Basic Principles of Physiology Stoelting’s Pharmacology Physiology in Anesthe.pdf

Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library Lippincott® Clinical Context  Health Library Aneshesiology Search... This Book This Book  Advanced Search   Texts Updates Stoelting’s Pharmacology & Physiology in A...

Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library Lippincott® Clinical Context  Health Library Aneshesiology Search... This Book This Book  Advanced Search   Texts Updates Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e  Video  Chapter Contents Cases More Collections CHAPTER 1: Basic Principles of Physiology  Pamela Flood, Lisa Wise-Faberowski, Steven L. Shafer Text Figures Tables  Views   Share   Get Permissions This chapter reviews the basic principles of the composition of the body, and the sructure of cells. Although very basic, these principles are essential for everything that follows.  https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library Body Composition  Listen   Water is the medium in which all metabolic reactions occur. Water accounts for about 60% of the weight in an adult man and about 50% of the body weight in an adult woman (Figure 1.1). In a neonate, total body water may represent 70% of body weight. Total body water is less in women and obese individuals, refecting the decreased water content of adipose tissue. Advanced age is also associated with increased fat content and decreased total body water (Table 1.1). Body fuids can be divided into intracellular and extracellular fuid depending on their location relative to the cell membrane (see Figure 1.1). Approximately two-thirds of the total body fuid in an adult are contained inside the esimated 100 trillion cells of the body. The fuid in these cells, despite individual diferences in consituents, is collectively designated intracellular fuid. Extracellular fuid, one-third of fuid outside the cells, is divided into intersitial fuid and plasma (intravascular fuid) by the capillary membrane (see Figure 1.1). FIGURE 1.1 https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library  View Original Download Slide (.ppt) Body fluid compartments and the percentage of body weight represented by each compartment. The location relative to the capillary membrane divides extracellular fuid into plasma or intersitial fuid. Arrows represent fuid movement between compartments. Reprinted with permission from Gamble JL. Chemical Anatomy, Physiology, and Pathology of Extracellular Fluid: A Lecture Syllabus. 6th ed. Cambridge, MA: Harvard University Press; 1954:9. Copyright © 1954 by the President and Fellows of Harvard College. https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library TABLE 1.1 Total body water by age and gender Total body water Age (y) Men (%) Women (%) 18-40 61 51 41-60 55 47 >60 52 46 Intersitial fuid is present in the spaces between cells. An esimated 99% of this fuid is held in the gel sructure of the intersitial space. Plasma is the noncellular portion of blood. The average plasma volume is 3 L, a little over half of the blood volume of 5 L. Plasma is in dynamic equilibrium with the intersitial fuid through pores in the capillaries, the intersitial fuid serving as a reservoir from which water and electrolytes can be mobilized into the circulation. Loss of plasma volume from the intravascular space is minimized by colloid osmotic pressure exerted by the plasma proteins. Other extracellular fuid that may be considered as part of the intersitial fuid includes cerebrospinal fuid, gasrointesinal fuid (because it is mosly resorbed), and fuid in potential spaces (pleural space, pericardial space, peritoneal cavity, synovial cavities). Excess amounts of fuid in the intersitial space manifes as peripheral edema. The normal daily intake of water (drink and internal product of food metabolism) by an adult averages 2.5 L, of which about 1.5 L is excreted as urine, 100 mL is los in sweat, and 100 mL is present in feces. Insensible water losses occur with respiration and difusion through the skin. Inhaled air, saturated with water vapor (47 mm Hg at 37°C), is subsequently exhaled, accounting for an average daily water loss through the lungs of 300 to 400 mL. The water content of inhaled air decreases with decreases in ambient air temperature such that more endogenous https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library water is required to achieve a saturated water vapor pressure at body temperature. As a result, insensible water loss from the lungs is greates in cold environments and leas in warm temperatures. The remaining 400 mL of insensible losses is by difusion through the skin and is not perceived as sweat. Insensible water loss is limited by the mosly impermeable cornifed layer of the skin. When the cornifed layer is removed or interrupted, as after burn injury, the loss of water through the skin is greatly increased. Blood Volume Blood contains extracellular fuid, the plasma, and intracellular fuid, mosly held in erythrocytes. The body has multiple sysems to maintain intravascular fuid volume, including renin-angiotensin sysem, and arginine vasopressin (antidiuretic hormone), that increase fuid resorption in the kidney and evoke changes in the renal tubules that lead to resoration of intravascular fuid volume (see Chapter 16). The average blood volume of an adult is 5 L, compromising about 3 L of plasma and 2 L of erythrocytes. These volumes vary with age, weight, and gender. For example, in nonobese individuals, the blood volume varies in direct proportion to the body weight, averaging 70 mL/kg for lean men and women. The greater the ratio of fat to body weight, however, the less is the blood volume in milliliter per kilogram because adipose tissue has a decreased vascular supply. The hematocrit or packed cell volume is approximately the erythrocyte fraction blood. The normal hematocrit is about 45% for men and posmenopausal women and about 38% for mensruating women, with a range of approximately ±5%. Consituents of Body Fluid Compartments The consituents of intracellular and extracellular fuid are identical, but the quantity of each subsance varies among the compartments (Figure 1.2). The mos sriking diferences are the low protein content in intersitial fuid compared with intracellular fuid and plasma and the fact that sodium and chloride ions are largely extracellular, whereas mos of the potassium ions (approximately 90%) are intracellular. This unequal disribution of ions results in esablishment of a potential (voltage) diference across cell membranes. FIGURE 1.2 https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library  View Original Download Slide (.ppt) Electrolyte composition of body fluid compartments. The consituents of extracellular fuid are carefully regulated by the kidneys so that cells are bathed in an osmotically neutral fuid containing the proper concentrations of electrolytes and nutrients. The normal amount of sodium and potassium in the body is about 58 mEq/kg and 45 mEq/kg, respectively. Trauma is associated with progressive loss of potassium through the kidneys. For example, a patient undergoing surgery excretes about 100 mEq of potassium in the frs 48 hours posoperatively and, after this period, about 25 mEq daily. Plasma potassium concentrations are not good indicators of total body potassium content because mos potassium is intracellular. There is a correlation, however, between the potassium and hydrogen ion content of plasma, the two increasing and decreasing together. In metabolic acidosis, there is net efux of potassium out of the cells to compensate for the hydrogen ion infux and preserve the resing potential. As metabolic acidosis is treated, the plasma potassium will fall from that measured in the acidotic sate. Osmosis Osmosis is the movement of water (solvent molecules) across a semipermeable membrane from a compartment in which the nondifusible solute (ion) concentration is lower to a compartment in which the solute concentration is higher (Figure 1.3). The lipid bilayer that surrounds all cells is freely permeable to water but is impermeable to ions. As a result, water rapidly moves across the cell membrane to esablish osmotic equilibration, which happens almos insantly. FIGURE 1.3 https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library  View Original Download Slide (.ppt) Diagrammatic representation of osmosis depicting water molecules (open circles) and solute molecules (solid circles) separated by a semipermeable membrane. Water molecules move across the semipermeable membrane to the area of higher concentration of solute molecules. Osmotic pressure is the pressure that would have to be applied to prevent continued movement of water molecules. Republished with permission of McGraw Hill LLC from Ganong WF. Review of Medical Physiology. 21s ed. New York, NY: Lange Medical Books/McGraw-Hill; 2003; permission conveyed through Copyright Clearance Center, Inc. Cells control their size by controlling intracellular osmotic pressure. The maintenance of a normal cell volume and pressure depends on sodium-potassium adenosine triphosphatase (ATPase) (sodium-potassium exchange pump), which maintains the intracellular-extracellular ionic balance by removing three sodium ions from the cell for every two potassium ions brought into the cell. The sodium-potassium pump also maintains the transmembrane electrical potential and the sodium and potassium concentration gradients that power many cellular processes, including neuronal conduction. The osmotic pressure exerted by nondifusible particles in a solution is determined by the number of particles in the solution (with each ion counting as 1 unit) and not the type of particles (molecular weight) (see Figure 1.3). Thus, a 1-mol solution of glucose or albumin and 0.5-mol solution of sodium chloride exert the same osmotic pressure because the sodium chloride exiss as independent sodium and chloride ions, each having a concentration of 0.5 mol. Osmole is the unit used to express osmotic pressure in solutes, but the denominator for osmolality is kilogram of water. Osmolarity is the correct terminology when osmole concentrations are expressed in liters of body fuid (eg, plasma) rather than kilograms of water. Because it is much easier to express body fuids in liters of fuid rather https://anesthesiology-lwwhealthlibrary-com.ezproxylocal.library.nova.edu/content.aspx?sectionid=250487752&bookid=3088[5/13/2024 8:11:06 AM] Basic Principles of Physiology | Stoelting’s Pharmacology & Physiology in Anesthetic Practice, 6e | Anesthesiology | Health Library than kilograms of free water, almos all physiology calculations are based on osmolarity. Plasma osmolarity is important in evaluating dehydration, overhydration, and electrolyte abnormalities. Normal plasma has an osmolarity of about 290 mOsm/L. All but about 20 mOsm of the 290 mOsm in each liter of plasma are contributed by sodium ions and their accompanying anions, principally chloride and bicarbonate. Proteins normally contribute