Question

The next three questions go together 21. Expired air is saturated with water vapor and generally contains more of it than does inspired air. There is usually a net loss of water from the body due to breathing. Water vapor pressure in the at saturation in the lungs (at 37°C) is 47 mmHg, while in expired air at 33°C it is 37.7 mmHg. Consider the case of someone who is breathing dry air in and moist air out at a rate of 15,000 L/day. This corresponds to a mean respiratory minute volume of 10.4 L/min, and to a fairly tvpical daily metabolic rate. The atmospheric pressure is 760 mmHg (i.e., standard pressure). Since 1 mol (18 g) of water vapor occupies 22.4 L at standard temperature and pressure, 1 L has a mass of 18/22.4-0.80 g. Ignoring air volume corrections, what is the daily rate of water loss (in g) if the air is inspired completely dry and expired saturated with water vapor at (a) 37°C and (b) 33°C? Note that solving this problem involves several steps. The professor and TA will help with this if you have difficulty with it. Recall from chapter 21 that you can determine the fractional concentration of a gas if you know its partial pressure and the total pressure (PFPotal). So you can determine the fractional concentration of water vapor in exhaled air at each temperature. Next, because you now the total volume of air exhaled in a day, you can determine the amount of water vapor exhaled per day. Finally, because you know the mass of water vapor per liter, you can determine the mass of exhaled water vapor. As always, be careful that the units work out properly.

Answer 1

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