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MP - Lecture 26

Pulmonary Gas Exchange and O2 Transport

Medical Physiology – Lecture 26 Pulmonary Gas Exchange and O2 Transport
Diffusion of O2 from air to blood consists of two reversible processes: O2 diffusion from air to RBC interior and dissolved O2 binding to Hb
Both processes of O2 diffusion rapidly reach: Equilibrium
At equilibrium, PO2 = PO2 in air
At equilibrium, dissolved O2 and HbO2 depends on: PO2
P50 represents: Pressure at which Hb is 50% saturated
P50 increase means O2 affinity: Decreased (right-shift)
P50 can be increased by: High temperatures, 2,3-DPG, high CO2, and low pH
Pulse oximeter measures: Absorbance ratio at two different wavelengths (HbO2 and Hb) to determine saturation
Pulse oximter does not measure: Arterial O2 content
O2 content depends on: Hb saturation and amount
Hb saturation depends on: PO2 and dissociation curve
O2 content (mL O2 / 100 mL blood) = (1.34) x (Hb saturation) x (Hb content)
O2 content if 100% saturation and 15g/100 mL blood = (1.34) x (1.0) x (15) = 20
O2 content if PO2 = 26 mmHg and 15g/100 mL blood = (1.34) x (0.5) x (15) = 10
Amount of O2 dissolved in blood at PO2 = 100 is: 0.3, minor contribution
Amount of dissolved O2 in blood at PO2 = 1000 is: 3, significant contribution
Rate of diffusion between air and blood is proportional to: Surface area
Rate of diffusion between air and blood is inversely proportional to: Thickness of barrier
PO2 at the pulmonary capillary end equals: PAO2
CO2 partial pressure difference is smaller compared to O2, but same amount of diffusion because: CO2 has higher solubility
Inspired air PO2 150 mmHg
Inspired air PCO2 ~0 mmHg
Alveolar air PO2 100 mmHg
Alveolar air PCO2 40 mmHg
Why don’t alveolar PO2 and PCO2 not change with each breath? They do but very subtle changes
Steady state and gas composition is determined by: Alveolar ventilation
VCO2 = (VA) x (PACO2/760)
Hyperventilation increases O2 content by: Small increase
Hypoventilation causes: PACO2 increase, PO2 decrease
Alveolar PO2 and PCO2 change in ___ direction with ___ magnitudes. Opposite, similar
If O2 content and VA change by the same factor, PAO2 and PACO2: Do not change
Alveolar Gas Equation PAO2 = PIO2 – (PACO2/R), where R = 0.8 in balanced diet
During hypoventilation, supplemental O2: Corrects hypoxemia, but not hypercapnia
Created by: emyang



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