? Ventilation-Perfusion Ratio describes the imbalance between alveolar ventilation and alveolar bloodflow VA/ Q When both parameters are normal, the ratio is 1 WhenVA is adequate but there is no bloodflow Q ( = 0), there is no gas exchange. Similarly, when there is no ventilation VA ( = 0), but bloodflow ( ) is Q present, there will also be no gas exchange. Ventilation-Perfusion and Alveolar Gas Concentration Ventilation-Perfusion Summarized Use this diagram to help you contextualize the discussion from the previous few slides Remember: in an upright individual, both ventilation and perfusion (blood flow) increase from apex to base of the lung, but blood flow increases faster than ventilation Berne and Levy, Fig. 23.7 Ventilation-Perfusion Abnormalities Normal 2-arteriole model Anatomic shunt model • Ventilation normal • A portion of cardiac output bypasses lung and mixes with oxygenated blood Ventilation-Perfusion Abnormalities Normal 2-arteriole model Physiological shunt model • Perfusion equally distributed • Ventilation goes to 0 in the affect lung region • Atelectasis—incl mucous plugs, foreign bodies, airway tumors Ventilation-Perfusion Abnormalities Normal 2-arteriole model Uneven ventilation model • Perfusion equally distributed • Ventilation unevenly distributed • Some ventilation is wasted—more oxygen than can be transported by blood • When combined with the anatomic dead space, this is called physiological dead space