When you have completed this section, you should be able to
• list the levels of human structure from the most complex to the simplest;
• discuss the value of both reductionistic and holistic viewpoints to understanding human form and function; and
• discuss the clinical significance of anatomical variation among humans.
Earlier in this chapter, we observed that human anatomy is studied by a variety of techniques—dissection, palpation, and so forth. In addition, anatomy is studied at several levels of detail, from the whole body down to the molecular level.
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.
Anatomists, surgeons, and students must be constantly aware of how much one body can differ from another. A quick look around any classroom is enough to show that no two humans are exactly alike; on close inspection, even identical twins exhibit differences. Yet anatomy atlases and textbooks can easily give you the impression that everyone's internal anatomy is the same. This simply is not true. Books such as this one can only teach you the most common structure—the anatomy seen in about 70% or more of people. Someone who thinks that all human bodies are the same internally would make a very confused medical student or an incompetent surgeon.
Some people lack certain organs. For example, most of us have a palmaris longus muscle in the forearm and a plantaris muscle in the lower leg, but these are absent from some people. Most of us have five lumbar vertebrae (bones of the lower spine), but some people have six and some have four. Most of us have one spleen and two kidneys, but some have two spleens or only one kidney. Most kidneys are supplied by a single renal artery, but some have two renal arteries. Figure 1.10 shows some common variations in human anatomy, and insight 1.2 describes a particularly dramatic and clinically important variation.
Insight 1.2 Clinical Application
In most people, the spleen, pancreas, sigmoid colon, and most of the heart are on the left, while the appendix, gallbladder, and most of the liver are on the right. The normal arrangement of these and other internal organs is called situs (SITE-us) solitus. About 1 in 8,000 people, however, are born with an abnormality called situs inversus—the organs of the thoracic and abdominal cavities are reversed between right and left. A selective right-left reversal of the heart is called dextrocardia. In situs perversus, a single organ occupies an atypical position—for example, a kidney located low in the pelvic cavity instead of high in the abdominal cavity.
Conditions such as dextrocardia in the absence of complete situs inversus can cause serious medical problems. Complete situs inversus, however, usually causes no functional problems because all of the viscera, though reversed, maintain their normal relationships to each other. Situs inversus is often discovered in the fetus by sonography, but many people remain unaware of their condition for decades until it is discovered by medical imaging, on physical examination, or in surgery. You can easily imagine the importance of such conditions in diagnosing appendicitis, performing gallbladder surgery, interpreting an X ray, or auscultating the heart valves.
_Think About It_
People who are allergic to aspirin or penicillin often wear Medic Alert bracelets or necklaces that note this fact in case they need emergency medical treatment and are unable to communicate. Why would it be important for a person with situs inversus to have this noted on a Medic Alert bracelet?
Before You Go On
Answer the following questions to test your understanding of the preceding section:
10. In the hierarchy of human structure, what is the level between organ system and tissue? Between cell and molecule?
11. How are tissues relevant to the definition of an organ?
12. Why is reductionism a necessary but not sufficient point of view for fully understanding a patient's illness?
13. Why should medical students observe multiple cadavers and not be satisfied to dissect only one?
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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.