Our main objective in writing this textbook is to provide students with a sound basis for understanding human physiology. Two principles guided us throughout in attempting to meet this goal. The first is comprehensiveness: We have included all topics regarded by most physiologists as essential to a basic understanding of all functions of the human body. The second principle is simplicity: We have developed step-by-step, easy to comprehend explanations of each of these key concepts. All explanations, even those at the molecular level, are expressed as clearly and simply as possible. Two main themes have been used to organize the text. The first of these themes is homeostasis. Throughout, students are shown how dynamic counterbalancing forces interact to maintain the internal environment in an optimum condition to support life. The second theme, which flows directly from the first, is disorders, the results of homeostatic imbalances. Clinical conditions are presented and then contrasted with specific normal processes. A sufficient amount of anatomy is presented to enable students better to understand both normal physiology and disorders. We hope students will ultimately grasp that the human body functions by employing simple control mechanisms to keep trillions of cells in harmony with each other. Beginning students often feel overwhelmed by the volume of seemingly complicated information that is presented to them. After all, the human body is a very complex organism. However, the search for the answer to the question of how the body functions has often led to breakthroughs that replace complex and confusing ideas with simple, easy-to-understand models of operations, such as the double helix model of DNA or the amplification cascade mechanism for regulation of cellular activity. It is our greatest hope that some of our students may permanently be attracted into the marvelous laboratory of the human body.