Review of Major Themes

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To close this chapter, let's distill a few major points from it. These themes can provide you with a sense of perspective that will make the rest of the book more meaningful and not just a collection of disconnected facts. These are some key unifying principles behind all study of human anatomy and physiology:

• Cell theory. All structure and function result from the activity of cells. Every physiological concept in this book ultimately must be understood from the standpoint of how cells function. Even anatomy is a result of cellular function. If cells are damaged or destroyed, we see the results in disease symptoms of the whole person.

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Physiology: The Unity of Anatomy and Physiology Companies, 2003 Form and Function, Third Edition

22 Part One Organization of the Body

• Homeostasis. The purpose of most normal physiology is to maintain stable conditions within the body. Human physiology is essentially a group of mechanisms that produce stable internal conditions favorable to cellular function. Any serious departure from these conditions can be harmful or fatal to cells.

• Evolution. The human body is a product of evolution. Like every other living species, we have been molded by millions of years of natural selection to function in a changing environment. Many aspects of human anatomy and physiology reflect our ancestors' adaptations to their environment. Human form and function cannot be fully understood except in light of our evolutionary history.

• Hierarchy of structure. Human structure can be viewed as a series of levels of complexity. Each level is composed of a smaller number of simpler subunits than the level above it. These subunits are arranged in different ways to form diverse structures of higher complexity. For example, all the body's organs are made of just four primary classes of tissue, and the thousands of proteins are made of various combinations of just 20 amino acids. Understanding these simpler components is the key to understanding higher levels of structure.

• Unity of form and function. Form and function complement each other; physiology cannot be divorced from anatomy. This unity holds true even down to the molecular level. Our very molecules, such as DNA and proteins, are structured in ways that enable them to carry out their functions. Slight changes in molecular structure can destroy their activity and threaten life.

_Think About It_

Architect Louis Henri Sullivan coined the phrase, "Form ever follows function." What do you think he meant by this? Discuss how this idea could be applied to the human body and cite a specific example of human anatomy to support it.

Insight 1.5 Clinical Application

Medical Imaging

The development of techniques for looking into the body without having to do exploratory surgery has greatly accelerated progress in medicine. A few of these techniques are described here.


X rays, a form of high-energy radiation, were discovered by William Roentgen in 1885. X rays can penetrate soft tissues of the body and darken photographic film on the other side. They are absorbed, however, by dense tissues such as bone, teeth, tumors, and tuberculosis nodules, which leave the film lighter in these areas (fig. 1.15a). The process of examining the body with X rays is called radiography. The term X ray also applies to a photograph (radiograph) made by this method. Radiography is commonly used in dentistry, mammography, diagnosis of fractures, and examination of the chest. Hollow organs can be visualized by filling them with a radiopaque substance that absorbs X rays. Barium sulfate is given orally for examination of the esophagus, stomach, and small intestine or by enema for examination of the large intestine. Other substances are given by injection for angiography, the examination of blood vessels (fig. 1.15b). Some disadvantages of radiography are that images of overlapping organs can be confusing and slight differences in tissue density are not easily detected. Nevertheless, radiography still accounts for over half of all clinical imaging. Until the 1960s, it was the only method widely available.


Sonography19 is the second oldest and second most widely used method of imaging. It is an outgrowth of sonar technology developed in World War II. A handheld device held firmly to the skin produces high-frequency ultrasound waves and receives the signals that echo back from internal organs. Although sonography was first used medically in the 1950s, images of significant clinical value had to wait until computer technology had developed enough to analyze differences in the way tissues reflect ultrasound. Sonography is not very useful for examining bones or lungs, but it is the method of choice in obstetrics, where the image (sonogram) can be used to locate the placenta and evaluate fetal age, position, and development. Sonography avoids the harmful effects of X rays, and the equipment is inexpensive and portable. Its primary disadvantage is that it does not produce a very sharp image (fig. 1.16).

Computed Tomography (CT)

The CT scan, formerly called a computerized axial tomographic20 (CAT) scan, is a more sophisticated application of X rays developed in 1972. The patient is moved through a ring-shaped machine that emits low-intensity X rays on one side and receives them with a detector on the opposite side. A computer analyzes signals from the detector and produces an image of a "slice" of the body about as thin as a coin (fig. 1.17). The computer can "stack" a series of these images to construct a three-dimensional image of the body. CT scanning has the advantage of imaging thin sections of the body, so there is little overlap of organs and the image is much sharper than a conventional X ray. CT scanning is useful for identifying tumors, aneurysms, cerebral hemorrhages, kidney stones, and other abnormalities. It has virtually eliminated exploratory surgery.

Positron Emission Tomography (PET)

The PET scan, developed in the 1970s, is used to assess the metabolic state of a tissue and to distinguish which tissues are most active at a given moment (fig. 1.18). The procedure begins with an injection of radioactively labeled glucose, which emits positrons (electron-like particles with a positive charge). When a positron and electron meet, they annihilate each other and give off a pair of gamma rays that can be detected by sensors and analyzed by computer. The computer displays a color image that shows which tissues were using the most glucose at the moment. In cardiology, PET scans can show the extent of damaged

19sono = sound + graphy = recording process

20tomo = section, cut, slice + graphic = pertaining to a recording

Saladin: Anatomy & 1. Major Themes of Text

Physiology: The Unity of Anatomy and Physiology Form and Function, Third Edition

Angiogram Cerebral Blood Vessels
Figure 1.15 Radiography. (a) An X ray (radiograph) of the head and neck. (b) A cerebral angiogram, made by injecting a substance opaque to X rays into the circulation and then taking an X ray of the head to visualize the blood vessels. The arteries are enhanced with false color in this photograph.

Chapter 1 Major Themes of Anatomy and Physiology 23

Week Old Fetus
Figure 1.16 Fetal Sonogram. Shows the head and right arm of a 28-week-old fetus sucking its thumb.

heart tissue. Since it consumes little or no glucose, the damaged tissue appears dark. The PET scan is an example of nuclear medicine—the use of radioactive isotopes to treat disease or to form diagnostic images of the body.

Magnetic Resonance Imaging (MRI)

MRI, once known as nuclear magnetic resonance (NMR) imaging, was developed in the 1970s as a technique superior to CT scanning for visualizing soft tissues. The patient lies within a cylindrical chamber surrounded by a large electromagnet that creates a magnetic field 3,000 to 60,000 times as strong as the earth's. Hydrogen atoms in the tissues align themselves with the magnetic field. The patient is then bombarded with radio waves, which cause the hydrogen atoms to absorb additional energy and align in a different direction. When the radio waves are turned off, the hydrogen atoms abruptly realign themselves to the magnetic field, giving off their excess energy at different rates that depend on the type of tissue. A computer analyzes the emitted energy to produce an image of the body. MRI can "see" clearly through the skull and spinal column to produce images of the nervous tissue. Moreover, it is better than CT for distinguishing between soft tissues such as the white and gray matter of the nervous system (fig. 1.19).

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Physiology: The Unity of Anatomy and Physiology Form and Function, Third Edition

24 Part One Organization of the Body

Themes For Companies

Figure 1.17 Computed Tomographic (CT) Scan of the Head at the Level of the Eyes. The eyes and skin are shown in blue, bone in red, and the brain in green.

Figure 1.17 Computed Tomographic (CT) Scan of the Head at the Level of the Eyes. The eyes and skin are shown in blue, bone in red, and the brain in green.

Pet Brain Glucose Metabolism
Figure 1.18 Positron Emission Tomographic (PET) Scan of the Brain of an Unmedicated Schizophrenic Patient. Red areas indicate high glucose consumption (high metabolism). In this patient, the visual center of the brain (rear of head, bottom of photo) was especially active when the scan was made.
Pet Scan Brain Visual And Spatial
Figure 1.19 Magnetic Resonance Image (MRI) of the Head at the Level of the Eyes. The optic nerves appear in red and the muscles that move the eyes appear in green.

Functional MRI (fMRI) is a variation on this technique that visualizes moment-to-moment changes in tissue function. fMRI scans of the brain, for example, show shifting patterns of activity as the brain applies itself to a specific task. fMRI has lately replaced the PET scan as the most important method for visualizing brain function. The use of fMRI in brain imaging is further discussed in chapter 14.

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Essentials of Human Physiology

Essentials of Human Physiology

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.

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