45 - Understanding ABGs part 2

KYJ 45 - Blood Gas series -part 2.

In part one of this series on understanding Blood gases, we reviewed the principle of partial pressure (Dalton's Law).

We also touched on Henry's law that (in part) helps us understand gas dissolving into plasma.
 This episode we look at oxygen and CO2 in blood.

Abbreviations
pp= partial pressure
A = Alveoli
a = Arterial
v = Venous (upper case V= ventilation)
p = pressure
O2 = oxygen
CO2 = carbon dioxide
mmHg = millimeters of Mercury
Diffusion = gas particles moving from one area to another eg lungs to blood or blood to cells.

Normal values
The partial pressure (pp) of oxygen in blood varies between arterial and venous blood. When freshly oxygenated in the lungs, arterial blood has a normal value of 80-100 mmHg.  In depleted venous blood, the oxygen pressure (tension) is approximately 35-45 mmHg.
Clearly you see that the arterial oxygen tension is more than twice that of venous blood.

The opposite effect happens with Carbon dioxide.  Normal tension in arterial blood is 35-45 mmHg, but in venous blood rises to  42-52mmHg. Slightly higher but not as dramatic as the difference in venous vs arterial Oxygen.

For the purpose of this series we will focus on Arterial blood as measured during Arterial Blood Gases.

First Oxygen:
When oxygen is inhaled, it is drawn in with a partial pressure of 159 mmHg (room air 21% of 760 mmHg)
After humidification and mixing with gases inside the lung's alveoli, the partial pressure (pp) has reduced to about 105-110 mmHg.  This pressure is abbreviated as pAO2 (uppercase "A" = Alveolar) .

Oxygen diffuses into the blood stream where it dissolves first into plasma exerting pressure in the blood.  Any given volume of blood only comes into close contact with alveoli for 0.7 of a second, so time to fully equilibrate gas pressure in both blood and lung is not possible.

Given that blood entering the lung capillaries has a pp oxygen in venous blood of ~40mmHg, the difference in pressure between blood and lung is steep (40 in venous blood vs 105 in alveoli).
Gas diffuses from high pressure to low pressure, so oxygen will migrate from lungs to blood in attempt to balance the pressure.
After a split second, the oxygen pressure in blood has risen from 40 ish to 90 ish, and makes its way back to the left heart to be pumped out to all those starving cells in the body.
At the exodus from the lungs arterial blood has a predictable oxygen tension (paO2) of 80-100mmHg.
Less than 80 is deemed Hypoxaemia. Less than 60 is called Respiratory Failure.
The difference between oxygen pp in alveoli and arterial blood is called the Aa Gradient, and it should be about 7-12 mmHg difference. It slightly increases with age, but a high Aa gradient is an indicator of lung deterioration.

CO2.
CO2 is the waste gas produced by cell metabolism and must be excreted. Most efficiently, carbon dioxide is exhaled.  Transported by venous blood, to the lungs, and blown off by breathing.  Some is also excreted by kidneys converted into bicarbonate, but let's not get ahead of ourselves.

Diffusion of CO2 from blood  into alveoli happens simultaneously with oxygen diffusion.

With almost no measurable CO2 in the air you breathe (shhh don't tell the Greenies) and high CO2 pressure in blood returning to the lungs (42-52), it isn't any surprise that CO2 will diffuse out of the blood and into the lungs to be breathed off.  Arterial blood therefore has slightly less CO2 at 35-45mmHg.  If CO2 accumulates in our blood, it causes the blood to become acidic (Acidosis). Breathing therefore allows us to maintain a beautiful balance.
We will leave this session here, and tomorrow look at acidosis in greater detail.
Memorise your paO2 80-100
And paCO2 35-45 normal values.