(MEDC 1301)
C H A P T E R 1 / 3 / 08 / Functional Organization of the Human Body
C H A P T E R 2 / 11 / 16 / The Cell and Its Functions
C H A P T E R 4 / 45 / 12 / Transport of Substances Through the Cell Membrane
C H A P T E R 5 / 57 / 15 / Membrane Potentials and Action Potentials
C H A P T E R 6 / 72 / 13 / Contraction of Skeletal Muscle
C H A P T E R 7 / 85 / 07 / Excitation of Skeletal Muscle: Neuromuscular Tra
C H A P T E R 8 / 92 / 11 / Contraction and Excitation of Smooth Muscle
C H A P T E R 32 / 419 / 20 / Red Blood Cells, Anemia, and Polycythemia
C H A P T E R 33 / 429 / 22 / Resistance of the Body to Infection: I Leukocytes, Granulocyt
C H A P T E R 34 / 439 / 22 / Resistance of the Body to Infection: II Immunity & Allergy
C H A P T E R 35 / 451 / 6 / Blood Types; Transfusion; Tissue and Organ Transplantation
C H A P T E R 36 / 457 / 17 / Hemostasis and Blood Coagulation -- Bilirubin & Jaundice
C H A P T E R 45 / 555 / 17 / Organization of the Nervous System, Basic Functions of Syna
C H A P T E R 60 / 748 / 13 / The Autonomic Nervous System and the Adrenal Medulla
3 hours
C H A P T E R 1
Functional Organization of
theHuman Body and Control of the“InternalEnvironment”
The goal of physiology is to explain the physical andchemical factors that are responsible for the origin,development, and progression of life. Each type oflife, from the simple virus to the largest tree or thecomplicated human being, has its own functionalcharacteristics. Therefore, the vast field of physiologycan be divided into viral physiology, bacterialphysiology, cellular physiology, plant physiology,human physiology, and many more subdivisions.Human Physiology. In human physiology, we attempt to explain the specific characteristicsand mechanisms of the human body that make it a living being. Thevery fact that we remain alive is almost beyond our control, for hunger makesus seek food and fear makes us seek refuge. Sensations of cold make us lookfor 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 BodyThe basic living unit of the body is the cell. Each organ is an aggregate of manydifferent cells held together by intercellular supporting structures.Each type of cell is specially adapted to perform one or a few particularfunctions. For instance, the red blood cells, numbering 25 trillion in eachhuman being, transport oxygen from the lungs to the tissues. Although the redcells are the most abundant of any single type of cell in the body, there areabout 75 trillion additional cells of other types that perform functions differentfrom those of the red cell. The entire body, then, contains about 100 trillioncells.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 allcells, oxygen reacts with carbohydrate, fat, and protein to release the energyrequired for cell function. Further, the general chemical mechanisms for changingnutrients into energy are basically the same in all cells, and all cells deliverend products of their chemical reactions into the surrounding fluids.Almost all cells also have the ability to reproduce additional cells of theirown kind. Fortunately, when cells of a particular type are destroyed from onecause or another, the remaining cells of this type usually generate new cells untilthe supply is replenished.
Extracellular Fluid—The “Internal
Environment”
About 60 per cent of the adult human body is fluid, mainly a water solution ofions and other substances. Although most of this fluid is inside the cells and iscalled intracellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid. This extracellular fluid is inconstant motion throughout the body. It is transportedrapidly in the circulating blood and then mixedbetween the blood and the tissue fluids by diffusionthrough the capillary walls.In the extracellular fluid are the ions and nutrientsneeded by the cells to maintain cell life. Thus, all cellslive in essentially the same environment—the extracellularfluid. For this reason, the extracellular fluid isalso called the internal environment of the body, or themilieu intérieur, a term introduced more than 100 yearsago by the great 19th-century French physiologistClaude Bernard.Cells are capable of living, growing, and performingtheir special functions as long as the proper concentrationsof oxygen, glucose, different ions, amino acids,fatty substances, and other constituents are availablein this internal environment.
Differences Between Extracellular and Intracellular Fluids.The extracellular fluid contains large amounts ofsodium, chloride, and bicarbonate ions plus nutrientsfor the cells, such as oxygen, glucose, fatty acids, andamino acids. It also contains carbon dioxide that isbeing transported from the cells to the lungs to beexcreted, plus other cellular waste products that arebeing transported to the kidneys for excretion.The intracellular fluid differs significantly fromthe extracellular fluid; specifically, it contains largeamounts of potassium, magnesium, and phosphate ionsinstead of the sodium and chloride ions found in theextracellular fluid. Special mechanisms for transportingions through the cell membranes maintain the ionconcentration differences between the extracellularand intracellular fluids. These transport processes arediscussed in Chapter 4.
“Homeostatic” Mechanisms ofthe Major Functional Systems
Homeostasis
The term homeostasis is used by physiologists to meanmaintenance of nearly constant conditions in the internalenvironment. Essentially all organs and tissues ofthe body perform functions that help maintain theseconstant conditions. For instance, the lungs provideoxygen to the extracellular fluid to replenish theoxygen used by the cells, the kidneys maintain constantion concentrations, and the gastrointestinalsystem provides nutrients.A large segment of this text is concerned with themanner in which each organ or tissue contributes tohomeostasis. To begin this discussion, the differentfunctional systems of the body and their contributionsto homeostasis are outlined in this chapter; then webriefly outline the basic theory of the body’s controlsystems that allow the functional systems to operate insupport of one another.
Extracellular Fluid Transport andMixing System—The BloodCirculatory System
Extracellular fluid is transported through all parts ofthe body in two stages. The first stage is movement ofblood through the body in the blood vessels, and thesecond is movement of fluid between the blood capillariesand the intercellular spaces between the tissuecells.Figure 1–1 shows the overall circulation of blood.
All the blood in the circulation traverses the entire circulatorycircuit an average of once each minute whenthe body is at rest and as many as six times each minutewhen a person is extremely active.As blood passes through the blood capillaries,continual exchange of extracellular fluid also occursbetween the plasma portion of the blood and theinterstitial fluid that fills the intercellular spaces. Thisprocess is shown in Figure 1–2. The walls of the capillariesare permeable to most molecules in the plasmaof the blood, with the exception of the large plasmaprotein molecules. Therefore, large amounts of fluidand its dissolved constituents diffuse back and forthbetween the blood and the tissue spaces, as shown bythe arrows. This process of diffusion is caused bykinetic motion of the molecules in both the plasma andthe interstitial fluid. That is, the fluid and dissolvedmolecules are continually moving and bouncing in alldirections within the plasma and the fluid in the intercellularspaces, and also through the capillary pores.Few cells are located more than 50 micrometers froma capillary, which ensures diffusion of almost any substancefrom the capillary to the cell within a fewseconds.Thus, the extracellular fluid everywhere in thebody—both that of the plasma and that of the interstitialfluid—is continually being mixed, therebymaintaining almost complete homogeneity of theextracellular fluid throughout the body.
Origin of Nutrients in theExtracellular FluidRespiratory System. Figure 1–1 shows that each time theblood passes through the body, it also flows throughthe lungs. The blood picks up oxygen in the alveoli,thus acquiring the oxygen needed by the cells. Themembrane between the alveoli and the lumen of thepulmonary capillaries, the alveolar membrane, is only0.4 to 2.0 micrometers thick, and oxygen diffuses bymolecular motion through the pores of this membraneinto the blood in the same manner that water and ionsdiffuse through walls of the tissue capillaries.Gastrointestinal Tract. A large portion of the bloodpumped by the heart also passes through the walls ofthe gastrointestinal tract. Here different dissolvednutrients, including carbohydrates, fatty acids, andamino acids, are absorbed from the ingested food intothe extracellular fluid of the blood.
Liver and Other Organs That Perform Primarily Metabolic Functions.
Not all substances absorbed from the gastrointestinaltract can be used in their absorbed form by thecells. The liver changes the chemical compositions ofmany of these substances to more usable forms, andother tissues of the body—fat cells, gastrointestinalmucosa, kidneys, and endocrine glands—help modifythe absorbed substances or store them until they areneeded.
Musculoskeletal System. Sometimes the question isasked, How does the musculoskeletal system fit intothe homeostatic functions of the body? The answer isobvious and simple: Were it not for the muscles, thebody could not move to the appropriate place at theappropriate time to obtain the foods required fornutrition. The musculoskeletal system also providesmotility for protection against adverse surroundings,without which the entire body, along with its homeostaticmechanisms, could be destroyed instantaneously.
Removal of Metabolic End Products
Removal of Carbon Dioxide by the Lungs.
At the same timethat blood picks up oxygen in the lungs, carbon dioxideis released from the blood into the lung alveoli; the respiratorymovement of air into and out of the lungscarries the carbon dioxide to the atmosphere. Carbondioxide is the most abundant of all the end productsof metabolism.
Kidneys. Passage of the blood through the kidneysremoves from the plasma most of the other substancesbesides carbon dioxide that are not needed by thecells. These substances include different end productsof cellular metabolism, such as urea and uric acid; theyalso include excesses of ions and water from the foodthat might have accumulated in the extracellular fluid.The kidneys perform their function by first filteringlarge quantities of plasma through the glomeruli intothe tubules and then reabsorbing into the blood thosesubstances needed by the body, such as glucose, aminoacids, appropriate amounts of water, and many of theions. Most of the other substances that are not neededby the body, especially the metabolic end productssuch as urea, are reabsorbed poorly and pass throughthe renal tubules into the urine.
Regulation of Body FunctionsNervous System. The nervous system is composed ofthree major parts: the sensory input portion, the centralnervous system (or integrative portion), and the motoroutput portion. Sensory receptors detect the state ofthe body or the state of the surroundings. For instance,receptors in the skin apprise one whenever an objecttouches the skin at any point. The eyes are sensoryorgans that give one a visual image of the surroundingarea. The ears also are sensory organs. The centralnervous system is composed of the brain and spinalcord. The brain can store information, generatethoughts, create ambition, and determine reactionsthat the body performs in response to the sensations.Appropriate signals are then transmitted through themotor output portion of the nervous system to carryout one’s desires.A large segment of the nervous system is called theautonomic system. It operates at a subconscious leveland controls many functions of the internal organs,including the level of pumping activity by the heart,movements of the gastrointestinal tract, and secretionby many of the body’s glands.
Hormonal System of Regulation.
Located in the body areeight major endocrine glands that secrete chemicalsubstances called hormones. Hormones are transportedin the extracellular fluid to all parts of the bodyto help regulate cellular function. For instance, thyroidhormone increases the rates of most chemical reactionsin all cells, thus helping to set the tempo of bodilyactivity. Insulin controls glucose metabolism; adrenocorticalhormones control sodium ion, potassium ion,and protein metabolism; and parathyroid hormonecontrols bone calcium and phosphate. Thus, the hormonesare a system of regulation that complementsthe nervous system. The nervous system regulatesmainly muscular and secretory activities of the body,whereas the hormonal system regulates many metabolicfunctions.
ReproductionSometimes reproduction is not considered a homeostaticfunction. It does, however, help maintain homeostasisby generating new beings to take the place ofthose that are dying.This may sound like a permissiveusage of the term homeostasis, but it illustrates that, inthe final analysis, essentially all body structures areorganized such that they help maintain the automaticityand continuity of life.
Control Systems of the BodyThe human body has thousands of control systems init. The most intricate of these are the genetic controlsystems that operate in all cells to help control intracellularfunction as well as extracellular function. Thissubject is discussed in Chapter 3.Many other control systems operate within theorgans to control functions of the individual partsof the organs; others operate throughout the entirebody to control the interrelations between the organs.For instance, the respiratory system, operating inassociation with the nervous system, regulates theconcentration of carbon dioxide in the extracellularfluid. The liver and pancreas regulate the concentrationof glucose in the extracellular fluid, and thekidneys regulate concentrations of hydrogen, sodium,potassium, phosphate, and other ions in the extracellularfluid.
Examples of Control Mechanisms
Regulation of Oxygen and Carbon Dioxide
Concentrations in theExtracellular Fluid.
Because oxygen is one of the majorsubstances required for chemical reactions in the cells,it is fortunate that the body has a special controlmechanism to maintain an almost exact and constantoxygen concentration in the extracellular fluid. Thismechanism depends principally on the chemical characteristicsof hemoglobin, which is present in all redblood cells. Hemoglobin combines with oxygen as theblood passes through the lungs. Then, as the bloodpasses through the tissue capillaries, hemoglobin,because of its own strong chemical affinity for oxygen,does not release oxygen into the tissue fluid if toomuch oxygen is already there. But if the oxygen concentrationin the tissue fluid is too low, sufficientoxygen is released to re-establish an adequate concentration.Thus, regulation of oxygen concentrationin the tissues is vested principally in the chemicalcharacteristics of hemoglobin itself. This regulation iscalled the oxygen-buffering function of hemoglobin.Carbon dioxide concentration in the extracellularfluid is regulated in a much different way. Carbondioxide is a major end product of the oxidative reactionsin cells. If all the carbon dioxide formed in thecells continued to accumulate in the tissue fluids, themass action of the carbon dioxide itself would soonhalt all energy-giving reactions of the cells. Fortunately,a higher than normal carbon dioxide concentrationin the blood excites the respiratory center,causing a person to breathe rapidly and deeply. Thisincreases expiration of carbon dioxide and, therefore,removes excess carbon dioxide from the blood andtissue fluids. This process continues until the concentrationreturns to normal.
Regulation of Arterial Blood Pressure. Several systems contributeto the regulation of arterial blood pressure.One of these, the baroreceptor system, is a simple andexcellent example of a rapidly acting control mechanism.In the walls of the bifurcation region of thecarotid arteries in the neck, and also in the arch of theaorta in the thorax, are many nerve receptors calledbaroreceptors, which are stimulated by stretch of thearterial wall.When the arterial pressure rises too high,the baroreceptors send barrages of nerve impulses tothe medulla of the brain. Here these impulses inhibitthe vasomotor center, which in turn decreases thenumber of impulses transmitted from the vasomotorcenter through the sympathetic nervous system to theheart and blood vessels. Lack of these impulses causesdiminished pumping activity by the heart and alsodilation of the peripheral blood vessels, allowingincreased blood flow through the vessels. Both of theseeffects decrease the arterial pressure back towardnormal.Conversely, a decrease in arterial pressure belownormal relaxes the stretch receptors, allowing thevasomotor center to become more active than usual,thereby causing vasoconstriction and increased heartpumping, and raising arterial pressure back towardnormal.
NormalRanges and Physical Characteristicsof Important ExtracellularFluid Constituents
Table 1–1 lists the more important constituents andphysical characteristics of extracellular fluid, alongwith their normal values, normal ranges, and maximumlimits without causing death. Note the narrowness ofthe normal range for each one. Values outside theseranges are usually caused by illness.Most important are the limits beyond which abnormalitiescan cause death. For example, an increasein the body temperature of only 11°F (7°C) abovenormal can lead to a vicious cycle of increasing cellularmetabolism that destroys the cells. Note also thenarrow range for acid-base balance in the body, witha normal pH value of 7.4 and lethal values onlyabout 0.5 on either side of normal. Another importantfactor is the potassium ion concentration, becausewhenever it decreases to less than one third normal,a person is likely to be paralyzed as a result of thenerves’ inability to carry signals. Alternatively, ifthe potassium ion concentration increases to two ormore times normal, the heart muscle is likely to beseverely depressed. Also, when the calcium ion concentrationfalls below about one half of normal, aperson is likely to experience tetanic contraction ofmuscles throughout the body because of the spontaneousgeneration of excess nerve impulses in theperipheral nerves. When the glucose concentrationfalls below one half of normal, a person frequentlydevelops extreme mental irritability and sometimeseven convulsions.These examples should give one an appreciation forthe extreme value and even the necessity of thevast numbers of control systems that keep the bodyoperating in health; in the absence of any one of thesecontrols, serious body malfunction or death can result.