Spirometer and Lung Capacity

After measuring our vital capacity through the use of a spirometer, our class got a variety of readings ranging from 2-5, or 2000-5000. My vital capacity reading was 3700 which represents 3.7 gallons of air that my lungs are capable of holding.
 I think the lung capacity of each person is often a good reflection of their physical health. People who exercise regularly will have a better lung capacity, especially if their sport of choice is swimming. Someone with a history of swimming and exercising their lung capacity would likely have a greater reading on the spirometer. There are other ways to expand your lung capacity such as through concentrated breathing exercises, which is all but synonymous to playing a wind instrument. I think that my years of playing clarinet were beneficial to me in this exercise. Another sizable factor is just that - size. The taller the person, the larger their lungs thus allowing more air capacity.
Avg. spirometer readings:
Girl at age 17: 2750
Boy at age 17: 4100
The average person's regular breath is 500 mls. However, we breathe deeper while exercising or experiencing increased heart rate. The opposite is true for times when our heart rate is slower, such as while sleeping.
Breathing out forcefully is different from regular exhalation because when we exhale naturally, that is our diaphragm relaxing and contracting. When exhaling forcefully muscles such as the abdomen contract creating pressure in the thoracic cavity which pushes air out.