School of Medicine SUNYAB PGY452 Respiratory Lectures March 24 2015 Daniel D Swartz Ph D Lecture 5 Gas Pressure and Diffusion Reading Assignment Berne Levy chapter 22 pages 444 458 Partial Pressures of Gases in the Alveoli and the Blood The partial pressure of any gas PO2 PCO2 etc either in the alveoli or in the blood equals The fractional concentration of the gas F times the total barometric pressure PB PO2 FO2 x PB PIO2 0 21 x 760 mm Hg 160 mm Hg I inspired air Calculation of Gas Partial Pressures Chap 22 pg 444 Partial Pressures of Gases and Alveolar Gas Composition Chap 22 pg 445 Diffusion and Transport of O2 and CO2 200 ml min at CO of 5 L min 250 ml min at CO of 5 L min Respiratory exchange ratio Normal 0 8 Fig 23 1 B L Pulmonary Gas Exchange O2 F IO2x x 760 P B P IO2 159 0 2 x PIO2 FIO2 x PB PIO2 P I0 21 mm Hg 760 150 mm Hg P IN2 601 610 PIN2 P IO2 150 PIO2 160 P ICO2 0 PICO2 0 PAO2 100 PACO2 40 P 100 573 P AO2 AN2 P ACO2P 40 47 AH2O Total 760 760 PB PB 760 mm Hg at sea level mm Hg H 2O H2O P A O2 100 P AO2 100 40 P A CO2 40 LUNG Lung PACO2 PvO2 40 P vO2 40 v CO2 46 46 PPvCO2 Q P B 760 mmHg PP H2O atat 37 C37 47omm Hg 47 C H2O PB P AN2 573 P AH2O 47 PARTIAL PRESSURES PPaO2 100 aO2 100 aCO2 40 PPaCO2 40 Pulmonary arteries Pulmonary Pulmonar y arteries Pulmonary veins veins Left Left Heart Heart Right Right Heart Heart Q Systemic veins Systemic veins Systemic arteries Systemic arteries P vCO2 46 P O2 40 46 P vvCO2 PvO2 40 P t O2 40 40 P tO2 consumed consumed P t CO2 4646 P tCO2 produced produced Driving pressure for O Driving pressure for CO 2 across the lung P 2 across the lung P AO2 a CO2 40 PPPaCO2 40 a O2 100 PaO2 100 P vO2 100 40 60 mm Hg vCO2 P ACO2 46 40 6 mm Hg TISSUES Tissues Driving pressures for O2 and CO2 across the lung in mm Hg Alveolar Time Constant Time constant t airway resistance R x compliance C Alveolar units with long time constants fill and empty slowly Thus an alveolar unit with increased airway resistance and or increased compliance will take longer to fill and empty compared to a normal unit In a normal human breathing at 12 BPM inspiratory time is 2 seconds and expiratory time is 3 seconds This timing is sufficient for the alveoli to fill almost completely 100 Chap 22 pg 447 t R X C Fig 22 3 B L Diffusion of Oxygen and Carbon Dioxide across the Pulmonary Capillary Membrane Fick s Law Gas Diffusion Surface area Diffusion constant Pressure gradient Thickness Fig 23 2 B L DIFFUSION OF O2 CO2 Causes of Hypoxemia low PaO2 80mmHg Cause Arterial Po2 Anatomic Shunt Physiologic Shunt Decreased Decreased AaDO2 PaO2 Response to 100 O2 Increased No significant change Increased Decreased Diffusion Abnor Decreased Increased Increased Decreased FIO2 Decreased Normal Increased Hypoxic vasoconstriction Table 22 3 B L Diffusion of N2O O2 and CO from the start to the end of a single capillary Anesthetic gases nitrous oxide ether Insoluble in blood perfusion limited Carbon monoxide low membrane solubility but high blood solubility binds to hemoglobin diffusion limited Fig 23 3 B L Transit time decreases Emphysema increase vascular resistance 1 Decrease capillaries 2 Blocked airways physiologic shunt 3 Hypoxic vaso constriction 4 Increase PVR and PAP 5 Pulmonary edema increase membrane thickness
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