The training of a general practitioner requires scientific knowledge of the metabolic behavior of tissues and systems in conditions of health in order to establish a correct diagnosis and appropriate medical guidance in the course of diseases. Therefore, a central purpose of this book is to present the characteristics of the digestive system, liver, gallbladder, and bile duct metabolism, as well as adaptation mechanisms between these and other tissues in physiological health conditions. In the course of diseases, there are new metabolic adaptations that are, in general, the cause of clinical complications, and since it is not possible to cover them all, the most frequent ones have been selected at a global level. Hopefully, this book contributes to a systematic and integrated knowledge of human metabolism that every physician must possess, which will help to achieve the best diagnosis and management of their patients.