Respiratory II: Gas Exchange Lecture Notes PDF

Respiratory II: Gas Exchange ANSC 3080 G. Bedecarrats Learning Objective  Describe the mechanism of gas exchange between the alveoli and pulmonary capillaries Properties of Gases  Gas molecules constantly moving (collide) = ex...

Respiratory II: Gas Exchange ANSC 3080 G. Bedecarrats Learning Objective  Describe the mechanism of gas exchange between the alveoli and pulmonary capillaries Properties of Gases  Gas molecules constantly moving (collide) = exert pressure on the container  Total pressure: determined by the total number of gas molecule per volume unit  Partial pressure of gases: in a mixture = total pressure x % of that gas  Independent of other gases present in the mixture  Example:  O2 = 20.94% of atmospheric air  Total pressure (sea level) = 760 mm Hg  Partial pressure of O2 (PO2) = 0.21x760 = 159mm Hg 1  Definitions (continued)  Air (dry) P = PN2 + PO2 + PCO2 = 760 mm Hg  BUT air in lungs is NOT dry:  Inspired air is 100% saturated with water when it reaches the respiratory zone  Some of the gas molecules colliding with water dissolve in water (slightly different than for dry air)  At equilibrium the relative amount of dissolved gas is constant  Depends of the solubility of the gas  Partial pressure of the gas  Presence of water “dilutes” gas content  At 37 C, water vapor pressure is 47 mm Hg  PO2 = (0.21 x 760)-(0.21x47) = 150 mm Hg  PO2 = (amount present in dry air) – (amount dissolved in water) Gas Exchange = Diffusion  Passive movement of gas molecules from regions of high concentration (partial pressure) to regions of low concentration (partial pressure)  In alveoli the air velocity = 0 (due to large cross-sectional area)  Movement from alveoli to alveolar-capillary membrane by diffusion only In the Alveoli At constant atmospheric partial pressures Diffusion of gases will depend on the alveolar and blood partial pressures Depends on ventilation for alveoli (how much air replaced in the alveoli) Depends on the tissue consumption for blood At higher altitude pO2 in atmosphere  Significant impact on diffusion pCO2 in atmosphere is way lower than in alveoli and blood changes in atmospheric pressure does not impact CO2 diffusion dramatically (except in enclosed spaced) 2  Effect of ventilation on partial pressures in the alveoli Hyperventilation Hypoventilation In the Lung  Gas exchange occurs between alveoli and blood capillary network  Blood from right ventricle flows in capillaries  Note that lungs also have some capillaries from the left side of the heart to bring oxygen to lung tissue  Blood entering alveolar capillaries has low pO2  O2 diffuses from alveoli to the blood  At the end of capillary, pO2 is the same in blood and alveoli  For CO2: pCO2 higher in blood entering capillary then diffuses out toward alveoli Main determinants:  Driving partial pressure gradient  PalveoO2 – PcapO2  Surface area available for diffusion (A)  Increased area = more exchange  Capillaries can open during exercises to increase exchange  Thickness of the air-blood barrier (X)  Deep inspiration during exercises reduces the distance between alveoli and capillary epitheliums  Physical properties of the gas (D) VO2 = D x A x (PAO2 – PcapO2)/X 3  Ventilation - Perfusion  Perfusion correspond to the blood entering the lung  Need to match ventilation with the blood flow for optimum gas exchange  For bipeds (human) at rest: not optimum Gravity = perfusion (blood flow) lowest at the top of the lung (capillaries can collapse) Not so pronounced for ventilation  Not so bad for quadrupeds  V/Q = term used for ventilation/perfusion (0.8 in humans)  V/Q becomes more uniform during exercise (more blood pump throughout the lung) Hypoxic Vasoconstriction Certain diseases can affect either the ventilation or perfusion of certain alveoli = V/Q inequality Natural mechanism to minimize the impact: hypoxic vasoconstriction closing of the poorly ventilated alveoli redirecting blood towards the well ventilated Mechanism initiated by a reduction of pO2 and/or increase in pCO2 in the interstitial fluid of affected areas Impaired Pulmonary Gas Exchange  Thickening of alveolar-capillary membrane  Increases the time for diffusion across the membrane and decreases rate of diffusion  High altitude or low air pO2  Decrease alveolar oxygen pressure, hence decrease driving pressure  Hypoventilation  Inadequate ventilation of lung  Ventilation-perfusion inequality  Ventilated alveoli with no blood supply or vice versa 4  Gas Exchange in Tissues Follows the same principle as in the alveoli Diffusion driven by difference in partial pressure Cells consume O2 and produce CO2 Blood from the left heart loaded with O2 Diffusion of O2 will occur from blood to interstitial fluid to the cells Diffusion of CO2 will occur from the cell to the interstitial fluid to the blood 5