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 typical 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 (Px = Fx × Ptotal). 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. Biology 482, fall 2017, Homework Problem Set 2 Name: 22. From these two answers, how much water is saved daily, when the inspired air is dry, by breathing out air at 33°C rather than at 37°C? 23. In fact, water vapor pressure of atmospheric air is not generally zero, as in the two previous calculations, and it varies considerably. For the next calculation, a representative value of 13 mmHg can be used, which corresponds to saturation near 15°C, or under-saturation at higher temperatures. What is the daily rate of water loss if the inspired air has a water vapor pressure of 13 mmHg and the expired air, saturated at 33°C, has a water vapor pressure of 37.7 mmHg? Because air is rarely completely dry, the answer to Question 21 is not completely realistic. Hint: If air has some water vapor in it, then the gradient of water vapor is lessened between exhaled air and ambient air (how it is lessened is really all that you have to figure out in order to answer this question; the rest of the calculation is the same as for Q21) and water loss will be reduced.

Answer 1

21.

Generally expired air is saturated with water and thus contain more water vapour than inspired air. Thus there is net loss of water vapour from body.

Now actual water loss depend upon metabolic rate and also on environmental conditions. Thus in case of small mammal their respiratory rate is high due to high metabolic rate.

On the other hand generally water vapour pressure of environment is not zero.thus it show under saturation at higher temperatures.

22.

Normally rate of expired air is expected around 400ml/day.

Beside this rate of vapour loss also depends upon metabolic rate. Thus greater loss represent high metabolic rate.