CHAPTER 1 Functional Organization of the Human Body and Control of the “Internal Environment” Physiology is the science that seeks 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 links the basic sciences with clin- ical medicine and integrates multiple functions of mol- ecules and subcellular components, the cells, tissues, and organs into the functions of the living human being. This integration requires communication and coordina- tion by a vast array of control systems that operate at every level, from the genes that program synthesis of molecules to the complex nervous and hormonal sys- tems that coordinate functions of cells, tissues, and organs throughout the 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 intercellu- lar supporting structures. The entire body contains 35 to 40 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 pro- tein to release energy that is required for 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. Microorganisms Living in the Body Outnumber Human Cells. Communities of microorganisms, often called microbiota, live on the skin and in the mouth, gut, and nose. The gastrointestinal tract, for example, normally contains a complex and dynamic population of 400 to 3 4 UNIT I Introduction to Physiology: The Cell and General Physiology 1000 species of microorganisms that outnumber our human cells. These microorganisms can cause diseases, but they usually live in harmony with their human hosts and provide vital functions, including immunity and digestion of foodstuffs, that are essential for our survival. MECHANISMS OF HOMEOSTASIS— MAINTENANCE OF NEARLY CONSTANT INTERNAL ENVIRONMENT (P. 4) Essentially all the organs and tissues of the body per- form functions that help maintain the constituents of the extracellular fluid so they are relatively stable, a con- dition called homeostasis. Much of our discussion of physiology focuses on mechanisms by which the cells, tissues, and organs contribute to homeostasis. Extracellular Fluid Transport and Mixing System: The Blood Circulatory System About 50% to 70% of the adult human body is fluid, with approximately two- thirds inside the cells and one- third in the extracellular fluid surrounding the cells and cir- culating in the blood. Extracellular fluid is transported throughout the body in two stages. The first stage is movement of blood throughout 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 inter- stitial 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 facili- tates absorption of various nutrients, including car- bohydrates, 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, Functional Organization of the Human Body and Control of the 5 “Internal Environment” and other tissues of the body (e.g., fat cells, kidneys, endocrine glands) help modify the absorbed sub- stances or store them until they are needed. • The musculoskeletal system consists of skeletal mus- cles, 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 protects internal organs and supports the body. Removal of Metabolic End Products (p. 6) • The respiratory system not only provides oxygen to the extracellular fluid but also removes carbon diox- ide, which is produced by the cells, released from the blood into the alveoli, and then released to the exter- nal environment. • The kidneys excrete most of the waste products other than carbon dioxide. The kidneys play a major role in regulating extracellular fluid composition by controlling 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. • The liver eliminates certain waste products pro- duced in the body, as well as toxic substances that are ingested. • The gastrointestinal tract eliminates undigested materials and some waste products of metabolism in the feces. 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 inter- nal environment and to be intelligent beings so we can obtain the most advantageous conditions for survival. • The hormone systems control many metabolic func- tions 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 regu- late cell function. 6 UNIT I Introduction to Physiology: The Cell and General Physiology 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 dehy- dration. The skin also serves to regulate body temperature. Reproduction The reproductive system provides for formation of new beings like ourselves. Even this function can be consid- ered 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. 7) The human body has thousands of control systems that are essential for homeostasis. For example, genetic systems operate in all cells to control intra- cellular and extracellular functions. Other control systems operate within the organs or throughout the entire body to control interactions among the organs. Regulation of oxygen and carbon dioxide concen- trations 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 carbon dioxide concen- tration in the blood increases above normal, the respi- ratory center is excited, causing the person to breathe rapidly and deeply. This breathing increases the expira- tion 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 important constituents of extracellular fluid along with their normal values, normal ranges, and maximum limits that can be Functional Organization of the Human Body and Control of the 7 “Internal Environment” Table 1-1 Some Important Constituents and Physical Characteristics of the Extracellular Fluid, Normal Range of Control, and Approximate Nonlethal Limits for Short Periods Average Approximate Normal Normal Nonlethal Parameter Units Value Range Limit Oxygen mm Hg 40 25–40 10–1000 (venous) Carbon diox- mm Hg 45 41–51 5–80 ide (venous) Sodium ion mmol/L 142 135–145 115–175 Potassium mmol/L 4.2 3.5–5.3 1.5–9.0 ion Calcium ion mmol/L 1.2 1.0–1.4 0.5–2.0 Chloride ion mmol/L 106 98–108 70–130 Bicarbonate mmol/L 24 22–29 8–45 ion Glucose mg/dl 90 70–115 20–1500 Body tem- ° F (° C) 98.4 98–98.8 65–110 perature (37.0) (37.0) (18.3–43.3) Acid- base pH 7.4 7.3–7.5 6.9–8.0 endured for short periods without the occurrence of death. Note the narrowness of the ranges; levels out- side these ranges are usually the cause or the result of illness. Characteristics of Control Systems Many Control Systems of the Body Operate by Nega- tive Feedback. For regulation of carbon dioxide concen- tration, as discussed, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation, which decreases