Gas Laws and Pulmonary Diffusion PDF

DIFFUSSION OF PULMONARY GASES Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005  GAS LAWS IDEAL GAS LAW PV=nRT P= Pressure V= Volume n= Number of molecules present R= Gas constant (.08) T= Temperature in Kelvin scale If n remains constant: P...

DIFFUSSION OF PULMONARY GASES Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005  GAS LAWS IDEAL GAS LAW PV=nRT P= Pressure V= Volume n= Number of molecules present R= Gas constant (.08) T= Temperature in Kelvin scale If n remains constant: P1xV1 = P2 x V2 T1 T2  BOYLE’S LAW (P1 x V1 = P2 x V2) If T remains constant, P will vary inversely to V. Example: Container with 200 mL P of 10 cm H2O. Volume is reduced 50% New P= 20 cmH2O P2 = P1 x V1 = 10 cmH2O x 200 mL = 20 cmH2O V2 100 mL  CHARLES’ LAW (V1 / T2 = V2 / T2) If P remains constant, V will vary proportional to T. Example: Balloon with 3L T increased from 250K to 300K. Volume is increased to 3.6L V2 = V1 x T2 = 3L x 300K = 3.6L T1 250K  GAY-LUSSAC’S LAW (P1 / T1 = P2 / T2) If V remains constant, P will vary proportional to T. Example: Container with 50 cmH2O T increased from 275K to 375K. Pressure in the container is increased to 68 cmH2O P2 = P1 x T2 = 50 cmH2O x 375K = 68 cm H2O T1 275K  DALTON’S LAW In a mixture of gases, the total P is equal to the sum of partial P of each separate gas.  PARTIAL P OF ATHMOSPHERIC GASES Atmospheric gases surrounding earth exert a P at sea level of ~760 mmHg = barometric P. PB decreases with altitude. Gases that compose PB  PARTIAL P OF O2 and CO2 PBO2 (159 mmHg) > PAO2(100 mmHg). Alveolar O2 mixes with alveolar CO2 pressure (PACO2= 40 mmHg) and alveolar water vapor pressure (PH2O= 47 mmHg). Partial P of Gases in the Air 3-2  WATER VAPOR PRESSURE Alveolar gas is 100% humidified @ body temperature. Absolute humidity 44 mg/dL Water vapor P 47 mmHg  ALVEOLAR GAS EQUATION (PAO2) PAO2 = [PB-PH2O] FiO2 – PaCO2 (1.25) PAO2 =.21 – 50 PAO2 =.21 – 50 PAO2 = 100 mmHg IF FIO2 is >= 60% omit 1.25: PAO2 = [PB-PH2O] FiO2 – PaCO2  DIFFUSION of PULMONARY GASES Passive movement of gas molecules from high P to low P. In the lungs, gas diffuses through the ACM. ACM (0.36 – 2.5μ ) Liquid lining of the intra-alveolar lining. Alveolar epithelial cell. Basement membrane of the capillary endothelium. Plasma in the capillary blood. RBC membrane. Intracellular fluid of the RBC until Hgb molecule is encountered. ACM 3-2 O2 and CO2 DIFFUSION Venous blood: PVO2 = 40 mmHg and PVCO2 = 46 mmHg. ( ΔP=6 mmHg). As it passes through capillary system PAO2 = 100 mmHg and PACO2 = 40 mmHg. ( ΔP=60 mmHg). Diffusion Across ACM 3-3 Pressure Gradient ACM 3-4 O2 and CO2 DIFFUSION Equilibrium is reached in ~ 0.25 sec; transit time through ACM is 0.75 sec. In exercise, transit time is shorter = less time for gas diffusion. O2 and CO2 DIFFUSION O2 and CO2 DIFFUSION  GAS DIFFUSION FICK’S LAW Rate of gas diffusion is proportional to the surface area of the tissue and inversely proportional to the thickness. Clinically: A: area reduced (alveolar collapse or fluid). P1-P2: decreased PAO2 (high altitude or alveolar hypoventilation). T: thickness (alveolar fibrosis or edema) What can you do to help? Fick’s Law  Henry’s Law Amount of gas that dissolves in a liquid is proportional to the partial P of the gas. Solubility coefficient @ 37°C: O2 = 0.024 ml/mmHg/ml H 2O CO2 = 0.59 ml/mmHg/ml H2O 24:1  Graham’s Law Rate of diffusion of gas through a liquid is proportional to the solubility coefficient of the gas and inversely proportional to the molecular weight of the gas. GMW: O2 = 32 CO2 = 44 O2 moves faster than CO2. Combining Graham’s and Henry’s Laws = CO2 diffuses 20 times faster than O2.  PERFUSION-LIMITED GAS FLOW Rate of diffusion of gas through alveolar wall depends on amount of blood that flows pasts the alveoli. PERFUSION-LIMITED GAS FLOW  DIFFUSION-LIMITED GAS FLOW Rate of diffusion of gas through alveolar wall depends on integrity of the ACM. DIFFUSION-LIMITED GAS FLOW O2 and DIFFUSION vs. PERFUSSION-LIMITED GAS FLOW Rate of diffusion of gas through alveolar wall depends on integrity of the ACM. DIFFUSION-LIMITED GAS FLOW DIFFUSION-LIMITED GAS FLOW  Summary O2 cascade Dry gas 159 mmHg Reason for change Conducting airways149 Addition of water vapor End exp gas 114 Mixing of deadspace Alveolar 101 Addition of CO2 Arterial blood 97 Intrapulmonary shunting Mean systemic Capillary pressure 40 O2 diffusion into cell Cellular cytoplasm