Gas Exchange PDF

Gas exchange Gas exchange is the diffusion of (O2, CO2) across the respiratory membrane. Following pulmonary ventilation O2 diffuses into pulmonary capillaries & CO2 diffuses into the alveoli across the respiratory membrane.  The Partial pressure The partial pressure of a gas in a mixt...

Gas exchange Gas exchange is the diffusion of (O2, CO2) across the respiratory membrane. Following pulmonary ventilation O2 diffuses into pulmonary capillaries & CO2 diffuses into the alveoli across the respiratory membrane.  The Partial pressure The partial pressure of a gas in a mixture of gases is the pressure exerted by that gas in the mixture = Total pressure X fractional concentration of the gas Partial pressure of Gases dissolved in solution The partial pressure of a gas in blood is produced by the dissolved gas that is free in the solution Partial pressure of Gases dissolved in solution The partial pressure of a gas in blood dose not include any gas that is present in a bound form O2 or CO2 bound to Hb do not participate in the partial pressure  Partial pressures of Gases 1ST - Dry inspired air contains 21% O2 (PO2 =160 mmHg) and almost no CO2 (PCO2=0 mmHg) Partial pressures of Gases 3rd - Alveolar Air: In the alveoli the composition of air differs from the inspired air because of three factors: 1- Saturation with water vapour 2- loading of O2 into the blood 3- Adding CO2 from the blood Partial pressures of Gases 3rd - Alveolar Air: PAO2 decreases to 100 mmHg PACO2 increases to 40 mm Partial pressure of Gases dissolved in Blood Partial pressure of O2 and CO2 in mixed venous blood: venous blood reaching the pulmonary capillaries PV O2 =40 mmH PV CO2 =45 mmHg Loading of O2 PAO2 = 100 mmHg 40 100 mmHg mmHg  Loading of O2 O2 diffuses along its partial pressure gradient from the alveolus into the blood until equilibrium is reached within the first third of the capillary  Loading of O2 Blood leaving the alveolar capillaries has been oxygenated PO2 is equal to PAO2 = 100 mmHg Unloading of CO2 PACO2 =40 mmHg PvCO2= 40 mmHg 45 mmHg Unloading of CO2 CO2 diffuses along its partial pressure gradient from the blood into the alveolus until equilibrium is reached  Blood leaving the pulmonary capillaries has a PCO2 equal to PACO2 = 40 mmHg  Partial pressure of Gases dissolved in Blood Partial pressure of O2 and CO2 in end alveolar capillary blood: If Gas exchange in the alveoli is normal The partial pressure of gases in end capillary blood : Equal to alveolar Partial pressures PaO2 =PAO2 =100 mmHg PaCO2=PACO2= 40 mm  Components of respiratory membrane The respiratory membrane consists of 6 layers: 1- The fluid lining layer of the alveoli. 2- Alveolar epithelium. 3- Alveolar epithelial basement membrane. 4- Thin interstitial layer between alveolar epithelium and capillary membrane. 5- Capillary basement membrane. 6- Capillary endothelial membrane. Factors affecting gas exchange Thickness of respiratory membrane decrease diffusion. Surface area of respiratory membrane increase diffusion. Diffusion coefficient of the gas - Is directly proportion to the solubility of the gas in the membrane and inversely proportion to the square root of the molecular weight of the gas. The pressure difference across the respiratory membrane, from high pressure area into lower pressure area