The digested food, after absorption, all passes into the blood, either directly or indirectly, and mixes with it. The materials which are to serve the purposes of nutrition are now taken up by a stream of liquid that is in constant motion throughout the minutest divisions of every part of the animal. Flowing in regular channels, the blood reaches not only the bones and muscular tissues, but it passes through several special organs and glands, where the nutrients it is carrying and certain of its own constituents meet with profound changes. It is here that we discover the manner in which food is applied to use, and what are some of the transformations which the proteins, carbohydrates, and fats undergo in performing their functions.

In order to follow intelligently this most interesting phase of nutrition, we must know something of the blood and of the organs - the lungs, liver, and kidneys - through which it passes.