The seventh edition team of this medical textbook consists of several section authors who have continued from the previous editions, including Drs. Kalman Rubinson and Eric Lang for the nervous system, Dr. James Watras for muscle, Dr. Achilles Pappano for the cardiovascular system, Drs. Michelle Cloutier and Roger Thrall for the respiratory system, Drs. Kim Barrett and Helen Raybould for the gastrointestinal system, and Dr. Bruce White for the endocrine and reproductive systems. Additionally, we welcome two new authors, Dr. Withrow Gil Wier for the cardiovascular system and Dr. John Harrison for the endocrine and reproductive systems.

In this edition, as in the previous ones, we have aimed to emphasize broad concepts and minimize the collection of isolated facts. Each chapter has been written with the intention of providing a lucid, accurate, and up-to-date text. We have included both clinical and molecular information in each section, as feedback has indicated that this information offers clinical context and insights into physiological phenomena at the cellular and molecular levels. A new addition to this edition is a list of sources that readers can refer to for further information on the topics covered in each chapter, which we hope you find valuable.

The human body consists of billions of cells that are organized into tissues such as muscle, epithelia, and nervous tissue, as well as organ systems including the nervous, cardiovascular, respiratory, renal, gastrointestinal, endocrine, and reproductive systems. For these tissues and organ systems to function properly and support daily activities, certain general conditions must be met. The survival of cells within the body is of utmost importance, which requires adequate cellular energy supplies, maintenance of an appropriate intracellular environment, and defense against external threats. Once cell survival is ensured, cells can perform their designated functions, such as contraction by skeletal muscle cells. Ultimately, the coordination and regulation of cellular, tissue, and organ functions are essential. These fundamental aspects of physiology are presented throughout this book. The following is a brief introduction to these general concepts.

Cells rely on a constant supply of energy, which is derived from the hydrolysis of adenosine triphosphate (ATP). Without continuous replenishment, the cellular ATP supply would be depleted in most cells within a minute. Therefore, ATP synthesis must occur continuously. This process, in turn, necessitates a steady supply of cellular fuels. However, the levels of these fuels (such as glucose, fatty acids, and ketoacids) in the blood can only support cellular metabolism for a few minutes. To maintain adequate levels of cellular fuels in the blood, precursors (such as carbohydrates, proteins, and fats) need to be ingested. Furthermore, these fuels can be stored and mobilized when precursors cannot be ingested. Triglycerides (stored in adipose tissue), glycogen (stored in the liver and skeletal muscle), and protein serve as the storage forms of these fuels. The complex process of maintaining sufficient levels of cellular fuels in the blood involves various tissues, organs, and organ systems:

- The liver converts precursors into fuel storage forms (e.g., glucose → glycogen) during food ingestion and converts storage forms to cellular fuels during fasting (e.g., glycogen → glucose and amino acids → glucose).
- Skeletal muscle, similar to the liver, stores fuel (glycogen and protein) and converts glycogen and protein into fuels (e.g., glucose) or fuel intermediates (e.g., protein → amino acids) during fasting.
- The gastrointestinal tract digests and absorbs fuel precursors.
- Adipose tissue stores fuel during feeding (e.g., fatty acids → triglycerides) and releases the fuels during fasting.
- The cardiovascular system transports the fuels to cells and to and from their storage sites.
We are pleased that the following section authors have continued as members of the seventh edition team: Drs. Kalman Rubinson and Eric Lang (nervous system), Dr. James Watras (muscle), Dr. Achilles Pappano (cardiovascular system), Drs. Michelle Cloutier and Roger Thrall (respiratory system), Drs. Kim Barrett and Helen Raybould (gastrointestinal system), and Dr. Bruce White (endocrine and reproductive systems). We also welcome the following authors: Dr. Withrow Gil Wier (cardiovascular system), and Dr. John Harrison (endocrine and reproduction systems).
As in the previous editions of this textbook, we have attempted to emphasize broad concepts and to minimize the compilation of isolated facts. Each chapter has been written to make the text as lucid, accurate, and current as possible. We have included both clinical and molecular information in each section, as feedback on these features has indicated that this information serves to provide clinical context and new insights into physiologic phenomena at the cellular and molecular levels. New to this edition is a list of sources that the reader can consult for further information on the topics covered in each chapter. We hope that you find this a valuable addition to the book.
The human body consists of billions of cells that are organized into tissues (e.g., muscle, epithelia, and nervous tissue) and organ systems (e.g., nervous, cardiovascular, respiratory, renal, gastrointestinal, endocrine, and reproductive). For these tissues and organ systems to function properly and thus allow humans to live and carry out daily activities, several general conditions must be met. First and foremost, the cells within the body must survive. Survival requires adequate cellular energy supplies, maintenance of an appropriate intracellular milieu, and defense against a hostile external environment. Once cell survival is ensured, the cell can then perform its designated or specialized function (e.g., contraction by skeletal muscle cells). Ultimately, the function of cells, tissues, and organs must be coordinated and regulated. All of these functions are the essence of the discipline of physiology and are presented throughout this book. What follows is a brief introduction to these general concepts.
Cells need a constant supply of energy. This energy is derived from the hydrolysis of adenosine triphosphate (ATP). If not replenished, the cellular ATP supply would be depleted in most cells in less than 1 minute. Thus, ATP must be continuously synthesized. This in turn requires a steady supply of cellular fuels. However, the cellular fuels (e.g., glucose, fatty acids, and ketoacids) are present in the blood at levels that can support cellular metabolism only for a few minutes. The blood levels of these cellular fuels are maintained through the ingestion of precursors (i.e., carbohydrates, proteins, and fats). In addition, these fuels can be stored and then mobilized when ingestion of the precursors is not possible. The storage forms of these fuels are triglycerides (stored in adipose tissue), glycogen (stored in the liver and skeletal muscle), and protein. The maintenance of adequate levels of cellular fuels in the blood is a complex process involving the following tissues, organs, and organ systems:
Liver: Converts precursors into fuel storage forms (e.g., glucose → glycogen) when food is ingested, and converts storage forms to cellular fuels during fasting (e.g., glycogen → glucose and amino acids → glucose).
Skeletal muscle: Like the liver, stores fuel (glycogen and protein) and converts glycogen and protein to fuels (e.g., glucose) or fuel intermediates (e.g., protein → amino acids) during fasting.
Gastrointestinal tract: Digests and absorbs fuel precursors.
Adipose tissue: Stores fuel during feeding (e.g., fatty acids → triglycerides) and releases the fuels during fasting.
Cardiovascular system: Delivers the fuels to the cells and to and from their storage sites.