S3 - gas exchange & lung function tests Flashcards
Describe factors affecting the rate of diffusion across the air blood interphase
pressure difference solubility of gas in solution cross-sectional area of the fluid distance molecules must diffuse molecular weight of the gas temperature of the fluid
what does diffusion of gases depend on?
solubility of gas
CO2 20x more soluble than O2
what is Fick’s law?
molecules across a barrier is proportional to the permeability of the molecules x transfer SA over which diffusion can occur x concentration gradient
what is proportional to diffusion coefficient? what can it be used to determine?
solubility / √(MW) is proportional to diffusion coefficient
used to determine relative rates at which different gases will diffuse across same membrane at same pressures
(CO2 diffuse 20x more rapidly due to solubility)
what is the difference in diffusion coefficient compensated by?
the differences in partial pressure
larger partial pressure difference compensates for slower diffusion of O2
Describe the layers making up the diffusion barrier at the airblood interphase
5 cell membranes, 3 layers of cytoplasm, 2 layers of tissue fluid:
epithelial cell of alveolus (2 membranes, 1 cytoplasm)
tissue fluid
endothelial cell of capillary (2 membranes, 1 cytoplasm)
plasma
RBC membrane (1 membrane)
Describe the role of diffusion resistance in gas exchange. State and explain the difference in the diffusion rates of O2 and CO2
depends on:
- nature of barrier:
- permeability is diffusion coefficient of molecules in the membrane (mobility once dissolved) x solubility
Describe the role of diffusion resistance in gas exchange. (what does diffusion resistance depend on?)
depends on:
- nature of barrier:
- permeability
- size of molecule - nature of gas
how do you work out permeability of a membrane across a lipid bilayer?
permeability = diffusion coefficient of molecules in the membrane (mobility once dissolved) x solubility in lipid bi-layer x thickness of membrane
relationship between diffusion coefficient and size of molecule?
larger molecules, smaller diffusion coefficients, giving lower permeability
State and explain the difference in the diffusion rates of O2 and CO2
CO2 diffuses much faster than O2 due to a higher solubility of 20x
therefore, exchange of O2 always limiting step
(although both molecules small, so fast diffusion)
Explain the reason for the partial pressures of O2 and CO2 observed in inspired air
inspired air always saturated with water as it passes along moist airways
water content depends on temperature & core boyd temp (37)
O2 is slightly diluted
Explain the reason for the partial pressures of O2 and CO2 observed in alveolar air
State the normal PO2 and PCO2 in alveolar air
pCO2 - 5.3kPa
pO2 - 13.3kPa
pO2 lower in alveoli compared to external:
- continuous diffusion of oxygen across alveolar wall
- diluting effect of CO2 entering alveoli as it travels in opposite direction to O2
pO2 in alveoli still higher than capillaries, so oxygen diffuse down conc gradient
(blood leaving alveolar capillaries is in equilibrium with alveolar air, so has same pO2 & pCO2)
Explain the reason for the partial pressures of O2 and CO2 observed in mixed venous blood
State the normal PO2 and PCO2 in venous blood
pCO2 - 6.1 kPa
pO2 - 5.3 kPa
there is a higher concentration of CO2 (diffusion from capillaries) and a lower concentration of oxygen (diffused down conc gradient into capillaries)
pCO2 is higher in capillaries than alveoli, therefore diffuses into alveoli to be exhaled
Explain the reason for the partial pressures of O2 and CO2 observed in arterial blood
State the normal PO2 and PCO2 in arterial blood
pCO2 - 5.3kPa
pO2 - 13.3kPa
same as in alveolar air as the equilibrium has been reached post gas exchange and travelling from lungs to body, hasn’t supplied body via systemic circulation yet
(blood leaving alveolar capillaries is in equilibrium with alveolar air, so has same pO2 & pCO2)
