CHAPTER 1 Functional Organization of the Human Body and Control of the “Internal Environment”

The goal of physiology is to understand the function of living organisms and their parts. In human physiology, we are concerned with the characteristics of the human body that allow us to sense our environment, move about, think and communicate, reproduce, and perform all of the functions that enable us to survive and thrive as living beings.

Human physiology is a broad subject that includes the functions of molecules and subcellular components; tissues; organs; organ systems, such as the cardiovascular system; and the interaction and communication among these components. A distinguishing feature of physiology is that it seeks to integrate the functions of all of the parts of the body to understand the function of the entire human body. Life in the human being relies on this total function, which is considerably more complex than the sum of the functions of the individual cells, tissues, and organs.

Cells Are the Living Units of the Body

Each organ is an aggregate of many cells held together by intercellular supporting structures. The entire body contains about 75 to 100 trillion cells, each of which is adapted to perform special functions. These individual cell functions are coordinated by multiple regulatory systems operating in cells, tissues, organs, and organ systems.

Although the many cells of the body differ from each other in their special functions, all of them have certain basic characteristics. For example, (1) oxygen combines with breakdown products of fat, carbohydrates, or protein to release energy that is required for normal function of the cells; (2) most cells have the ability to reproduce, and whenever cells are destroyed the remaining cells often regenerate new cells until the appropriate number is restored; and (3) cells are bathed in extracellular fluid, the constituents of which are precisely controlled.

“Homeostatic” Mechanisms of the Major Functional Systems (p. 4)

Essentially all of the organs and tissues of the body perform functions that help maintain the constituents of the extracellular fluid relatively constant, a condition called homeostasis. Much of our discussion of physiology focuses on the mechanisms by which the cells, tissues, and organs contribute to homeostasis.

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Extracellular Fluid Transport and Mixing System—The Blood Circulatory System

Extracellular fluid is transported throughout the body in two stages. The first stage is movement of blood around the circulatory system, and the second stage is movement of fluid between the blood capillaries and cells. The circulatory system keeps the fluids of the internal environment continuously mixed by pumping blood through the vascular system. As blood passes through the capillaries, a large portion of its fluid diffuses back and forth into the interstitial fluid that lies between the cells, allowing continuous exchange of substances between the cells and the interstitial fluid and between the interstitial fluid and the blood.

Origin of Nutrients in the Extracellular Fluid

• The respiratory system provides oxygen for the body and removes carbon dioxide.

• The gastrointestinal system digests food and absorbs various nutrients, including carbohydrates, fatty acids, and amino acids, into the extracellular fluid.

• The liver changes the chemical composition of many of the absorbed substances to more usable forms, and other tissues of the body (e.g., fat cells, kidneys, endocrine glands) help modify the absorbed substances or store them until they are needed.

• The musculoskeletal system consists of skeletal muscles, bones, tendons, joints, cartilage, and ligaments. Without this system, the body could not move to the appropriate place to obtain the foods required for nutrition. This system also provides protection of internal organs and support of the body.

Removal of Metabolic End Products (p. 5)

• The respiratory system not only provides oxygen to the extracellular fluid but also removes carbon dioxide, which is produced by the cells, released from the blood into the alveoli, and then released to the external environment.

• The kidneys excrete most of the waste products other than carbon dioxide. The kidneys play a major role in regulating the extracellular fluid composition by controlling the excretion of salts, water, and waste products of the chemical reactions of the cells. By controlling body fluid volumes and compositions, the kidneys also regulate blood volume and blood pressure.

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• The liver eliminates certain waste products produced in the body as well as toxic substances that are ingested.

Regulation of Body Functions

• The nervous system directs the activity of the muscular system, thereby providing locomotion. It also controls the function of many internal organs through the autonomic nervous system, and it allows us to sense our external and internal environment and to be intelligent beings so we can obtain the most advantageous conditions for survival.

• The hormone systems control many of the metabolic functions of the cells, such as growth, rate of metabolism, and special activities associated with reproduction. Hormones are secreted into the bloodstream and are carried to tissues throughout the body to help regulate cell function.

Protection of the Body

• The immune system provides the body with a defense mechanism that protects against foreign invaders, such as bacteria and viruses, to which the body is exposed daily.

• The integumentary system, which is composed mainly of skin, provides protection against injury and defense against foreign invaders as well as protection of underlying tissues against dehydration. The skin also serves to regulate body temperature.

Reproduction

The reproductive system provides for formation of new beings like ourselves. Even this can be considered a homeostatic function because it generates new bodies in which trillions of additional cells can exist in a well-regulated internal environment.

Control Systems of the Body (p. 6)

The human body has thousands of control systems that are essential for homeostasis. For example, genetic systems operate in all cells to control intracellular as well as extracellular functions. Other control systems operate within the organs or throughout the entire body to control interactions among the organs.

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Regulation of oxygen and carbon dioxide concentrations in the extracellular fluid is a good example of multiple control systems that operate together. In this instance, the respiratory system operates in association with the nervous system. When the carbon dioxide concentration in the blood increases above normal, the respiratory center is excited, causing the person to breathe rapidly and deeply. This increases the expiration of carbon dioxide and therefore removes it from the blood and the extracellular fluid until the concentration returns to normal.

Normal Ranges of Important Extracellular Fluid Constituents

Table 1–1 shows some of the important constituents of extracellular fluid along with their normal values, normal ranges, and maximum limits that can be endured for short periods of time without the occurrence of death. Note the narrowness of the ranges; levels outside these ranges are usually the cause or the result of illnesses.

Table 1–1 Some Important Constituents and Physical Characteristics of the Extracellular Fluid, Normal Range of Control, and Approximate Nonlethal Limits for Short Periods

Characteristics of Control Systems

Most Control Systems of the Body Operate by Negative Feedback

For regulation of carbon dioxide concentration as discussed, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation, which decreases the carbon dioxide concentration toward normal levels. This is an example of negative feedback; any stimulus that attempts to change the carbon dioxide concentration is counteracted by a response that is negative to the initiating stimulus.

The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of the negative feedback. The gain is calculated according to the following formula.

Some control systems, such as those that regulate body temperature, have feedback gains as high as –33, which simply means that the degree of correction is 33 times greater than the remaining error.

Feed-Forward Control Systems Anticipate Changes

Because of the many interconnections between control systems, the total control of a particular body function may be more complex than can be accounted for by simple negative feedback. For example, some movements of the body occur so rapidly that there is not sufficient time for nerve signals to travel from some of the peripheral body parts to the brain and then back to the periphery in time to control the movements. Therefore, the brain uses feed-forward control to cause the required muscle contractions. Sensory nerve signals from the moving parts apprise the brain in retrospect of whether the appropriate movement, as envisaged by the brain, has been performed correctly. If it has not, the brain corrects the feed-forward signals it sends to the muscles the next time the movement is required. This is also called adaptive control, which is, in a sense, delayed negative feedback.

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Positive Feedback Can Sometimes Cause Vicious Cycles and Death, and Other Times Can Be Useful

A system that exhibits positive feedback responds to a perturbation with changes that amplify the perturbation and therefore leads to instability rather than stability. For example, severe hemorrhage may lower blood pressure to such a low level that blood flow to the heart is insufficient to maintain normal cardiac pumping; as a result, blood pressure falls even lower, further diminishing blood flow to the heart and causing still more weakness of the heart. Each cycle of this feedback leads to more of the same, which is a positive feedback or a vicious cycle.

In some cases the body uses positive feedback to its advantage. An example is the generation of nerve signals. When the nerve fiber membrane is stimulated the slight leakage of sodium ions into the cell causes opening of more channels, more sodium entry, more change in membrane potential, and so forth. Therefore, a slight leak of sodium into the cell becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential.

Summary—Automaticity of the Body (p. 9)

The body is a social order of about 75 to 100 trillion cells organized into various functional structures, the largest of which are called organs. Each functional structure, or organ, has a role in maintaining a constant internal environment. So long as homeostasis is maintained, the cells of the body continue to live and function properly. Thus, each cell benefits from homeostasis and, in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this loss happens, all cells of the body suffer. Extreme dysfunction leads to death, whereas moderate dysfunction leads to sickness.

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