The chapter opens with a description of the small intestine, which is a large part of the digestive system. It is a place where the body absorbs nutrients from the outside, and it is also a place in which the body disposes of wastes, such as uric acid, that are produced by the body when it breaks down purines, a type of food. The chapter continues with a discussion of the role of the white blood cell in the body's defense against foreign invaders. White blood cells are responsible for killing foreign invaders, including bacteria and viruses, because they are able to recognize and destroy these invaders.