Lecture 5: Ventilation-Perfusion Relation Flashcards
Lung mismatch
Ventilation-perfusion inequality, resulting in decreased arterial PO2
Dead space
Ventilation to alveoli that are not perfused
Shunt
Perfusion to alveoli that are not ventilated
Homeostatic lung response for ventilation-perfusion mismatches
Vasoconstriction around low PO2 alveoli (low vent.)
Bronchoconstriction around low PCO2 alveoli (low perfusion)
Hypoxia and its 4 general causes
Tissue level O2 deficiency
1. Hypoxic hypoxia
2. Anemic/CO hypoxia
3. Ischemic hypoxia
4. Histotoxic hypoxia
Hypoxic hypoxia
Low arterial PO2
Anemic/CO hypoxia
Low total O2 content but normal arterial PO2; deficiency in RBCs/Hb
Ischemic hypoxia
Low blood flow
Histotoxic hypoxia
Toxic agent preventing cells from using O2
Hypercapnia
Increased arterial PCO2
Why does a vent.-perfusion inequality affect O2 more than CO2?
Because of the sigmoidal O2 binding curve, good alveoli can only add so much O2 to blood; linear curve of CO2 means compensatory ventilation always increases/reduces CO2 effectively
Emphysema
Loss of elastic tissue and alveolar collapse increasing compliance
Partial pressures of H2O, O2, CO2 for air entering lungs at body temp (pre-equilibration)
H2O = 47 mmHg
O2 = 150 mmHg
CO2 = 0 mmHg
Equilibrates to O2 100 mmHg, CO2 40 mmHg
Why are VO2 and VCO2 similar despite a 10X difference in partial pressure change at the lungs?
Due to solubility
CO2 has high solubility, so even with small ΔP a large amount exchanges in and out of solution; O2 has low solubility so a larger ΔP is needed
Effect of gravity on ventilation
Intrapleural pressure is more negative at the top, less at the bottom, resulting in a TPP gradient. Alveoli at the top are bigger and less compliant, so more air goes to the bottom (bigger change in volume)
:: More vent. at bottom
Effect of gravity on perfusion
Above heart decreases hydrostatic P and vice versa
:: More perfusion at bottom
Effect of gravity on V:P
Gravity gradients for vent. and perfusion don’t change at the same rate
Overall, top = high V/Q, base = low V/Q
Blood partial Ps from a shunt
Shunt blood equilibrates to near mixed venous (PO2 = 40 mmHg, CO2 = 46 mmHg)
Blood partial Ps from dead space
Dead space blood equilibrates to near inspired air (PO2 = 150 mmHg, PCO2 = 0 mmHg)
Mixed arterial blood composition
Low V:Q blood is majority of volume (more Q, more drainage)
Δ(A-aO2)
Alveolar-arterial O2 difference; normal =< 10. Widening suggests more low V:Q units (closer to venous)
Right-to-left shunt
Extreme V/Q mismatch where blood goes from right heart to left without any gas exchange
