CHA PTE R Functional Organization of the Human Body and control of the Internal environment'' The goal of physiology is to explain the physical and chemical factors that are responsible for the origin development, and progression of life. Each type of life, from the simple virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiol ogy can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, human physiology, and many more subdivisions. Human Physiology. In human physiology, we attempt to explain the specific char acteristics and mechanisms of the human body that make it a living being. The very fact that we remain alive is almost beyond our control, for hunger makes us seek food and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. Thus, the human being is actually an automaton, and the fact that we are sensing feeling, and knowledgeable beings is part of this automatic sequence of life: hese special attributes allow us to exist under widely varying conditions. Cells as the Living Units of the Body The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. For instance, the red blood cells, numbering 25 trillion in each human being, transport oxygen from the lungs to the tissues. Although the red cells are the most abundant of any single type of cell in the body, there are about 75 trillion additional cells of other types that perform functions different from those of the red cell. The entire body, then, contains about 100 trillion cells Although the many cells of the body often differ markedly from one another. all of them have certain basic characteristics that are alike. for instance. in all cells, oxygen reacts with carbohydrate, fat, and protein to release the energy equired for cell function. Further, the general chemical mechanisms for chang ing nutrients into energy are basically the same in all cells, and all cells deliver end products of their chemical reactions into the surrounding fluid Almost all cells also have the ability to reproduce additional cells of their own kind. Fortunately, when cells of a particular type are destroyed from one cause or an nother, the remaining cells of this type usually generate new cells until he supply is replenished Extracellular fluid-The "internal Environment About 60 per cent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is lled intracellular fluid, about one third is in the spaces outside the cells and
CHAPTER 1 3 Functional Organization of the Human Body and Control of the “Internal Environment” The goal of physiology is to explain the physical and chemical factors that are responsible for the origin, development, and progression of life. Each type of life, from the simple virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, human physiology, and many more subdivisions. Human Physiology. In human physiology, we attempt to explain the specific characteristics and mechanisms of the human body that make it a living being. The very fact that we remain alive is almost beyond our control, for hunger makes us seek food and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. Thus, the human being is actually an automaton, and the fact that we are sensing, feeling, and knowledgeable beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions. Cells as the Living Units of the Body The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. For instance, the red blood cells, numbering 25 trillion in each human being, transport oxygen from the lungs to the tissues. Although the red cells are the most abundant of any single type of cell in the body, there are about 75 trillion additional cells of other types that perform functions different from those of the red cell. The entire body, then, contains about 100 trillion cells. Although the many cells of the body often differ markedly from one another, all of them have certain basic characteristics that are alike. For instance, in all cells, oxygen reacts with carbohydrate, fat, and protein to release the energy required for cell function. Further, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver end products of their chemical reactions into the surrounding fluids. Almost all cells also have the ability to reproduce additional cells of their own kind. Fortunately, when cells of a particular type are destroyed from one cause or another, the remaining cells of this type usually generate new cells until the supply is replenished. Extracellular Fluid—The “Internal Environment” About 60 per cent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called intracellular fluid, about one third is in the spaces outside the cells and
Unit I Introduction to Physiology: The Cell and General Physiology is called extracellular fluid. This extracellular fluid is in Extracellular Fluid Transport and constant motion throughout the body. It is transported Mixing System-The Blood apidly the culating blood and then mixed between the blood and the tissue fluids by diffusion Circulatory System In the extracellular fiuid are the ions and nutrients the body in two stages. The first stage is movement of blood through the body in the blood vessels, and the live in essentially the same environment-the extra- second is movement of fluid between the blood capil cellular fluid. For this reason, the extracellular fluid is laries and the intercellular spaces between the tissue also called the internal environment of the body, or the milieu interieur, a term introduced more than 100 years ago by the great 19th-century French physiologist All the blood in the circulation traverses the entire cir- lla heir s ecea apabteos asingngrow inge, and per forming the body is at rest and as many as six times each minute trations of oxygen, glucose, different ions, amino acids, en a person is extremel As blood passes through the blood capillaries, fatty substances, and other constituents are available continual exchange of extracellular fluid also occurs in this internal environment between the plasma portion of the blood and the Differences between extracellular and Intracellular fluids The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients Lungs for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion The intracellular fluid differs significantly from the extracellular fluid; specifically, it contains large amounts of potassium, magnesium, and phosphate ions Left instead of the sodium and chloride ions found in the heart extracellular fluid. Special mechanisms for transport pump ng ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4 hOmeostatic Mechanisms of the Major Functional Systems Nutrition and excretion Kidneys Homeostasis The term homeostasis is used by physiologists to mean maintenance of nearly constant conditions in the inter nal environment. Essentially all organs and tissues of Regulation Excretion the body perform functions that help maintain these constant conditions. For instance, the lungs provide electrolytes oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain con- Venous stant ion concentrations, and the gastrointestinal system provides nutrients A large segment of this text is concerned with the manner in which each organ or tissue contributes to homeostasis. To begin this discussion, the different functional systems of the body and their contributions to homeostasis are outlined in this chapter; then we briefly outline the basic theory of the body's control systems that allow the functional systems to operate in Figure 1-1 support of one another. General organization of the circulatory system
4 Unit I Introduction to Physiology: The Cell and General Physiology is called extracellular fluid. This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and the tissue fluids by diffusion through the capillary walls. In the extracellular fluid are the ions and nutrients needed by the cells to maintain cell life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the internal environment of the body, or the milieu intérieur, a term introduced more than 100 years ago by the great 19th-century French physiologist Claude Bernard. Cells are capable of living, growing, and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment. Differences Between Extracellular and Intracellular Fluids. The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion. The intracellular fluid differs significantly from the extracellular fluid; specifically, it contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4. “Homeostatic” Mechanisms of the Major Functional Systems Homeostasis The term homeostasis is used by physiologists to mean maintenance of nearly constant conditions in the internal environment. Essentially all organs and tissues of the body perform functions that help maintain these constant conditions. For instance, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients. A large segment of this text is concerned with the manner in which each organ or tissue contributes to homeostasis. To begin this discussion, the different functional systems of the body and their contributions to homeostasis are outlined in this chapter; then we briefly outline the basic theory of the body’s control systems that allow the functional systems to operate in support of one another. Extracellular Fluid Transport and Mixing System—The Blood Circulatory System Extracellular fluid is transported through all parts of the body in two stages. The first stage is movement of blood through the body in the blood vessels, and the second is movement of fluid between the blood capillaries and the intercellular spaces between the tissue cells. Figure 1–1 shows the overall circulation of blood. All the blood in the circulation traverses the entire circulatory circuit an average of once each minute when the body is at rest and as many as six times each minute when a person is extremely active. As blood passes through the blood capillaries, continual exchange of extracellular fluid also occurs between the plasma portion of the blood and the Right heart pump Left heart pump Gut Lungs Kidneys Regulation Excretion of electrolytes Venous end Arterial end Capillaries Nutrition and excretion O2 CO2 Figure 1–1 General organization of the circulatory system
Chapter 1 Functional Organization of the Human Body and Control of the"Internal Environment Arteriole the gastrointestinal tract. Here different dissolved nutrients,including carbohydrates, fatty acids, and amino acids, are absorbed from the ingested food into he extracellular fluid of the blood Liver and other Organs That Perform Primarily Metabolic Func- tions. Not all substances absorbed from the gastroin testinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, other tissues of the body-fat cells, gastrointestinal engle kidne leys, and endocrine glands--help modify the absorbed substances or store them until they are needed Musculoskeletal System. Sometimes the question is Figure 1-2 asked, How does the musculoskeletal system fit into the homeostatic functions of the body? The answer is Diffusion of fluid and dissolved constituents through the capillary obvious and simple: Were it not for the muscles, the walls and through the interstitial spaces. body could not move to the appropriate place at the appropriate time to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, interstitial fluid that fills the intercellular spaces. This without which the entire body, along with its homeo- process is shown in Figure 1-2. The walls of the capil- static mechanisms, could be destroyed instantaneously laries are permeable to most molecules in the plasma of the blood, with the exception of the large protein molecules. Therefore, large amounts Removal of metabolic end products and its dissolved constituents difuse back an between the blood and the tissue spaces, as shown by Removal of Carbon Dioxide by the Lungs. At the same time the arrows. This process of diffusion is caused by hat blood picks up oxygen in the lungs, carbon dioxide kinetic motion of the molecules in both the plasma and is released from the blood into the lung alveoli; the res the interstitial fluid. That is, the fluid and dissolved piratory movement of air into and out of the lungs molecules are continually moving and bouncing in all carries the carbon dioxide to the atmosphere Carbon directions within the plasma and the fluid in the inter- dioxide is the most abundant of all the end products cellular spaces, and also through the capillary pores. of metabolism Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any sub- Kidneys. Passage of the blood through the kidneys stance from the capillary to the cell within a few from the plasma most of the other substances seconds. Thus, the extracellular fluid everywhere in the besides carbon dioxide that are not needed by the body--both that of the plasma and that of the inter- cells. These substances include different end products stitial fluid--is continually being mixed, thereby of cellular metabolism, such as urea and uric acid; they maintaining almost complete homogeneity of the also include excesses of ions and water from the food extracellular fluid throughout the body. that might have accumulated in the extracellular fluid The kidneys perform their function by first filtering large quantities of plasma through the glomeruli into Origin of Nutrients in the he tubules and then reabsorbing into the blood those Extracellular Fluid substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the Respiratory System. Figure 1-1 shows that each time the ions. Most of the other substances that are not needed blood passes through the body, it also flows through by the body, especially the metabolic end products the lungs. The blood picks up oxygen in the alveoli, such as urea, are reabsorbed poorly and pass through thus acquiring the oxygen needed by the cells. The membrane between the alveoli and the lumen of the ulmonary capillaries, the alveolar membrane, is only to 2.0 micrometers thick, and oxygen diffuses by Regulation of Body Functions molecular motion through the pores of this membrane into the blood in the same manner that water and ions Nervous System. The nervous system is composed of diffuse through walls of the tissue capillaries. three major parts: the sensory input portion, the central nervous system(or integrative portion), and the motor Gastrointestinal Tract. A large portion of the blood output portion. Sensory receptors detect the state of pumped by the heart also passes through the walls of he body or the state of the surroundings. For instance
Chapter 1 Functional Organization of the Human Body and Control of the “Internal Environment” 5 interstitial fluid that fills the intercellular spaces. This process is shown in Figure 1–2. The walls of the capillaries are permeable to most molecules in the plasma of the blood, with the exception of the large plasma protein molecules. Therefore, large amounts of fluid and its dissolved constituents diffuse back and forth between the blood and the tissue spaces, as shown by the arrows. This process of diffusion is caused by kinetic motion of the molecules in both the plasma and the interstitial fluid. That is, the fluid and dissolved molecules are continually moving and bouncing in all directions within the plasma and the fluid in the intercellular spaces, and also through the capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few seconds.Thus, the extracellular fluid everywhere in the body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby maintaining almost complete homogeneity of the extracellular fluid throughout the body. Origin of Nutrients in the Extracellular Fluid Respiratory System. Figure 1–1 shows that each time the blood passes through the body, it also flows through the lungs. The blood picks up oxygen in the alveoli, thus acquiring the oxygen needed by the cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the alveolar membrane, is only 0.4 to 2.0 micrometers thick, and oxygen diffuses by molecular motion through the pores of this membrane into the blood in the same manner that water and ions diffuse through walls of the tissue capillaries. Gastrointestinal Tract. A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including carbohydrates, fatty acids, and amino acids, are absorbed from the ingested food into the extracellular fluid of the blood. Liver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. Musculoskeletal System. Sometimes the question is asked, How does the musculoskeletal system fit into the homeostatic functions of the body? The answer is obvious and simple: Were it not for the muscles, the body could not move to the appropriate place at the appropriate time to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed instantaneously. Removal of Metabolic End Products Removal of Carbon Dioxide by the Lungs. At the same time that blood picks up oxygen in the lungs, carbon dioxide is released from the blood into the lung alveoli; the respiratory movement of air into and out of the lungs carries the carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the end products of metabolism. Kidneys. Passage of the blood through the kidneys removes from the plasma most of the other substances besides carbon dioxide that are not needed by the cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that might have accumulated in the extracellular fluid. The kidneys perform their function by first filtering large quantities of plasma through the glomeruli into the tubules and then reabsorbing into the blood those substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially the metabolic end products such as urea, are reabsorbed poorly and pass through the renal tubules into the urine. Regulation of Body Functions Nervous System. The nervous system is composed of three major parts: the sensory input portion, the central nervous system (or integrative portion), and the motor output portion. Sensory receptors detect the state of the body or the state of the surroundings. For instance, Venule Arteriole Figure 1–2 Diffusion of fluid and dissolved constituents through the capillary walls and through the interstitial spaces
6 Unit I Introduction to Physiology: The Cell and General Physiology receptors in the skin apprise one whenever an object concentration of carbon dioxide in the extracellular touches the skin at any point. The eyes are sensory fluid. The liver and pancre gulate the concentra- organs that give one a visual image of the surrounding tion of glucose in the extracellular fluid, and the area. The ears also are sensory organs. The central kidneys regulate concentrations of hydrogen, sodium, nervous system is composed of the brain and spinal potassium, phosphate, and other ions in the extracel cord. The brain can store information, generate lular fluid thoughts, create ambition, and determine reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the Examples of Control Mechanisms motor output portion of the nervous system to carry out ones desires. Regulation of Oxygen and carbon Dioxide concentrations in the e. A large segment of the nervous system is called the Extracellular Fluid. Because oxygen is one of the major tonomic system. It operates at a subconscious level substances required for chemical reactions in the cells, and controls many functions of the internal organs, it is fortunate that the body has a special control including the level of pumping activity by the heart mechanism to maintain an almost exact and constant movements of the gastrointestinal tract, and secretion oxygen concentration in the extracellular fluid. This by many of the bodys gland mechanism depends principally on the chemical char acteristics of hemoglobin, which is present in all red Hormonal System of Regulation. Located in the body are blood cells. Hemoglobin combines with oxygen as the eight major endocrine glands that secrete chemical blood passes through the lungs. Then, as the blood substances called hormones. Hormones are trans- passes through the tissue capillaries, hemoglobin, ported in the extracellular fluid to all parts of the body because of its own strong chemical affinity for oxygen, to help regulate cellular function. For instance, thyroid does not release oxygen into the tissue fluid if too hormone increases the rates of most chemical reac- much oxygen is already there. But if the oxygen con tions in all cells, thus helping to set the tempo of bodily centration in the tissue fluid is too low, sufficient activity. Insulin controls glucose metabolism; adreno- oxygen is released to re-establish an adequate con cortical hormones control sodium ion, potassium ion, centration. Thus, regulation of oxygen concentration annt po tein metc mi a nh parate poids tbe bore ih the tissues is vested pin cipsally ihi the chemical mones are a system of regulation that complements called the oxygen buffering function of hemoglobin. the nervous system. The nervous system regulates Carbon dioxide concentration in the extracellular mainly muscular and secretory activities of the body, fluid is regulated in a much different way. Carbon whereas the hormonal system regulates many meta- dioxide is a major end product of the oxidative reac- bolic functions. tions in cells. if all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, the mass action of the carbon dioxide itself would soon Reproduction halt all energy-giving reactions of the cells. Fortu- ately, a higher than normal carbon dioxide concen- Sometimes reproduction is not considered a homeo- tration in the blood excites the respiratory center, static function. It does, however, help maintain home- causing a person to breathe rapidly and deeply. This stasis by generating new beings to take the place of increases expiration of carbon dioxide and, therefore those that are dying. This may sound like a permissive removes excess carbon dioxide from the blood and usage of the term homeostasis, but it illustrates that, in tissue fluids. This process continues until the concen- the final analysis, essentially all body structures are tration returns to normal organized such that they help maintain the automatic ity and continuity of life Regulation of Arterial Blood Pressure. Several systems con tribute to the regulation of arterial blood pressure One of these, the baroreceptor system, is a simple and Control Systems of the body excellent example of a rapidly acting control mecha nism. In the walls of the bifurcation region of the The human body has thousands of control systems in arotid arteries in the neck and also in the arch of the it. The most intricate of these are the genetic control aorta in the thorax, are many nerve receptors called systems that operate in all cells to help control intra- baroreceptors, which are stimulated by stretch of the cellular function as well as extracellular function. This arterial wall. When the arterial pressure rises too high, subject is discussed in Chapter 3 the baroreceptors send barrages of nerve impulses to Many other control systems operate within the the medulla of the brain. Here these impulses inhibit organs to control functions of the individual parts the vasomotor center, which in turn decreases the of the organs; others operate throughout the entire number of impulses transmitted from the vasomotor body to control the interrelations between the organs. center through the sympathetic nervous system to the For instance, the respiratory system, operating in heart and blood vessels. Lack of these impulses causes association with the nervous system, regulates the diminished pumping activity by the heart and also
6 Unit I Introduction to Physiology: The Cell and General Physiology receptors in the skin apprise one whenever an object touches the skin at any point. The eyes are sensory organs that give one a visual image of the surrounding area. The ears also are sensory organs. The central nervous system is composed of the brain and spinal cord. The brain can store information, generate thoughts, create ambition, and determine reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one’s desires. A large segment of the nervous system is called the autonomic system. It operates at a subconscious level and controls many functions of the internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body’s glands. Hormonal System of Regulation. Located in the body are eight major endocrine glands that secrete chemical substances called hormones. Hormones are transported in the extracellular fluid to all parts of the body to help regulate cellular function. For instance, thyroid hormone increases the rates of most chemical reactions in all cells, thus helping to set the tempo of bodily activity. Insulin controls glucose metabolism; adrenocortical hormones control sodium ion, potassium ion, and protein metabolism; and parathyroid hormone controls bone calcium and phosphate. Thus, the hormones are a system of regulation that complements the nervous system. The nervous system regulates mainly muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions. Reproduction Sometimes reproduction is not considered a homeostatic function. It does, however, help maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term homeostasis, but it illustrates that, in the final analysis, essentially all body structures are organized such that they help maintain the automaticity and continuity of life. Control Systems of the Body The human body has thousands of control systems in it. The most intricate of these are the genetic control systems that operate in all cells to help control intracellular function as well as extracellular function. This subject is discussed in Chapter 3. Many other control systems operate within the organs to control functions of the individual parts of the organs; others operate throughout the entire body to control the interrelations between the organs. For instance, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas regulate the concentration of glucose in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. Examples of Control Mechanisms Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid. Because oxygen is one of the major substances required for chemical reactions in the cells, it is fortunate that the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of hemoglobin, which is present in all red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. But if the oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues is vested principally in the chemical characteristics of hemoglobin itself. This regulation is called the oxygen-buffering function of hemoglobin. Carbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of the oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, the mass action of the carbon dioxide itself would soon halt all energy-giving reactions of the cells. Fortunately, a higher than normal carbon dioxide concentration in the blood excites the respiratory center, causing a person to breathe rapidly and deeply. This increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal. Regulation of Arterial Blood Pressure. Several systems contribute to the regulation of arterial blood pressure. One of these, the baroreceptor system, is a simple and excellent example of a rapidly acting control mechanism. In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called baroreceptors, which are stimulated by stretch of the arterial wall. When the arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here these impulses inhibit the vasomotor center, which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and also
Chapter 1 Functional Organization of the Human Body and Control of the"Internal Environment dilation of the peripheral blood vessels, allowing vast numbers of control systems that keep the bod increased blood flow through the vessels. Both of these operating in health; in the absence of any one of these effects decrease the arterial pressure back toward controls, serious body malfunction or death can result normal Conversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the Characteristics of Control Systems vasomotor center to become more active than usual thereby causing vasoconstriction and increased heart The aforementioned examples of homeostatic control pumping, and raising arterial pressure back toward mechan isms are only a few of the many thousands in he body, all of which have certain characteristics in common. These characteristics are explained in th Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents Negative Feedback Nature of Most Table lists the more important constituents and Control Systems physical characteristics of extracellular fluid, along Most control systems of the body act by negative feed with their normal values, normal ranges, and maximum back, which can best be explained by reviewing some of the homeostatic control systems mentioned pre- the normal range for each one Values outside these viously. In the regulation of carbon dioxide concen anges are usually caused by illness. Most important are the limits beyond which abnor tration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. in the body temperature of only 11F(7C)above dioxide concentration because the lungs expire greater normal can lead to a vicious cycle of increasing cellu amounts of carbon dioxide from the body. In other lar metabolism that destroys the cells. Note also the words, the high concentration of carbon dioxide initi narrow range for acid-base balance in the body, with ates events that decrease the concentration toward about O5 on either side of normal. Another important Conversely, if the carbon dioxide concentration falls factor is the potassium ion concentration, because too low this causes feedback to increase the concen- whenever it decreases to less than one third normal, tration This response also is negative to the initiating a person is likely to be paralyzed as a result of the stimulus. nerves inability to carry signals. Alternatively, if In the arterial pressure-regulating mechanisms, a the potassium ion concentration increases to two or high pressure causes a series of reactions that promote more times normal, the heart muscle is likely to be a lowered pressure, or a low pressure causes a series severely depressed. Also, when the calcium ion con- of reactions that promote an elevated pressure. In both centration falls below about one half of normal, a instances, these effects are negative with respect to the person is likely to experience tetanic contraction of initiating stimulus. muscles throughout the body because of the sponta Therefore, in general, if some factor becomes exces- neous generation of excess nerve impulses in the sive or deficient, a control system initiates negative peripheral nerves. When the glucose concentration feedback, which consists of a series of changes that falls below one half of normal, a person frequently return the factor toward a certain mean value. thus develops extreme mental irritability and sometimes maintaining homeostasis. These examples should give one an appreciation for Gain"of a Control System. The degree of effectiveness he extreme value and even the necessity of the with which a control system maintains constant Table 1-1 mportant Constituents and Physical Characteristics of Extracellular Fluid Normal value Normal Range Nonlethal Limit arbon dioxide 35-45 Sodium io Potassium ion 3.8-5.0 10-1.4 0.5-20 75-95 1500 Body temperature 984(37.0) (37.0) 65-110(183-433) Acid-base 73-7.5
Chapter 1 Functional Organization of the Human Body and Control of the “Internal Environment” 7 dilation of the peripheral blood vessels, allowing increased blood flow through the vessels. Both of these effects decrease the arterial pressure back toward normal. Conversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping, and raising arterial pressure back toward normal. Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents Table 1–1 lists the more important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are usually caused by illness. Most important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Another important factor is the potassium ion concentration, because whenever it decreases to less than one third normal, a person is likely to be paralyzed as a result of the nerves’ inability to carry signals. Alternatively, if the potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one half of normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in the peripheral nerves. When the glucose concentration falls below one half of normal, a person frequently develops extreme mental irritability and sometimes even convulsions. These examples should give one an appreciation for the extreme value and even the necessity of the vast numbers of control systems that keep the body operating in health; in the absence of any one of these controls, serious body malfunction or death can result. Characteristics of Control Systems The aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have certain characteristics in common. These characteristics are explained in this section. Negative Feedback Nature of Most Control Systems Most control systems of the body act by negative feedback, which can best be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases the extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. In other words, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is negative to the initiating stimulus. Conversely, if the carbon dioxide concentration falls too low, this causes feedback to increase the concentration. This response also is negative to the initiating stimulus. In the arterial pressure–regulating mechanisms, a high pressure causes a series of reactions that promote a lowered pressure, or a low pressure causes a series of reactions that promote an elevated pressure. In both instances, these effects are negative with respect to the initiating stimulus. Therefore, in general, if some factor becomes excessive or deficient, a control system initiates negative feedback, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis. “Gain” of a Control System. The degree of effectiveness with which a control system maintains constant Table 1–1 Important Constituents and Physical Characteristics of Extracellular Fluid Normal Value Normal Range Approximate Short-Term Unit Nonlethal Limit Oxygen 40 35–45 10–1000 mm Hg Carbon dioxide 40 35–45 5–80 mm Hg Sodium ion 142 138–146 115–175 mmol/L Potassium ion 4.2 3.8–5.0 1.5–9.0 mmol/L Calcium ion 1.2 1.0–1.4 0.5–2.0 mmol/L Chloride ion 108 103–112 70–130 mmol/L Bicarbonate ion 28 24–32 8–45 mmol/L Glucose 85 75–95 20–1500 mg/dl Body temperature 98.4 (37.0) 98–98.8 (37.0) 65–110 (18.3–43.3) ∞F (∞C) Acid-base 7.4 7.3–7.5 6.9–8.0 pH