PHS 3342 Lecture Notes - Lecture 13: Vapor Pressure, Partial Pressure, Carboxyhemoglobin
28 Apr 2018
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March 16, 2018
Gas Exchange
Partial Pressures (Dalton’s Law)
pO2 = %O2 (FiO2) x atmospheric pressure = 21% x 760 mmHg = 160 mmHg
-NB: only true when at sea level (21% O2)
-FiO2 = fraction of oxygen in inspired air
pCO2 = %CO2 x atmospheric pressure = 0.03% x 760 mmHg = 0.2 mmHg ~ 0 mmHg
-Therefore, the CO2 measured in expired air originates from the alveoli
Correction for Water Vapour
When atmospheric air is inhaled, it is warmed and fully modified in the upper airways, such that we must now correct
our previous calculations for the presence of this additional gas (H2O vapour)
Rather than re-calculate the % oxygen in the new mixture, we prefer to simply subtract the partial pressure
attributable to water vapour before making the calculation using the known %O2 being inhaled
pVAP(H2O) = 47 mmHg at 37°C
-Doesn’t change because body temperature remains constant at 37°C
Therefore:
-pATM = 760 - 47 = 713 mmHg
-pO2 = 21% x 713 mmHg = 150 mmHg
pO2 inspired (piO2) = FiO2 x (atmospheric pressure - water vapour pressure) = 150 mmHg
Solubility of Gases (Henry’s Law)
When a gas and a liquid are in contact, the volume of gas dissolved in a liquid is proportional to the partial pressure
of that gas above the liquid
-In the body, you air going across blood
At equilibrium: pGAS (air) = pGAS (liquid)
-Partial pressure in the airway = partial pressure in the blood
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March 16, 2018
Gas Exchange: Partial pressures of Oxygen
pO2 becomes 100 mmHg once it gets down to the alveoli, so paO2 is 100 mmHg when it enters the blood
Once it returns back through the veins, pvO2 has decreased to 40 mmHg because the oxygen has been used by the
tissues
Expired air is basically diluted alveolar air
Effect of Altitude on Inspired pO2
As altitude increases, atmospheric (barometric) pressure decreases
This results in decreases levels of piO2
-Recall: pO2 = FiO2 (pATM - pH2O)
pO2 is related to saturation of hemoglobin - directly related
Gas Exchange: Diffusion (Fick’s Law)
Net transfer of a gas molecule down its partial pressure gradient (passive process)
-Requires a partial pressure gradient
Transfer across short distances (μm)
Vgas = (Ad/T) x [ΔP]
-Vgas = rate of diffusion
-A = surface area
-T = thickness
-d = diffusibility = (solubility of the gas/√MW)
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-[ΔP] = pressure gradient
NB: diffusion of CO2 is 20x greater than that of O2 due to its high solubility
-Free oxygen is not very soluble in the blood
Diffusing capacity of the lung (DLCO) = Ad/T
-Measured using CO2
-Diffusion properties tend to show up with oxygen, but not in CO2 —> CO2 leaving doesn’t tend to be the problem,
O2 getting in is the problem
The respiratory system is optimized to maximize diffusion
-Large surface area (70m2): large number of spherical alveoli covered in networks of capillaries
-Minimal thickness (0.5 μm): 75 mL of blood spread over a very large area
Causes of Reduced Diffusion in Lungs
Thickened alveolar membrane: fibrosis, sarcoidosis, asbestosis
Decreased surface area: tumours, emphysema, edema
Decreased absorption by erythrocytes: anemia, low circulating blood volume, pulmonary embolism (blocking blood
flow to lung)
Uptake of Oxygen Along Capillary
O2 diffuses down its concentration gradient, saturates hemoglobin, and then very quickly reaches (near) equilibrium
with alveolar gas
At normal heart rates, contact time between blood and the lungs is ~0.75 seconds
Even during exercise (transit time = 0.25 seconds), red blood cells have enough time to replenish their oxygen supply
-Recall: arteriole end has low oxygen, venous end has high oxygen
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Document Summary
Partial pressures (dalton"s law) po2 = %o2 (fio2) x atmospheric pressure = 21% x 760 mmhg = 160 mmhg. Nb: only true when at sea level (21% o2) Fio2 = fraction of oxygen in inspired air pco2 = %co2 x atmospheric pressure = 0. 03% x 760 mmhg = 0. 2 mmhg ~ 0 mmhg. Therefore, the co2 measured in expired air originates from the alveoli. When atmospheric air is inhaled, it is warmed and fully modi ed in the upper airways, such that we must now correct our previous calculations for the presence of this additional gas (h2o vapour) Rather than re-calculate the % oxygen in the new mixture, we prefer to simply subtract the partial pressure attributable to water vapour before making the calculation using the known %o2 being inhaled pvap(h2o) = 47 mmhg at 37 c. Doesn"t change because body temperature remains constant at 37 c. Patm = 760 - 47 = 713 mmhg.
