Info

Figure

A thermostat that can signal an air conditioner and a furnace to turn on or off maintains a relatively stable room temperature. This system is an example of a homeostatic mechanism. The icon indicates how the actual value (black bar) compares to the normal range (green zone).

Response

Body temperature increases

Hypothalamus detects change and causes

1. Increased sweating

2. Dilation of skin blood vessels

Sweating and increased blood flow cause heat loss

Response

Body temperature increases

Hypothalamus Body Temperature Set Point

Body temperature returns toward normal

Body temperature returns toward

Hypothalamic set point

Body temperature decreases

Sweating and increased blood flow cause heat loss

Hypothalamic set point

Body temperature decreases

Response

Hypothalamus detects change and causes

1. Decreased sweating

2. Constriction of skin blood vessels

3. Shivering

Figure 1.5

The homeostatic mechanism that regulates body temperature is an example of homeostasis.

Decreased sweating and skin blood flow help retain heat; shivering produces heat

Body temperature returns toward normal

Body temperature returns toward involuntarily, an action called shivering. Such muscular contractions produce heat, which helps warm the body.

If a person becomes overheated, the hypothalamus triggers a series of changes that promote loss of body heat. For example, sweat glands in the skin secrete watery perspiration. As the water evaporates from the surface, heat is carried away and the skin is cooled. At the same time, blood vessels in the skin dilate. This allows the blood that carries heat from deeper tissues to reach the surface where more heat is lost to the outside. Body temperature regulation is discussed in more detail in chapter 6 (p. 182).

Another homeostatic mechanism regulates the blood pressure in the blood vessels (arteries) leading away from the heart. In this instance, pressure-sensitive areas (sensory receptors) within the walls of these vessels sense changes in blood pressure and signal a pressure control center in the brain. If the blood pressure is above the pressure set point, the brain signals the heart, causing its chambers to contract less rapidly and with less force. Because of decreased heart action, less blood enters the blood vessels, and the pressure inside the vessels decreases. If the blood pressure is dropping below the set point, the brain center signals the heart to contract more rapidly and with greater force so that the pressure in the vessels increases. Chapter 15 (p. 611) discusses blood pressure regulation in more detail.

A homeostatic mechanism also regulates the concentration of the sugar glucose in blood. In this case, cells within an organ called the pancreas determine the set point. If, for example, the concentration of blood glucose increases following a meal, the pancreas detects this change and releases a chemical (insulin) into the blood. Insulin allows glucose to move from the blood into various body cells and to be stored in the liver and muscles. As this occurs, the concentration of blood glucose decreases, and as it reaches the normal set point, the pancreas decreases its release of insulin. If, on the other hand, the blood glucose concentration becomes abnormally low, the pancreas detects this change and releases a different chemical (glucagon) that causes stored glucose to be released into the blood. Chapter 13 (p. 530) discusses regulation of the blood glucose concentration in more detail (see fig. 13.34).

There are many other examples of homeostatic mechanisms. One is the increased respiratory activity that maintains blood levels of oxygen in the internal environment during strenuous exercise. Another is the nervous system creating the sensation of thirst, stimulating water intake when the internal environment has lost water. In each of these examples, homeostasis is the consequence of a self-regulating control system that operates by a mechanism called negative feedback (negcah-tiv fedcbak). Such a system receives signals (or feedback) about changes in the internal environment and then causes responses that reverse these changes (in the oppo site or negative direction) back toward the set point. Negative feedback mechanisms also control the rates of some chemical reactions and hormone secretion (chapter 13, p. 512).

Sometimes changes occur that stimulate still other similar changes. Such a process that causes movement away from the normal state is called a positive feedback mechanism.

Although most feedback mechanisms in the body are negative, a positive system operates for a short time when a blood clot forms, because the chemicals present in a clot promote still more clotting (see chapter 14, p. 564). Another illustration of positive feedback is milk production. If a baby suckles with greater force or duration, the mother's mammary glands respond by making more and more milk. These examples are unusual. Because positive feedback mechanisms usually produce unstable conditions, most examples are associated with diseases and may lead to death.

Homeostatic mechanisms maintain a relatively constant internal environment, yet physiological values may vary slightly in a person from time to time or from one person to the next. Therefore, both normal values for an individual and the idea of a normal range for the general population are clinically important. The normal range icons in figures 1.4 and 1.5 are intended to reinforce this concept. Numerous examples of homeostasis are presented throughout this book, and normal ranges for a number of physiological variables are listed in Appendix C, Laboratory Tests of Clinical Importance, page 1030.

H What requirements of organisms are provided from the external environment?

What is the relationship between oxygen use and heat production?

Why is homeostasis so important to survival? Describe three homeostatic mechanisms.

Levels of Organization

Early investigators, limited in their ability to observe small parts, focused their attention on larger body structures. Studies of small parts had to await invention of magnifying lenses and microscopes, which came into use about 400 years ago. These tools revealed that larger body structures were made up of smaller parts, which, in turn, were composed of even smaller ones.

Today, scientists recognize that all materials, including those that comprise the human body, are composed of chemicals. Chemicals consist of tiny, invisible

Tiny Organisms The Human Body

Organelle

Figure 1.6

A human body is composed of parts within parts, which increase in complexity from the level of the atom to the whole organism.

Organelle

Figure 1.6

A human body is composed of parts within parts, which increase in complexity from the level of the atom to the whole organism.

particles called atoms, which are commonly bound together to form larger particles called molecules; small molecules may combine to form larger molecules called macromolecules.

Within the human organism, the basic unit of structure and function is a cell. Although individual cells vary in size and shape, all share certain characteristics. Human cells contain structures called organelles (or<3gan-elzc) that carry on specific activities. These organelles are composed of aggregates of large molecules, including proteins, carbohydrates, lipids, and nucleic acids. All cells in a human contain a complete set of genetic instructions, yet use only a subset of them, allowing cells to develop specialized functions. All cells share the same characteristics of life and must meet requirements to continue living.

Cells are organized into layers or masses that have common functions. Such a group of cells forms a tissue. Groups of different tissues form organs—complex structures with specialized functions—and groups of organs that function closely together comprise organ systems. Interacting organ systems make up an organism.

A body part can be described at different levels. The heart, for example, contains muscle, fat, and nervous tis sue. These tissues, in turn, are constructed of cells, which contain organelles. All of the structures of life are, ultimately, composed of chemicals (fig. 1.6). Clinical Application 1.1 describes two technologies used to visualize differences among tissues.

Chapters 2-6 discuss these levels of organization in more detail. Chapter 2 describes the atomic and molecular levels; chapter 3 deals with organelles and cellular structures and functions; chapter 4 explores cellular metabolism; chapter 5 describes tissues; and chapter 6 presents membranes as examples of organs and the skin and its accessory organs as an example of an organ system. Beginning with chapter 7, the structures and functions of each of the organ systems are described in detail. Table 1.3 lists the levels of organization and some corresponding illustrations in this textbook.

H How does the human body illustrate levels of organization?

What is an organism?

How do body parts at different levels of organization vary in complexity?

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