This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
Blood is renewed, so far as ordinary food materials are concerned, by substances either directly absorbed by the blood-vessels of the alimentary canal, or taken up by the lymphatics of the digestive tract and afterwards poured into the blood. But in order that energy may be set free for use by the tissues of the body (Chap. IX.), oxidations must occur, and the continuance of these vital oxidations depends on a constant supply of oxygen. As their result, waste substances are produced, which are no longer of use to the body, but detrimental to it if present in large quantity. The most abundant of these wastes is carbon dioxide gas.
The function of respiration has for its objects (1) to renew the supply of oxygen in the blood, and (2) to get rid of the carbon dioxide produced in the different organs.