Consider for the moment an analogy to human structure: The English language, like the human body, is very complex, yet an endless array of ideas can be conveyed with a limited number of words. All words in English are, in turn,
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Form and Function, Third Edition composed of various combinations of just 26 letters. Between an essay and an alphabet are successively simpler levels of organization: paragraphs, sentences, words, and syllables. We can say that language exhibits a hierarchy of complexity, with letters, syllables, words, and so forth being successive levels of the hierarchy. Humans have an analogous hierarchy of complexity, as follows (fig. 1.9):
The organism is composed of organ systems, organ systems are composed of organs, organs are composed of tissues, tissues are composed of cells, cells are composed (in part) of organelles, organelles are composed of molecules, and molecules are composed of atoms.
The organism is a single, complete individual. An organ system is a group of organs with a unique collective function, such as circulation, respiration, or digestion. The human body has 11 organ systems, illustrated in atlas A immediately following this chapter: the integumentary, skeletal, muscular, nervous, endocrine, circulatory, lymphatic, respiratory, urinary, digestive, and
Figure 1.9 The Body's Structural Hierarchy.
reproductive systems. Usually, the organs of one system are physically interconnected, such as the kidneys, ureters, urinary bladder, and urethra, which compose the urinary system. Beginning with chapter 6, this book is organized around the organ systems.
An organ is a structure composed of two or more tissue types that work together to carry out a particular function. Organs have definite anatomical boundaries and are visibly distinguishable from adjacent structures. Most organs and higher levels of structure are within the domain of gross anatomy. However, there are organs within organs— the large organs visible to the naked eye often contain smaller organs visible only with the microscope. The skin, for example, is the body's largest organ. Included within it are thousands of smaller organs: each hair, nail, gland, nerve, and blood vessel of the skin is an organ in itself.
A tissue is a mass of similar cells and cell products that forms a discrete region of an organ and performs a specific function. The body is composed of only four primary classes of tissue—epithelial, connective, nervous, and muscular tissues. Histology, the study of tissues, is the subject of chapter 5.
Cells are the smallest units of an organism that carry out all the basic functions of life; nothing simpler than a cell is considered alive. A cell is enclosed in a plasma membrane composed of lipids and proteins. Most cells have one nucleus, an organelle that contains its DNA. Cytology, the study of cells and organelles, is the subject of chapters 3 and 4.
Organelles12 are microscopic structures in a cell that carry out its individual functions. Examples include mitochondria, centrioles, and lysosomes.
Organelles and other cellular components are composed of molecules. The largest molecules, such as proteins, fats, and DNA, are called macromolecules. A molecule is a particle composed of at least two atoms, the smallest particles with unique chemical identities.
The theory that a large, complex system such as the human body can be understood by studying its simpler components is called reductionism. First espoused by Aristotle, this has proven to be a highly productive approach; indeed, it is essential to scientific thinking. Yet the reduc-tionistic view is not the last word in understanding human life. Just as it would be very difficult to predict the workings of an automobile transmission merely by looking at a pile of its disassembled gears and levers, one could never predict the human personality from a complete knowledge of the circuitry of the brain or the genetic sequence of DNA. Holism13 is the complementary theory that there are "emergent properties" of the whole organism that cannot be predicted from the properties of its separate parts—human beings are more than the sum of their parts. To be most effective, a health-care provider does not treat merely a disease
Figure 1.9 The Body's Structural Hierarchy.
12 elle = little
'3holo = whole, entire
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14 Part One Organization of the Body or an organ system, but a whole person. A patient's perceptions, emotional responses to life, and confidence in the nurse, therapist, or physician profoundly affect the outcome of treatment. In fact, these psychological factors often play a greater role in a patient's recovery than the physical treatments administered.
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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.