Properties of gases and diffusion Flashcards
Describe the kinetic theory of gases
Goes are a collection of molecules moving randomly around a space.
Pressure is generated by collisions of molecules with a surface.
The more frequency and harder the collisions, the higher the pressure generated by the gas.
What is Boyle’s law?
‘Pressure of a gas is inversely proportional to its volume’
Inspiration and expiration are examples of Boyle’s Law
Whta is atmospheric pressure?
Pressure exerted by the weight of the air above the earth in the atmosphere.
At sea level = 101kPa = 1atm = 760mmHg
At high altitudes, atmospheric pressure is lower.
What happens when inspired gases come into contact with body fluids?
Gas molecules will enter water to dissolve liquid
Water molecules evaporate to enter air.
What is SVP?
This is the saturated vapour pressure.
This is because water molecules entering the air exert a ‘vapour pressure.’ At equilibrium, the rate of water evaporation = rate of water condensation so, altho the air is saturated with vapour, this value remains constant.
Value is: 6.28kPa
How does the water vapour affect the partial pressure of the other gases?
You still have the same ratio of gases, but you have to do 101 (air pressure) - 6.28 (SVP) and then work out pressures from the remaining value.
At what point is equilibrium reached between air and water?
Partial pressure of gas in liquid = partial pressure of gas in air above it
How many moles of oxygen will dissolve in plasma per litre?
Solubility coefficient of O2 in plasma = 0.01 mmol
When exposed to pO2 of 13.3kPa in alveolar air
0.01x13.3 = 0.13 mmol of oxygen will dissolve
Plasma has 0.13 mmol of dissolved oxygen per litre.
What happens to partial pressure of oxygen when Hb in blood?
Oxygen in the cells doesn’t contribute to partial pressure - reaction must be complete (all Hb must have four oxygen molecules bound to it) before equilibrium can be established.
Equilibrium when ppOxygen of plasma = ppOxygen of alveolar air
Define partial pressure
Pressure exerted by the dissolved gas in a liquid
Define concentration of dissolved gas
number of mmol of gas dissolved in a litre of liquid
What is the solubility coefficient?
A constant for each gas (solute) and solvent
What makes up total content of gas
dissolved gas + gas bound to or reacted with a component. (e.g. oxygen dissolved in plasma + oxygen bound to Hb)
Why is pO2 lower and pCO2 higher in alveoli than in inhaled air?
Inhaled air is mixed with residual volumes
Effect of O2 diffusion acts the alveolar wall
Effect of CO2 entering the alveoli
Alveolar air composition stays constant at 13.3kPa of oxygen and 5.3kPa of carbon dioxide - blood equilibrates to this level.
What is pO2 and pCO2 in mixed venous blood?
pO2 = 6.0 KPa
pCO2 = 6.1kPa
But, this varies with metabolism
What is pO2 and pCO2 in alveolar gas?
pO2 - 13.3kPa
pCO2 = 5.3kPa
What factors affect the rate of diffusion?
partial pressure difference across membrane
SA available
Thickness (distance traveled)
Diffusion coefficient - (solubility / √Molecular weight)
What is the difference between the diffusion of CO2 and O2?
CO2 is MUCH more soluble than O2 so, despite O2 having a lower molecular weight, the CO2 diffuses 20 times faster than O2.
The large difference in partial pressure compensates for slower diffusion of oxygen.
This difference means that, in a diseased lung, oxygen gas exchange is thus more impaired than CO2.
What must gases cross to diffuse from alveolar air to RBC in capillary?
Fluid film lining alveolus Epithelial cell of alveolus Interstitial space Endothelial cell of capillary Plasma Red cell membrane
5 Layers
2 Layers os cytoplasm
2 Layers of tissue fluid + plasma
In what diseases is thickness of membrane increased?
Oedema - fluid in interstitial space and in alveoli
Lung fibrosis - increased thickness of alveolar capillary membrane
In what diseases is surface area of membrane decreased?
Remove a lung
Emphysema
How do diseases affect the diffusion coefficient of gas change?
carbon dioxide always diffuses much faster than oxygen so the diffusion fo oxygen is affected(low pO2) and carbon dioxide not (normal pCO2).
What is the difference between partial pressure and amount of a dissolved gas?
Amount dissolved = partial pressure x solubility coefficient.
