Metabolism, the total chemical activity of a living organism, consists of thousands of individual chemical reactions. Chemical reactions result in the capture of matter and energy and its conversion to different forms, as we will see in Part One of this book. For an organism to function, these reactions, many of which are occurring simultaneously, must be coordinated. Genes provide that control. The nature of the genetic material that controls these lifelong events has been understood only within the last 100 years. Much of Part Two is devoted to the story of its discovery.
The external environment can change rapidly and unpredictably in ways that are beyond an organism's control. An organism can remain healthy only if its internal environment remains within a given range of physical and chemical conditions. Organisms maintain relatively constant internal en vironments by making metabolic adjustments to conditions such as changes in temperature, the presence or absence of sunlight, or the presence of foreign agents inside their bodies. Maintenance of a relatively stable internal condition, such as a human's constant body temperature, is called homeostasis.
The adjustments that organisms make to maintain home-ostasis are usually not obvious, because nothing appears to change. However, at some time during their lives, many organisms respond to changing conditions not by maintaining their status, but by undergoing a major reorganization. An early form of such reorganization was the evolution of resting spores, a well protected, inactive form in which organisms survived stressful environments. A striking example that evolved much later is seen in many insects, such as butterflies. In response to internal chemical signals, a caterpillar changes into a pupa and then into an adult butterfly (see Figure 1.1).
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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.