This includes the world around us from where we get our oxygen, so will also involve the atmosphere.
So lets break it down into the four A's:
To work out the pressure of oxygen in our atmosphere take the barometric pressure (760mmHg) at sea level, and multiply that by the concentration of oxygen in the atmosphere which is 21% (0.21).
As soon as this oxygen is inhaled it will lose pressure.
This is because there as water vapour which also exerts a pressure in the airways.
So now we have atmospheric pressure at sea level minus water vapour pressure multiplied by the percentage of oxygen.
The levels in the alveoli are further influenced by two other factors.
These are both related to the presence of CO2
This is leaving the body as the O2 is entering and the relationship between the two is known as the respiratory quotient.
The pressure of CO2 in the alveoli is approximately 40mmHg
So now to work out the pressure in the avleoli we need to take all the above into account.
We need to now subtract the partial pressure of CO2 (40mmHg) divided by the respiratory quotient.
Note the capital A here.
If there is no lung disease then the pressure of oxygen within the artery should also be 100mmHg.
Note the small 'a' here.
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