Book excerpt: This second volume of Defining Physiology: Principles, Themes, Concepts. continues on the same format as the first. In this new release, a selection of 44 essential topics in each major organ system is defined, then major themes, concept and principles surrounding these words in their physiologic scenarios are elaborated. For each keyword, a question is posed at the end of the text to test for a better understanding of the associated physiology of nervous and gastrointestinal systems. This book presents an easy reference guide for those just starting out in the area of physiology and for those who are interested in clear and succinct definitions of key terms.