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MCAT Question A Day - 8/16/14 - Answer!

Carbon dioxide partial pressure:

A.  increases in the blood as it travels from the systemic venules to the inferior vena cava.
B.  increases in the blood as it travels from the pulmonary arteries to the pulmonary veins.
C.  is greater in the blood in the systemic capillary beds than in the alveoli of the lungs.
D.  is greater in the blood in the systemic capillary beds than in the systemic tissues.

C is correct. Carbon dioxide is produced in the tissues. It is transported by the blood to the lungs, where it iexpelled by diffusing into the alveoli. Since the concentration gradient carries CO2 into the capillaries from the tissues and from the blood into the alveoli, we can reason that there is a higher concentration of CO, in the tissues than in the alveoli. A is wrong because blood COconcentration will not change in the veins; it has nowhere to go. B is wrong because CO2 is expelled into the lungs at the pulmonary capillaries, so CO2 that was present in the pulmonary arteries (before the capillaries) will largely be gone in the pulmonary veins (after the capillaries). D describes the opposite of the concentration gradient that actually exists for CO, between the systemic tissues and the systemic capillaries.

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