Gas Exchange.doc

Gas Exchange Defines the terms: minute volume (pulmonary ventilation), alveolar ventilation, anatomical dead space, alveolar dead space and physiological (total) dead space. minute volume (pulmonary ventilation) ? volume of air entering and leaving the nose or mouth per minute alveolar ventilation ? less than the minute volume because the last part of each inspiration remains in the conducting airways and does not reach the alveoli anatomical dead space ? first 16 generations of airways (conducting zone) with no alveoli alveolar dead space ? volume of gas that enters unperfused alveoli (ventilated but not perfused) physiological (total) dead space ? anatomical dead space plus the alveolar dead space Defines partial pressure and calculates it for oxygen or carbon dioxide in a gas, given the percentage of each in the gas and the total gas pressure. partial pressure ? pressure exterted by each individual gas independent of the pressures of other gases in the mixture (Pgas = % total gas x Ptot) PO2 = 0.2093 x 760 mmHg = 159 mmHg; PCO2 = 0.0004 x 760 mmHg = 0.3 mmHg Lists typical partial pressures of O2, CO2, and water in the air (at sea level), alveolar air, arterial blood, and mixed venous blood in a health individual. fractional concentration of CO2 in the alveoli is directly proportional to the carbon dioxide production by the body (VdotCO2) and inversely proportional to alveolar ventilation FACO2 is proportional to VdotCO2 /VdotA FACO2 = fractional concentration of carbon dioxide in the alveoli VdotCO2 = carbon dioxide by the body (rate) VdotA = alveolar ventilation (rate) as alveolar ventilation increases, PO2 will also increase, but it cannot double because if the alveolar PO2 is already 104, it can only maximally increase to 149 mmHg (breathing air at sea level) PAO2 = FIO2(PB ? PH20) ? PACO2/R PAO2 = alveolar partial pressure of oxygen FIO2 = fractional concentration of oxygen in inspired air PB = barometric pressure PH2O = partial pressure of water vapor in the inspired air (dilutes the oxygen) PACO2 = alveolar partial pressure of carbon dioxide R = respiratory exchange ration = VdotCO2/VdotO2 O2 CO2 partial pressures (mmHg) air (sea level) 149.0 0.3 alveolar gas PAO2 = FIO2(PB ? PH20) ? PACO2/R PACO2 is proportional to VdotCO2/VdotA arterial blood 104 40 mixed venous blood 40 45-46 Defines hypoventilation, hyperventilation, and hyperpnea. hypoventilation ? inadequate ventilation that causes an increase in PaCO2 hyperventilation ? excess ventilation that causes a decrease in PaCO2 hyperpnea ? rapid breathing States the difference in solubility in water of O2 and CO2 and the effect this difference has on gas diffusion across the alveolar wall. O2 is slightly less dense than CO2 solubility of CO2 is 24x that of O2 CO2 diffuses 20x more rapidly through the alveolar capillary barrier than O2 diffusion perfusion limited ? looks similar to oxygen diffusivity of CO2 is 20x higher than oxygen, but the partial pressure is significantly lower (so overall volume of gas diffusing is similar to oxygen Defines pulmonary diffusing capacity. States how it changes with exercise, alveolar-wall thickening, alveolar edema, and emphysema. (AgasD)/T = Vdotgas /(P1gas ? P2gas) = DLgas Vgas = volume of gas diffusing through the tissue barrier per time, in mL/min A = surface area availabule for diffusion D = diffusion coefficient of the gas (diffusivity) D ~ solubility/?MW T = thickness of the barrier P1 ? P2 = partial pressure difference of the gas factors that affect diffusing capacity decreased surface area: destruction of alveolar wall (emphysema), decreased access to erythrocytes increased barrier thickness (alveolar edema ? addition of fluid barrier) exercise - causes increased blood flow so transfer of gas is diffusion limited, but the diffusing capacity is the same States whether O2 and CO2 reach equilibrium across the alveolar wall by the end of the pulmonary capillaries, at rest and during exercise in normal and abnormally thickened alveoli. oxygen perfusion limited ? under normal conditions PcO2 reaches PaO2 about 1/3 of the distance through the capillary with exercise, blood spends less time in the capillary (becomes diffusion limited) abnormalities: thickened alveolar wall ? transfer is reduced (diffusion limitation) emphysema ? decreased surface area for transfer (diffusion limitation) exercise ? increased blood flow (diffusion limitation) at low alveolar O2 (ex. Denver) ? lowered alveolar PO2 flattens curves (increased diffusion limitation) carbon dioxide perfusion limited ? looks similar to oxygen diffusivity of CO2 is 20x higher than oxygen, but the partial pressure is significantly lower (so overall volume of gas diffusing is similar to oxygen) same effects of thickened alveolar wall, emphysema, and exercise