Human Physiology: An Integrated Approach (6th Edition)
Welcome to the fascinating study of the human body!For most of recorded history, humans have been interested in how their bodies work. Early Egyptian, Indian, and Chinese writings describe attempts by physicians to treat various diseases and to restore health. Although some ancient remedies, such as camel dung and powdered sheep horn, may seem bizarre, we are still using others, such as blood-sucking leeches and chemicals derived from medicinal plants. The way we use these treatments has changed through the centuries as we learned more about the human body.
There has never been a more exciting time in human physiology. Physiology is the study of the normal functioning of a living organism and its component parts, including all its chemical and physical processes. The term physiology literally means “knowledge of nature.” Aristotle (384–322 b.c.e.) used the word in this broad sense to describe the functioning of all living organisms, not just of the human body. However, Hippocrates (ca. 460–377 b.c.e.), considered the father of medicine, used the word physiology to mean “the healing power of nature,” and thereafter the field became closely associated with medicine. By the sixteenth century in Europe, physiology had been formalized as the study of the vital functions of the human body. Today the term is again used to refer to the study of animals and plants.
Today we benefit from centuries of work by physiologists who constructed a foundation of knowledge about how the human body functions. Since the 1970s, rapid advances in the fields of cellular and molecular biology have supplemented this work. A few decades ago we thought that we would find the key to the secret of life by sequencing the human genome, which is the collective term for all the genetic information contained in the DNA of a species. However, this deconstructionist view of biology has proved to have its limitations, because living organisms are much more than the simple sum of their parts.
Physiology Is an Integrative Science
Many complex systems—including those of the human body— possess emergent properties, which are properties that cannot be predicted to exist based only on knowledge of the system’s individual components. An emergent property is not a property of any single component of the system, and it is greater than the simple sum of the system’s individual parts. Emergent properties result from complex, nonlinear interactions of the different components. For example, suppose someone broke down a car into its nuts and bolts and pieces and laid them out on a floor. Could you predict that, properly assembled, these bits of metal and plastic would become a vehicle capable of converting the energy in gasoline into movement? Who could predict that the right combination of elements into molecules and assemblages
of molecules would result in a living organism? Among the most complex emergent properties in humans are emotion, intelligence, and other aspects of brain function. None of these properties can be predicted from knowing the individual properties of nerve cells.
When the Human Genome Project (www.genome.gov) began in 1990, scientists thought that by identifying and sequencing all the genes in human DNA, they would understand how the body worked. However, as research advanced, scientists had to revise their original idea that a given segment of DNA contained one gene that coded for one protein. It became clear that one gene may code for many proteins. The Human Genome Project ended in 2003, but before then researchers had moved beyond genomics to proteomics, the study of proteins in living organisms. Now scientists have realized that knowing that a protein is made in a particular cell does not always tell us the significance of that protein to the cell, the tissue, or the functioning organism. The exciting new areas in biological research are called functional genomics, systems biology, and integrative biology, but fundamentally these are all fields of physiology. The integration of function across many levels of organization is a special focus of physiology. (To integrate means to bring varied elements together to create a unified whole.)
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