This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Acts Of Inspiration take place at a rate usually varying from fifteen to twenty per minute; but, like the pulsations of the heart, they are much more frequent in childhood, being above forty per minute in the infant. Only a small portion of the air in the lungs is changed in each ordinary respiration; the quantity so changed, or the breathing air, as it is termed, being on an average twenty or twenty-five cubic inches, while the vital capacity, or amount of air which can be expelled after a full inspiration, and which therefore includes a complemental supply not usually taken in, and a reserve quantity not usually parted with, is estimated on an average at 225 cubic inches. Even after the most forcible expiration, a large amount of residual air remains in the chest; and indeed it is impossible, even by direct pressure, completely to expel the air from the healthy lung of an animal which has breathed, so that it shall sink in water, like the lung of an animal born dead, into which the air has never entered.