Bill is a marathoner and is half way through the new york marathon. his legs (and some other muscles) are burning huge amounts of atp (total metabolic rate has increased to around 7 times basal). his cardiac output has risen to five times the basal rate. his respiratory rate has increased from 10 breath/min at rest to about 20 breath/min. his tidal volume has risen from 700 ml at rest to about 1525 ml. he weighs about 70 kg (~150 lb). if no other control actions or reflexes occur, what will be the effect on po2 at the alveoli:

a. increased by 30 mmhg,
b. largely unchanged,
c. decreased by 30 mmhg