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Core_Equations_in_Respiratory_Phys5

# Core_Equations_in_Respiratory_Phys5 - Nora Laiken PhD CORE...

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Nora Laiken, PhD CORE EQUATIONS IN RESPIRATORY PHYSIOLOGY 1 1. Ideal gas law: PV = nRT Related laws: At constant T: P 1 V 1 = P 2 V 2 (Boyle’s law) At constant P: V 1 / V 2 = T 1 / T 2 (Charles’ law) 2. Partial pressure vs. concentration relationships: a. In a gas phase : P G = F G P (Dalton’s law) P G = partial pressure of gas G F G = fractional concentration of gas G P = total gas pressure b. In moist inspired gas : P I G = F I G (P B – P H 2 O ) P B = barometric pressure P H 2 O = water vapor pressure ( always 47 mmHg at body temperature) Important application: P I O 2 = F I O 2 (P B – P H 2 O ) When breathing air, F I O 2 = 0.21, regardless of altitude c. In a liquid phase : C G = k H G P G (Henry’s law) k H O 2 = 0.003 ml O 2 /100 ml blood · mmHg k H CO 2 = 0.067 ml CO 2 /100 ml blood · mmHg = 0.03 mmol CO 2 /liter blood · mmHg 3. Relation between alveolar ventilation (V . A ) and total ventilation (V . T ): V . A = V . T – V . D V . D = dead space ventilation or V . A = nV T – nV D n = breathing frequency V T = tidal volume V D = dead space volume 4. Diffusion equations: a. Fick’s law of diffusion (applied to the transfer of gas G from alveolar gas pulmonary capillary blood): V . G = (A/T) · D G · (P A G – Pc G ) V . G = rate of transfer of gas G from alveolar gas pulmonary capillary blood A = area of blood-gas barrier T = thickness of blood-gas barrier D G = diffusion constant of gas G (proportional to solubility/ MW) P A G

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