Respiratory System – I Partial pressures of gases 1. All of the air on earth is ~21% oxygen, ~79% nitrogen, and less than 1% carbon dioxide. In any given place on earth, we can figure out the partial pressure of a gas by multiplying its percentage by the total atmospheric pressure at that elevation. Try this on your whiteboard. ATM pressure 760 (sea level, ~ Sacramento) 649 (Lake Tahoe) 295 (Mt. Kilimanjaro) PO2 PCO2 PN2 BUT, that is just the amount in the surrounding air. We are usually more concerned with how much is in the alveoli (air sacs). As the air enters your lungs, it takes on some moisture (water) from your airways, and it also mixes with “old” air within the alveoli….this will drop the PO2 and increase the PCO2. (Note: we won’t discuss PN2 any more, as it’s normally inert). 2. We know that at sea level (normal conditions), the PO2 in the alveoli will be a constant 100 mm Hg; PCO2 will be 40 mm Hg. This assumes normal breathing, with higher levels of oxygen coming in with each breath, and oxygen constantly leaking into the capillaries. Draw an alveolus on your whiteboard, showing the normal values for oxygen and carbon dioxide. Then, draw a capillary near your alveolus, and indicate the values of PO2 and PCO2 in the pulmonary capillary. Which values below should be at the beginning of the capillary, and which should be at the end? Place them on your drawing, and indicate how gases are moving. PO2 = 100 mm Hg PO2 = 40 mm Hg PCO2 = 40 mm Hg PO2 = 46 mm Hg 3. What causes the gases to move in either direction? 4. What could you do (medically) to increase the movement of oxygen into blood? 5. Go back to your first table. 649 (Lake Tahoe) is 85% of 760. So, what is the PO2 in the alveoli at Lake Tahoe? What kind of an effect might this have on someone?