Stated most simply and broadly, physiology is the study of how living organisms work. As applied to human beings, its scope is extremely broad. At one end of the spectrum, it includes the study of individual molecules—for example, how a particular protein's shape and electrical properties allow it to function as a channel for sodium ions to move into or out of a cell. At the other end, it is concerned with complex processes that depend on the interplay of many widely separated organs in the body—for example, how the brain, heart, and several glands all work together to cause the excretion of more sodium in the urine when a person has eaten salty food.
What makes physiologists unique among biologists is that they are always interested in function and integration—how things work together at various levels of organization and, most importantly, in the entire organism. Thus, even when physiologists study parts of organisms, all the way down to individual molecules, the intention is always ultimately to have whatever information is gained applied to the function of the whole body. As the nineteenth-century physiologist Claude Bernard put it: "After carrying out an analysis of phenomena, we must . . . always reconstruct our physiological synthesis, so as to see the joint action of all the parts we have isolated ... ."
In this regard, a very important point must be made about the present status and future of physiology. It is easy for a student to gain the impression from a textbook that almost everything is known about the subject, but nothing could be farther from the truth for physiology. Many areas of function are still only poorly understood (for example, how the workings of the brain produce the phenomena we associate with the word "mind").
Indeed, we can predict with certainty a coming explosion of new physiological information and understanding. One of the major reasons is as follows. As you will learn in Chapters 4 and 5, proteins are molecules that are associated with practically every function performed in the body, and the directions for the synthesis of each type of protein are coded into a unique gene. Presently, only a fraction of all the body's proteins has been identified, and the roles of these known proteins in normal body function and disease often remain incompletely understood. But recently, with the revolution in molecular biology, it has become possible to add or eliminate a particular gene from a chapter to provide such an orientation to the subject of human physiology.
living organism (Chapter 5) in order to better study the physiological significance of the protein for which that gene codes. Moreover, the gaining of new physiological information of this type will expand enormously as the Human Genome Project (Chapter 5) continues its task of identifying all of the estimated 50,000 to 100,000 genes in the body, most of these genes coding for proteins whose functions are unknown.
Finally, a word should be said about the interaction of physiology and medicine. Disease states can be viewed as physiology "gone wrong," or pathophysiology, and for this reason an understanding of physiology is absolutely essential for the study and practice of medicine. Indeed, many physiologists are themselves actively engaged in research on the physiological bases of a wide range of diseases. In this text, we will give many examples of pathophysiology, always to illustrate the basic physiology that underlies the disease.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.