Gas exchange Flashcards
How O2 travels from alveolar air to capillary blood?
- enters alveoli airspace from atmosphere
- dissolves in Alevolar Lining Fluid
- diffuses via alveolar epithelium, basement membrance. capillary endothelial cells
- dissolves in plasma
- binds to Hb
Times for 🔴 pass via pul capillary for oxygenation?
- 75 s
0. 25 s (exercise) due to increased pul blood flow
What’s rate of diffusion determined by?
partial p gradient, diffusion distance, SA
rate of diffusion ∝ (alveolar SA/distance²) x (PA - P꜀)
(PA - P꜀) : partial p gradient between alveolar air, capillary blood
How specific pathologies affect the rate of gas exchange?
- hypoperfusion type I : ↓P꜀
- hypoventilation type II : ↓Pa
- emphysema : ↓SA
- fibrosis : ↑basement membrane thickness
- pul oedema : ↑thickness of fluid layer
How to modulate pp gradients?
changing rate of V̇a
What’s Hypoventilation?
excessive CO2 within arterial blood (PaCO2 < 4.9 kPa).
What’s Hyperventilation?
reduced CO2 within arterial blood (PaCO2 > 6.0 kPa)
Effect of increasing ventilation rate?
increases alevolar O2 pp (PAO2)
decreases alveolar CO2 pp (PACO2)
Normal ventilation/perfusion ratio?
1
because 1L blood carries 200ml O2 + 1L dry air carries 200ml O2
Define dead-space effect
ventilation w/o perfusion
V/Q >1
Define physiologic dead-space
affected alveoli where inspired O2 ‘wasted’ + no gas exchange due to reduced blood supply
Why doesn’t overall perfusion change when there’s a pul embolism?
blood diverted via other pul arteries/capillaries so increased ventilation of these areas compensates for reduction in gas exchange in others
Define pulmonary shunt/shunt-effect
Reduced ventilation of alveoli or limits to diffusion
Blood from R to L w/o taking part in gas exchange
V/Q <1
Resting ventilation + perfusion?
5L/min
How to reduce ventilation-perfusion mismatching?
Hypoxic vasoconstriction of capillaries divert blood from poor to well ventilated alveoli
What induces a reflex hyperventilation?
↑PACO2
clears excess CO2 but DOESN’T ↑O2
eg of physiologic dead-space?
heart failure : cardiac arrest
blocked vessels : pul embolism
loss/damage capillaries : emphysema
eg of pul shunt?
cardiac shunts
pneumonia, acute lung inury, respiratory distress syndrome, atelectasis
Dead space effects?
V̇a = normal
Q = ↓
V/Q = ↑
Response to O2 therapy = helps
Pul shunt effects?
V̇a = ↓
Q = normal
V/Q = ↓
Response to O2 therapy = poor
Why doesn’t O2 therapy work on pul shunts?
cannot increase O2 saturation in well-ventilated lung regions sufficiently to compensate for deoxygenated blood which it will mix with
How hypoxic vasoconstriction can pathologically effect COPD?
- from chronic hypoventilation –>prolonged, widespread pul vasoconstriction
- increases resistance within the pul vasculature
- pul hypertension–>R heart hypertrophy –> R heart failure
How to determine cause of hypoxaemia?
PAO2 but invasive so:
F₁O2 + PaCO2
Basis of F₁O2 + PaCO2?
Alveolar content = O2 inspired - O2 consumed
O2 inspired calculated by determining the O2 p of inspired air (total pressure x molar fraction)
O2 consumption measured indirectly by measuring PaCO2 (similar to PACO2 + quantity of CO2 in expired air)
Role of Respiratory Exchange Ratio (RER) + normal value?
describes relationship between CO2 production + O2
consumption in metabolism
0.8
How’s PaCO2 used to calculate PAO2?
dividing PaCO2 by RER indirectly provides measurement of O2 consumption
then Alveolar content = O2 inspired - O2 consumed
Alveolar gas equation?
PAO2 = F₁O2 x (PB - PH2O) - (PaCO2/RER)
F₁O2 : O2 content of inspired air
PB : 100kPa - sea level
PH2O : 6kPa - humidified air
PAO2 : alevolar content
F₁O2 x (PB - PH2O) : O2 inspired (19.74)
(PaCO2/RER) : O2 consumption
How is RER calculated?
vCO2 produced/vO2 consumed
assumed 0.8
What determines RER?
metabolic substrate eg carb = 1, FA = 0.7
What’s A-a O2 gradient + values?
PAO2-PaO2
normal=<2kPa
F₁O2 x (PB - PH2O) ambient air at sea level=19.74kPa
How to investigate decrease PaO2?
1) hypo or hyperventilation causing hypoxaemia?
is PaCO2>5kPa? - hypo
is PaCO2<5kPa - V/Q inequality
2) is O2 diffusing into blood at alveoli?
use AGE + ABG readings to calcukate A-a O2 gradient
>2kPa - gas exchange prob due to V/Q mismatch, diffusion defect, shunt
