Respiratory Physiology PDF 2025

# Respiratory Physiology ## Alveolar PO2 * Alveolar PO2 (100mmHg) is less than atmospheric PO2 (160mmHg) due to: * It becomes saturated with water vapor * Only 15% of alveolar air is replaced by atmospheric air during inspiration (350mL added to 22000mL) * O2 continuously diffuses into b...

# Respiratory Physiology ## Alveolar PO2 * Alveolar PO2 (100mmHg) is less than atmospheric PO2 (160mmHg) due to: * It becomes saturated with water vapor * Only 15% of alveolar air is replaced by atmospheric air during inspiration (350mL added to 22000mL) * O2 continuously diffuses into blood * Alveolar PO2 remains constant because: * New O2 arrives to the alveoli is equal to O2 leaves the alveoli into the blood ## Respiratory Gas Exchange * **Atmospheric air consists of:** * N2 = 79% * O2 = 21% * CO2 = 0.04% * **Atmospheric pressure** (barometric pressure) = 760mmHg at sea level * **Partial pressure of O2 in air** = PO2 = 760 x 21% = 160mmHg * **Gas exchange** between alveolar air (air) & pulmonary capillaries (blood) occurs by **simple diffusion:** * i.e. from areas of high partial pressure to areas with low partial pressure ## Gas exchange at lungs * **Venous blood** (poor in O2 & rich in CO2) comes in contact with alveolar air (rich in O2 & poor in CO2) * **a) O2 diffusion:** * From alveolar air (PO2 = 100 mmHg) to pulmonary capillary blood (PO2 = 40mmHg) down pressure gradient * Blood leaving the pulmonary capillary has PO2 = 100 mmHg * **b) CO2 diffusion:** * From pulmonary capillary blood (PCO2 46mmHg) to alveolar air (PCO2 40mmHg) down pressure gradient * Blood leaving the pulmonary capillary has PCO2 = 40 mmHg ## O2 Reserve: * Pulmonary venous blood is poor in O2, but it still has some O2 that was not used by tissues during resting state. ## Diffusion Reserve: * Pulmonary venous blood passes through the lungs, then back to the heart in 0.75 seconds during resting state. * O2 and CO2 diffusion (equilibrium) occurs in 0.25 seconds only. * So, there will be always sufficient time for gases diffusion (equilibrium) in the lungs, even during heavy exercise. ## CO2 is always present even in arterial blood: This helps in: * Stimulation of respiratory higher centers. * Regulation of acid-base balance. ## Alveo-capillary membrane (= Respiratory membrane) * **6 layers:** * Fluid lining alveoli & surfactant * Alveolar epithelium * Alveolar basement membrane * Interstitial space &fluid * Capillary basement membrane * Capillary endothelium * **Thickness of the membrane:** 0.5 u * **Surface area of the membrane:** 50-100 m² ## Factors affecting diffusion of gases: * **1) Pressure gradient:** the gradient (difference) between pressure of the gas in the alveoli (PA) & in the blood capillary (PC) * **2) Surface area of the pulmonary membrane (A). (70 m²)** * **3) Thickness of the respiratory system 0.5u (T)** * **4) Diffusion coefficient of the gas (D) = Solubility of the gas / Molecular weight of the gas** * **So, Rate of gas diffusion = DA (PA-PC) / T** * The molecular size of CO2 is larger than O2 by 1.4 times. * The solubility of CO2 is much more than O2 by 24 times * **Net effect:** * The diffusion rate of CO2 is 20 times faster than O2 * Respiratory diseases affect O2 diffusion more than CO2 diffusion * Equilibration of O2 diffusion across the respiratory membrane occurs of at the same time of equilibration of CO2 diffusion, because O2 pressure gradient is 60 mmHg while CO2 pressure gradient is 6 mmHg