Lecture 20: Gas Exchange PDF

Lecture 20: Gas Exchange Dr. Ann Revill [email protected] Office: Dr. Arthur G. Dobbelaere Science Hall 380E Review: Top Hat Question Join code: 437994 2  Lecture objectives By the end...

Lecture 20: Gas Exchange Dr. Ann Revill [email protected] Office: Dr. Arthur G. Dobbelaere Science Hall 380E Review: Top Hat Question Join code: 437994 2  Lecture objectives By the end of this lecture, you will be able to: 1. Define partial pressure 2. Calculate partial pressure of a gas and partial pressure of inspired oxygen 3. Describe the mechanisms that affect the partial pressures of oxygen and carbon dioxide at each stage of gas exchange 3  Overview of gas exchange We’ve discussed processes by which air is moved in and out of Bulk flow the lung to the gas exchange surface Diffusion – i.e. by bulk flow Also discussed how heart pumps blood and moves O2 and CO2 around the body Bulk flow – i.e. by bulk flow Next: consider diffusional processes governing gas exchange between the lung and blood, blood and tissues, and Diffusion then the reverse 4 Sherwood 13-26  To talk gas exchange we need to talk “partial pressure.” What does it mean? Partial pressure of a gas (Pgas) is pressure a single gas exerts in a mixture of gases Atmospheric pressure = barometric pressure of all gases in atmosphere What’s most abundant gas in our atmosphere? nitrogen What % of O2 in air? 21% What’s average atmospheric pressure at sea level? – ≈ 760 mmHg  How to calculate partial pressure of gas? Gas molecules are constantly in motion This motion exerts a pressure Magnitude of pressure is proportional to temperature, concentration of gases Dalton’s Law: in a mixture of gases (e.g. air) each gas operates independently, so total pressure is sum of individual pressures: Ptotal = P1 + P2 + P3 + Pn – For Air Patm = PN2 + PO2 + PH20 + PCO2 – To calculate partial pressure of a specific gas Px = partial pressure of a gas (mm Hg) PX = Patm x Fx F = fractional concentration of gas 6  Calculation of PO2 and PIO2 Given these values, what is PO2 at sea level in dry inspired air? – Patm = 760 mm Hg – FO2 = 0.21 PO2 = Patm x FO2 = 760 mm Hg x 0.21 = 160 mm Hg In alveoli, air is humidified, so must account for H2O pressure: Px = (Patm – PH2O) x Fx How does the partial pressure of O2 change in humidified air (partial pressure of inspired oxygen P IO2)? PH2O = 47 mm Hg @ 37˚C PIO2 = (Patm – PH2O) x FIO2 Partial pressure of = (760 mm Hg - 47 mm Hg) x 0.21 inspired oxygen, PIO2 = 150 mm Hg 7  Thought experiments on PIO2 Would PIO2 be higher or lower at the top of Mt. Humphrey’s compared to Phoenix? Lower. Why? Patm is lower at high elevation, FIO2 doesn’t change PIO2 = (Patm – PH2O) x FIO2 = (520 – 47) x 0.21 = 99.33 mmHg Would PIO2 be higher or lower when receiving supplemental oxygen (increasing the FI02 to 80%) through a non-rebreathing mask? Higher. Why? FIO2 increased, Patm is unchanged (FO2 also unchanged) PIO2 = (Patm – PH2O) x FIO2 = (760 – 47) x 0.8 = 570.4 mmHg 8  Concentrations of dissolved gases Higher solubility Lower solubility Equilibrium reached when Px (gas) = Px (liquid) Henry’s Law: Concentration of a GAS (in liquid) = Px x solubility Partial pressure of a gas in liquid phase equivalent to partial pressure in gas phase Concentration of a gas in solution is ONLY due to dissolved gas, not relevant if bound, e.g. to hemoglobin, or chemically modified, e.g. into HCO3- 9  Remember diffusion of gases and Fick’s Law? Vx = volume of gas diffused per unit time Vx = D x A x ΔP D = diffusion coefficient of the gas A = surface area ΔX ΔP = partial pressure difference of the gas ΔX = membrane thickness Diffusion directly proportional to: Diffusion coeffient Surface area Pressure gradient Diffusion inversely proportional to: Membrane thickness (diffusion distance) 10  Diffusion of gases Driving force for gas diffusion? Pressure gradient (ΔP) What does the diffusion coefficient depend on? – Molecular weight and solubility Diffusion coefficients vary for different gases – CO2 is 20x more soluble than O2 – i.e. for a given ΔP, CO2 diffuses ~20x faster than O2 11  Gas exchange Alveolar air PAO2 = 100 mm Hg,