Because it codes for specific proteins and contains the genetic instructions needed in order for an organism to survive, develop and reproduce
Age- younger people have smaller capacity
gender-males have larger capacity
muscle mass
aerobic fitness
diseases of the respitorary system
Well, the food is in the environment. I would like to compare it with how bacteriae feed (however my biology is quite rusty), that is, they engulf their food and they "digest" -- not the proper term, since there are no organs to do real digestion -- it.
Rainwater picks up carbon dioxide from the air and as it percolates through the soil, which turns into a weak acid. This slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves.
