Lecture 5 - Partial pressure and diffusion Flashcards
What would methotrexate induced pulmonary fibrosis do to FRC?
Decrease FRC
As elastic recoil increases and compliance decreases
Equation for pressure
P= (n x R x T)/ V
n = no. of moles present R= gas constant T = Temperature V = Volume
Dalton’s law
In a mixture of gases, each individual gas exerts a partial pressure in proportion to its volume percentage in the mixture
Atmospheric pressure
101.1 kPa
What happens to atmospheric pressure at higher altitudes?
Decreases as less weight of air pressing down
Equation for partial pressure of gas
PP = Total pressure x Proportion of gas in mixture
Equilibrium of gas in contact with liquid
Rate of gas dissolved = rate of gas coming out of the solution
pp of dissolved gas = pp of free gas
Equation for the amount of dissolved gas
Amount of gas dissolved = partial pressure of gas x solubility coefficient
Solubility coefficient
Amount of gas that dissolves in 1 L of plasma at body temperature (37degrees) when exposed to a given partial pressure
pO2 in atmosphere, conducting airways, alveoli, arteries and vein
Atmosphere - 21.3 kPa Conducting airways - 20 kPa Alveoli - 13.3 kPa Arteries - 13.3 kPa Veins - 6kPa
Why does pO2 decrease in alveoli and conducting airways
In the conducting airways, air is warmed and moistened therefore there is a water vapour pressure exerted.
In the alveoli, new air (10%) is mixed with old air. The old air loses oxygen and gains CO2
What percentage of the tidal volume is used by the anatomical dead space
30%
150ml
pCO2 in atmosphere, conducting airways, alveoli, arteries and vein
Atmosphere - 0.04 kPa Conducting airways - 0.04 kPa Alveoli - 5.3 kPa Arteries - 5.3 kPa Vein - 6.1 kPa
Solubility coefficient of O2
0.01 mmol/L/kPa
Content of gas
Amount of gas chemically bound + amount of free gas in solution
Why is O2 bound to Hb
More O2 can be carried in blood so more O2 can be dissolved.
Keeps partial pressure in blood low to maintain a strong diffusion gradient
Percentage of O2 bound to Hb
98-99%
Factors affecting diffusion
Surface area
Diffusion distance
Partial pressure gradient
Diffusion coefficient - depending on solubility and molecular weight of gas
Diffusion coefficient
Solubility / √ molecular weight
Molecular weight is square rooted therefore solubility matters more
Why does CO2 have a higher diffusion coefficient compared to O2?
More soluble - diffuses faster
Has a higher molecular weight but solubility matters more
CO2 diffuses 20 x faster than O2
What compensates for the low diffusion coefficient of O2
Higher partial pressure gradient
In a diseased state is CO2 or O2 affected more? why?
O2 as diffuses slower
CO2 affected in late-stage disease
Diffusion barriers
Fluid film lining the alveoli Epithelial cells of the alveoli Interstitial fluid Endothelium of the capillary Plasm RBC membrane
How has the lungs adapted for diffusion
Large SA
Short diffusion pathways as thin
100% oxygen exchange occurs through 1/3rd of the capillary length - plenty of reserve
What causes diffusion impairment?
Fibrotic lung - thickened alveolar membrane
Pulmonary oedema - Fluid in interstitial space
Emphysema - decreases alveolus SA
