Humans derive their food from plants, animals, and microorganisms, or a part of these. However, since we are multicellular organisms, our body cells are unable to utilize these organic masses as they are. They first need to be decomposed or digested in the alimentary canal into low-molecular-weight substances, which are then absorbed into the body and transported from head to foot through the circulatory system. Each peripheral tissue and cell absorb the substances and decompose them into substances with even lower molecular weight, which are concomitantly used for extraction of bioenergy, materials for biosynthesizing proteins, and other various types of metabolism.
The fact that humans eat cellular components of diverse organisms as food necessitates an accurate distinction between the tissues, cells, and cellular components of their own and those of food, otherwise we might end up digesting our own bodies. Moreover, the alimentary canal is vital since it comes in contact with external agents. The intestinal canal, in particular, possesses a developed biodefense system as well as a control system for this. The defense system must be active against extraneous organisms, while being tolerant toward its own cells simultaneously.
Aside from the alimentary tract, including the esophagus, stomach, duodenum, small intestine, and large intestine, the digestive organs include the mouth, which is responsible for the pretreatment of food called manducation; the pancreas and gallbladder, responsible for the production and secretion of digestive juices; the liver, responsible for the processing of absorbed substances; and the anus, responsible for the excretion of feces. Although the lumen of the alimentary canal is situated inside the body from the perspective of individuals, it can also be regarded as extracellular, namely outside the body, from the perspective of cells (Fig. 8-1). The so-called digestive enzymes (enzymes are macromolecular substances generally constituted by protein) are a group of hydrolases and have properties different from that of the degradative enzyme systems in cells.