Respiration

[mortenra]

The 40 ml CO2 released from each litre of blood passing the lungs may be useful to know about. It can be read from the textbook curves that gives the CO2 concentration in blood as a function of partial pressures. The difference between 40 and 46 mm Hg pCO2, the arterio-venous difference of pCO2, gives a difference in concentration of CO2 of approximately 40 ml O2 pr liter blood.

Another approach: Most physiology students will have read that arterial blood contains 200 ml O2 pr litre blood, and that we use about 25% of this (50 ml), so that mixed venous blood contains 150 ml O2 pr litre. This means that each litre of blood passing the lungs must take up 50 ml of O2. We also know that most humans produce 8 molecules of CO2 for every 10 molecules of O2 taken up (the respiratory exchange ratio of 0.8), and this means that we get rid of approximately 50 ml * 0.8 = 40 ml CO2 pr litre blood.

One can also breathe in a bag for a minute and see how much CO2 comes out of you. That is approximately 200 ml CO2 pr minute, or 40 ml pr litre blood if you have a cardiac output of 5 litre pr minute (which also means that 5 litre goes through the lungs pr minute).

This was three ways of finding out that we get rid of 40 ml CO2 for each litre of blood that flows through the lungs (typical at rest).


A last thing: I am not sure if there was a question about how percentage of gas corresponds to mm Hg, but if so the connection is as follows: 5.3% in alveolar air during expiration corresponds to 40 mm Hg because 5.3% of one atmosphere (760 mm Hg) is 40.28 mm Hg.