Johnson Essential Medical Physiology

3. Auflage 2003
ISBN: 978-0-08-047270-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 1008 Seiten

Essential Medical Physiology, Third Edition, deals with the principal subjects covered in a modern medical school physiology course. This thoroughly revised version includes chapters on general physiology as well as cardiovascular, respiratory, renal, gastrointestinal, endocrine, central nervous system, and integrative physiology. It contains clinical notes, chapter outlines with page numbers, 2-color figures throughout, and new chapters on Exercise, Diabetic Ketoacidosis, and Maternal Adaptations in Pregnancy. Among the contributors to this indispensible textbook are leading physiologists Leonard R. Johnson, Stanley G. Schultz, H. Maurice Goodman, John H. Byrne, Norman W. Weisbrodt, James M. Downey, D. Neil Granger, Frank L. Powell, Jr., James A. Schafer, and Dianna A. Johnson. This text is recommended for medical, graduate, and advanced undergraduate students studying physiology, physicians, and clinical specialists as well as anyone interested in basic human physiology.* Includes clinical notes
* 'Key Points' summarize most important information
* Contains chapter outlines with page numbers
* 2-color figures throughout
* New chapters on Exercise, Diabetic Ketoacidosis, and Maternal Adaptations in Pregnancy

Autoren/Hrsg.

Johnson, Leonard R.

Leseproben

CHAPTER 1 The Internal Environment STANLEY G. SCHULTZ Homeostasis: The Subject of Physiology Origin of Our Milieu Interieur Living Things Are Open Systems Body Fluid Compartments and Their Contacts with the Outside World Ionic Composition of the Major Fluid Compartments Body Fluid Osmolarity and pH Measuring the Sizes of the Body Fluid Compartments Plasma Volume Extracellular Fluid Volume Total Body Water Interstitial and Intracellular Fluid Compartments Alterations in Body Fluid Compartments Dehydration Infusion of 2 L of Isotonic Saline (150 mmol/L NaCl) Infusion of Isotonic Urea (300 mmol/L) Rapid Infusion of Pure Water Ingestion of 100 g of NaCl Tablets Infusion of Isosmotic Saline (NaCl) Solution Containing 20% Albumin Infusion of Isosmotic (5%) Glucose Solution KEY POINTS • Physiologic processes have evolved to maintain the constancy or stability of the internal environment. This tendency toward maintenance of physiologic stability is homeostasis. • Living things are open systems. • The total body water is contained within two major compartments—the intracellular compartment and the extracellular compartment. The intracellular compartment is further subdivided into the plasma and the interstitial fluid compartments. • The intracellular compartment is characterized by low intracellular Na+ and Cl- concentrations and a high K+ concentration. The intracellular compartment comprises approximately 25 L, which is approximately 35% of total body weight. • The extracellular compartment is divided into the plasma and interstitial compartments; the former comprises approximately 3 L and the latter approximately 12 L. These compartments are characterized by high Na+ and Cl- concentrations and a low K+ concentration. • Four organs provide the interfaces between the extracellular fluid compartment and the outside world: the skin, alimentary canal, lung, and kidney. The proper interactions among these organs and the external environment is essential for homeostasis. HOMEOSTASIS: THE SUBJECT OF PHYSIOLOGY
If one had to describe, with a single word, what physiology is all about, that word would be homeostasis. This word was coined by the great American physiologist, Walter B. Cannon, in his book entitled The Wisdom of the Body (1939) and refers to regulatory mechanisms by which biologic systems tend to maintain the internal stability necessary for survival while adjusting to internal or external threats to that stability. If homeostasis is successful, life continues; if it is unsuccessful, disease and perhaps death ensue. Cannon’s notion of homeostasis was an extension of the concept first introduced by one of the founders of modern physiology, Claude Bernard, a physician by training who made many seminal contributions to our early understanding of digestion, metabolism, vasomotor activity of nerves, neuromuscular transmission, and other areas of physiology. But perhaps his greatest lasting contribution was the notion that all physiologic processes are designed to maintain the internal environment, the milieu interieur, that bathes our cells, tissues, and organs. The following is from his opening lecture in a course in general physiology given at the College de France in 1887: The living body, though it has need of the surrounding environment, is nevertheless relatively independent of it. This independence which the organism has of its external environment derives from the fact that in the living being, the tissues are in fact withdrawn from direct external influences and are protected by a veritable internal environment which is constituted, in particular, by the fluids circulating in the body. In short, he suggested that we exist within our own “hot houses” and that our existence depends on our abilities to maintain that “hot house.” Although this notion may seem trivial now, at the time when it was introduced it was revolutionary. It proved to be prophetic and has influenced all physiologic thinking to this day. ORIGIN OF OUR MILIEU INTERIEUR
There is good evidence that our predecessors, the protovertebrates or prochordates, migrated from the seas into brackish or fresh water during the Cambrian period, almost 500 million years ago. During that migration, which lasted some 200 million years, they “locked” within themselves a fluid similar in composition to that of the seas from which they emerged. Some of these early vertebrates returned to the oceans, which had become saltier; others stayed in brackish or fresh water; and still others chose to live on land. But, regardless of their final habitats, the “sea” within all vertebrates is remarkably similar in ionic composition to the salinity of the Cambrian seas from which they emerged. This sea that bathes all cells is rich in Na+ and Cl-, in contrast with the intracellular fluid, which, in all living forms including microorganisms, is rich in K+ and poor in Na+ and Cl-. LIVING THINGS ARE OPEN SYSTEMS
Maintaining a constant internal environment would pose no problem if living things were closed systems, like a solution in a sealed bottle. Instead, however, all freely living forms are open systems constantly exchanging matter and energy with the environment. The remarkable thing is that despite this constant exchange, essential for life, the compositions of our intracellular and extracellular fluids are maintained remarkably constant; this is what homeostasis is all about. How is this accomplished? BODY FLUID COMPARTMENTS AND THEIR CONTACTS WITH THE OUTSIDE WORLD
The total body water (TBW) in higher animals is distributed among three major compartments: the blood plasma, the interstitial fluid (ISF), and the intracellular fluid (ICF). The plasma is separated from the ISF compartment by highly permeable capillaries; together, plasma and ISF constitute the extracellular fluid (ECF) compartment. This compartment is separated from the ICF compartment by cell membranes, which in most instances, as discussed in Chapter 3, are highly permeable to water but very selective with respect to the passage of solutes. A fourth, small compartment, called the transcellular fluid compartment, consists primarily of fluid in transit in the lumina of epithelial organs (e.g., the gall bladder, stomach, intestines, and urinary bladder), as well as the cerebrospinal fluid and the intraocular fluid. In an average human adult weighing approximately 70 kg, the TBW makes up approximately 60% of body weight or about 40 L. The distribution of this water among the plasma ISF and ICF compartments is shown in Fig. 1, and the way in which the sizes of these compartments are determined is discussed below. The transcellular fluid compartment in such an individual comprises approximately 2 to 4% of the TBW and contains approximately 1 to 2 L of water. FIGURE 1 Sizes of the major body fluid compartments and the four organs that interface the extracellular fluid and the external environment. Total body water makes up approximately 60% of body weight. The ECF of all vertebrates is in intimate contact with four organs that interface with the external environment. One is a tube that runs from mouth to anus—the alimentary canal. It is responsible for absorbing water, essential elements, and the metabolites that form our building blocks and fuel our activities; for the most part, however, it is indiscriminant with respect to what it will permit to enter the body. The second organ is the lungs, which are responsible for exchanging oxygen and carbon dioxide with the environment and, as discussed below, are a source of water loss. The third organ is the kidneys, which turn over the sea within us many times each day and correct for the indiscretions of our gastrointestinal tracts. In short, the composition of our body fluids is determined not by what the mouth takes in but by what our kidneys keep (Smith, 1961). The final organ in contact with the external environment is, of course, the skin, which plays a primary role in temperature regulation, but is also a source of water loss. It is the precise interplay of these interfaces (particularly the alimentary canal, lungs, and kidneys) and the external environment that maintains the constancy of our milieu interieur and permits us to live a relatively free and independent life. A large portion of this text is devoted to...