TBR Biology Ch 2, Passage 15, Question 98

[drechie]

98. Which of the statements below is FALSE regarding pC02 during exercise?

1. A) The alveolar pC02 determines arterial pC02-
   
   
2. B) Alveolar ventilation increases.
   
   
3. C) Venous pC02 increases.
   
   
4. D) Arterial pC02 increases.
   

I chose A. I thought that the Arteries directly release CO2 to the aveoli of the lung, while the aveoli of the lung exchange O2 to vein. Thus, I thought A was a definite false given the relationship. Can someone explain to me why A is "true." And also explain why D is the "false" answer



D is correct, arterial pC02 increases. During exercise, it is clear that the cells will be using more oxygen and thus producing more carbon dioxide. For that reason, the levels of venous pC02 will increase. Willthisleadtoarisein the arterial partial pressure of carbon dioxide? The answer is no, because the rate of ventilation will increase. We know that we breathe "harder" when we are exercising. In other words, the excess carbon dioxide :is released as a resultofthehigherrateofventilation. Theincreaseinthecarbondioxideproductionisequivalenttotheincreasein breathing rate. The result of this is that the arterial partial pressure of carbon dioxide does not change, making D a false statement. The correct choice is D.

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[EParker37]

For A, you are thinking of pulmonary arteries and veins, the question is referring to systemic arteries, in which case, the alveolar pco2 is going to be the same as the arterial pco2, because the co2 that is pulled out of circulation and exhaled is going to determine the arterial pco2. When we draw a blood gas, we take it from a systemic artery, usually the radial artery in the wrist. D is most definitely the correct answer, for the reasons stated by the book. With increased ventilation, we get increased gas exchange, and the increased venous pco2 is compensated for by this increase in breathing.