iv Textbook The second edition of Anatomy & Physiology Essentials has been revised and updated to reflect the latest progress in research and development related to the human body in health and wellness, as well as in various disease states. Many of the Research Notes features have been revised or replaced to reflect the newest available information. In addition, information about COVID-19 and other emerging diseases has been added as appropriate throughout the textbook. Another noticeable change is the addition of development information that traces each body system throughout the human life span. A new section in each body system chapter describes the development of cells, tissues, organs, and organ systems from conception through older adulthood. In addition to the new development sections and Research Notes features added to every chapter, content in individual chapters has been updated as follows: New to This Edition • Chapter 1 Foundations of Human Anatomy and Physiology has been completely revised to present the physical processes that sustain life more clearly, including metabolic processes and homeostasis. Information about the metric system has been moved to Appendix A. • Chapter 2 Molecules and Cells includes a new Clinical Case Study highlighting issues regarding diagnosis and treatment of COVID-19. • Chapter 9 The Endocrine System has been partially reorganized and tweaked to present information about this complex system in a clearer fashion. • Chapter 10 The Respiratory System has been updated to include information about COVID-19 and ongoing research efforts to conquer this pandemic disease. • Chapter 11 The Blood has been revised to include a clear explanation of the origin and development of blood cells and the process of hematopoiesis. • Chapter 13 The Lymphatic and Immune Systems now includes a Research Notes feature thoroughly discussing the virus that causes COVID-19 and the approaches being used to find suitable vaccines. Goodheart-Willcox and Body Scientific Augmented Reality See the cover in motion with Augmented Reality. With the help of Body Scientific, we have created a new way to get excited about anatomy and learning with the use of AR. To see our book cover in motion: 1. Open the camera app on your smartphone or tablet. 2. Hold your camera over the QR code and allow the new page to open. 3. Select iPhone or Android, depending on the device you are using. 4. Download the Body Scientific BSI AR DISPLAY app. 5. Open the BSI app and slowly move your device over G-W’s textbook cover.