Gas Transport

You saw back in the blood chapter that gas gets transported. In these few pages we will go into a bit more detail about how that works. Once you know the about the form in which gas is transported, it is easier to understand how gas is exchanged in the lungs.

I have put transport of oxygen and carbon monoxide together on one page, since they are carried similarly. Carbon dioxide has a few ways in which it is carried, so you can read about that separately on the carbon dioxide page.

One new concept in these pages is the idea of gas pressure for individual gases. You see, atmospheric pressure remains 760 mm Hg. But, only some of that 760 mm Hg is due to oxygen, some is due to carbon dioxide, some is due to nitrogen, etc. Each of the individual gases applies some of the total pressure. Each of these individual gas pressures is referred to as the partial pressure for that gas. The partial pressure of oxygen gas is symbolized by P_O₂ and the partial pressure of carbon dioxide is symbolized by P_CO₂. Air is 21% oxygen, only around 0.04% carbon dioxide, and mostly nitrogen (78%). So, oxygen only applies 21% of the total pressure of 760 mm Hg-- so the P_O₂ is 160 mm Hg. And the P_CO₂ is 0.3 mm Hg.

Why do you need to understand partial pressures? Because if carbon dioxide needs to get transported, it will do so in a manner to accommodate the pressure it exerts on the blood. The more carbon dioxide that is trying to enter blood, the more that the blood will have to deal with it and pick it up for transport. You will also use partial pressures to understand alveolar gas exchange.