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CHAPTER
26
Fluid, Electrolyte, and Acid-Base
Balance
Objectives
Body Fluids
1. List the factors that determine body water content and describe the effect of each factor.
Water Balance and ECF Osmolality
5. List the routes by which water enters and leaves the body.
Electrolyte Balance
9. Indicate routes of electrolyte entry and loss from the body.
10. Describe the importance of sodium in the body’s fluid and electrolyte balance, and
indicate its relationship to normal cardiovascular system functioning.
Acid-Base Balance
13. List important sources of acids in the body.
14. List the three major chemical buffer systems of the body and describe how they resist
pH changes.
17. Distinguish between acidosis and alkalosis resulting from respiratory and metabolic
factors. Describe the importance of respiratory and renal compensations to acid-base
balance.
Developmental Aspects of Fluid, Electrolyte, and Acid-Base Balance
Suggested Lecture Outline
I. Body Fluids (pp. 991–993; Figs. 26.1–26.3)
A. Body Water Content (p. 991)
1. Total body water is a function of age, body mass, gender, and body fat.
2. Body water declines throughout life, ultimately comprising about 45% of total body
mass in old age.
B. Fluid Compartments (p. 991; Fig. 26.1)
1. There are two main fluid compartments of the body: the intracellular compartment,
C. Composition of Body Fluids (pp. 991–993; Fig. 26.2)
1. Nonelectrolytes include most organic molecules, do not dissociate in water, and carry
no net electrical charge.
2. Electrolytes dissociate in water to ions and include inorganic salts, acids and bases,
and some proteins.
D. Fluid Movement Among Compartments (p. 993; Fig. 26.3)
1. Anything that changes solute concentration in any compartment leads to net water flows.
2. Substances must pass through both the plasma and interstitial fluid in order to reach
the intracellular fluid, and exchanges between these compartments occur almost
continuously, leading to compensatory shifts from one compartment to another.
II. Water Balance and ECF Osmolality (pp. 993–997; Figs. 26.4–26.7)
A. For the body to remain properly hydrated, water intake must equal water output
(p. 994; Fig. 26.4).
1. Most water enters the body through ingested liquids and food, but is also produced by
cellular metabolism.
B. Regulation of Water Intake (pp. 994–995; Fig. 26.5)
1. The thirst mechanism is triggered by a decrease in plasma osmolality, which results in
a dry mouth and excites the hypothalamic thirst center.
thirst center.
C. Regulation of Water Output (p. 995)
1. Drinking is necessary because there is obligatory water loss due to the insensible water
losses, water lost with food residues and feces, and a minimum 500 ml sensible water
D. Influence of ADH (p. 995; Fig. 26.6)
1. The amount of water reabsorbed in the renal collecting ducts is proportional to ADH
release.
2. ADH secretion is promoted or inhibited by the hypothalamus in response to changes in
solute concentration of extracellular fluid, large changes in blood volume or pressure,
or vascular baroreceptors.
E. Disorders of Water Balance (pp. 995–997; Fig. 26.7)
1. Dehydration occurs when water output exceeds water intake and may lead to weight
loss, fever, mental confusion, or hypovolemic shock.
III. Electrolyte Balance (pp. 997–1004; Figs. 26.8–26.10; Tables 26.1–26.2)
A. The Central Role of Sodium in Fluid and Electrolyte Balance (pp. 997–999; Table 26.1)
1. Sodium is the most important cation in regulation of fluid and electrolyte balance in
the body due to its abundance and osmotic pressure.
B. Regulation of Sodium Balance (pp. 999–1000; Figs. 26.8–26.10; Table 26.2)
1. When aldosterone secretion is high, nearly all the filtered sodium is reabsorbed in the
distal convoluted tubule and the collecting duct.
a. Absorption of sodium ions creates osmotic absorption of water, while sodium
excretion also causes water loss to the urine.
2. The most important trigger for the release of aldosterone is the renin-angiotensin-
aldosterone mechanism, initiated in response to sympathetic stimulation, a decrease in
filtrate osmolality, or decreased blood pressure.
3. Atrial natriuretic peptide, released in response to increased stretch of atrial cells,
reduces blood pressure and blood volume by inhibiting release of ADH, renin, and
aldosterone and directly causing vasodilation.
4. Estrogens are chemically similar to aldosterone and enhance reabsorption of salt by
6. Glucocorticoids enhance tubular reabsorption of sodium, but increase glomerular
filtration.
C. Regulation of Potassium Balance (pp. 1001–1003)
1. Potassium is critical to the maintenance of the membrane potential of neurons and
muscle cells and is a buffer that compensates for shifts of hydrogen ions in or out of
2. Potassium balance is chiefly regulated by renal mechanisms, which control the amount
of potassium secreted into the filtrate.
D. Regulation of Calcium and Phosphate Balance (pp. 1003–1004)
1. Calcium ion levels are closely regulated by parathyroid hormone and calcitonin; about
98% is reabsorbed.
E. Regulation of Anions (p. 1004)
1. Chloride is the major anion reabsorbed with sodium and helps maintain the osmotic
pressure of the blood.
IV. Acid-Base Balance (pp. 1004–1012; Figs. 26.11–26.14; Table 26.3)
A. Because of the abundance of hydrogen bonds in the body’s functional proteins, they are
strongly influenced by hydrogen ion concentration (p. 1004).
1. When arterial blood pH rises above 7.45, the body is in alkalosis; when arterial pH
falls below 7.35, the body is in physiological acidosis.
B. Chemical Buffer Systems (pp. 1004–1006; Fig. 26.11)
1. A chemical buffer is a system of one or two molecules that acts to resist changes in pH
by binding H+ when the pH drops or releasing H+ when the pH rises.
2. The bicarbonate buffer system is the main buffer of the extracellular fluid and consists
of carbonic acid and its salt, sodium bicarbonate.
a. When a strong acid is added to the solution, carbonic acid is mostly unchanged, but
3. The phosphate buffer system operates in the urine and intracellular fluid similarly to
the bicarbonate buffer system: Sodium dihydrogen phosphate is its weak acid, and
monohydrogen phosphate is its weak base.
4. The protein buffer system consists of organic acids containing carboxyl groups that
dissociate to release H+ when the pH begins to rise or bind excess H+ when the pH
declines.
C. Respiratory Regulation of H+ (p. 1006)
1. The respiratory and renal systems are physiological buffer systems that control pH by
regulating the amount of acid or base in the body.
a. Physiological buffer systems act more slowly than chemical buffer systems, but
have a much greater buffering power.
D. Renal Mechanisms of Acid-Base Balance (pp. 1006–1009; Figs. 26.12–26.14)
1. Only the kidneys can rid the body of acids generated by cellular metabolism, while
also regulating blood levels of alkaline substances and renewing chemical buffer
components.
E. Abnormalities of Acid-Base Balance (pp. 1009–1012; Table 26.3)
1. The partial pressure of CO2 in the blood is the most important indicator of adequate
respiratory function.
2. Metabolic acidosis and alkalosis is related to any factor except loss of CO2 in the lungs.
a. Metabolic acidosis is characterized by low blood pH and bicarbonate levels and is
due to excessive loss of bicarbonate ions or ingestion of too much alcohol.
b. Metabolic alkalosis is indicated by rising blood pH and bicarbonate levels and is the
result of vomiting or excessive base intake.
4. Respiratory rate and depth increase during metabolic acidosis and decrease during
metabolic alkalosis in order to change the rate of loss of CO2 in the lungs.
CO
V. Developmental Aspects of Fluid, Electrolyte, and Acid-Base Balance (p. 1012)
A. An embryo and young fetus are more than 90% water, but as solids accumulate, the
percentage declines to about 70–80% at birth.
B. Distribution of body water begins to change at 2 months of age and takes on adult
distribution by the time a child is 2 years of age.
Cross References
Additional information on topics covered in Chapter 26 can be found in the chapters listed below.
1. Chapter 2: Ions; water; acid-base reactions and pH
2. Chapter 3: Sodium-potassium pump; membrane transport (osmosis, diffusion)
5. Chapter 17: Plasma
6. Chapter 19: Baroreceptors; capillary exchange
7. Chapter 22: Carbon dioxide and bicarbonate; hemoglobin and pH control
Lecture Hints
1. Clearly define the boundaries of each fluid compartment, and stress the dynamic nature
2. Refer students to a review of osmosis and diffusion. A thorough understanding of the
movements of solute and solvent are crucial for comprehension of fluid/electrolyte
balance.
3. Stress the different solute compositions of intracellular and extracellular compartments.
4. Emphasize that water will always move with solutes whenever possible. Water cannot be
5. Remind the class of blood pressure control by nervous, renal, and hormonal mechanisms.
All of these control systems are highly integrated, and this is an opportunity to illustrate
the cooperative nature of body systems in maintaining homeostasis.
6. Stress the importance of acid-base balance and levels of intracellular potassium, espe-
7. Emphasize the importance of acidity or basicity on all chemical reactions.
9. Emphasize the difference between using strong acid-base combinations versus weak
acid-base combinations as buffering systems. Relating the two makes it easier for
students to realize the need for the latter.
10. Clearly distinguish between metabolic and respiratory acids and bases.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
2. Demonstrate the principles of osmosis and diffusion as a reminder to students about
those processes.
4. To help students visualize how solute imbalances affect cells, obtain some dialysis tubing
and create “cells” by filling sections of the tubing with saline solution and tying off the
Critical Thinking/Discussion Topics
1. Discuss the effects of IV therapy on the fluid and electrolyte balance in the body;
distinguish between the infant or small child and the adult.
Library Research Topics
1. Research the rationale behind taking arterial blood gas values to help determine acid-base
balance.
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 26.1 The major fluid compartments of the body.
Figure 26.4 Major sources of water intake and output.
Figure 26.5 The thirst mechanism for regulating water intake.
Figure 26.6 Mechanisms and consequences of ADH release.
maintain blood pressure homeostasis.
Figure 26.11 Dissociation of strong and weak acids in water.
Figure 26.12 Reabsorption of filtered HCO3
– is coupled to H+ secretion.
Table 26.2 Sodium Concentration and Sodium Content
Table 26.3 Causes and Consequences of Acid-Base Imbalances
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
14. The body fluid compartments include the intracellular fluid compartment, located inside
with fluid volume of approximately 15 liters. (p. 991)
15. A decrease in plasma volume of 10–15% and/or an increase in plasma osmolality of
2–3% results in a dry mouth and excites the hypothalamic thirst or drinking center.
Hypothalamic stimulation occurs because the osmoreceptors in the thirst center become
16. It is important to control the extracellular fluid (ECF) osmolality because the ECF
determines the ICF volume and underlies the control of the fluid balance in the body. The
17. Sodium is pivotal to fluid and electrolyte balance and to the homeostasis of all body
systems because it is the principal extracellular ion. While the sodium content of the body
may be altered, its concentration in the ECF remains stable because of immediate adjust-
ments in water volume. The regulation of the sodium-water balance is inseparably linked to
18. Respiratory system regulation of acid-base balance provides a physiological buffering
system. Falling pH, due to rising hydrogen ion concentration or 2
CO
P in plasma, excites
the respiratory center (directly or indirectly) to stimulate deeper, more rapid respirations.
When pH begins to fall, the respiratory center is inhibited. (p. 1006)
19. Chemical acid-base buffers prevent pronounced changes in H+ concentration by binding
to hydrogen ions whenever the pH of body fluids drops and releasing them when pH
rises. (p. 1005)
21. Factors that place newborn babies at risk for acid-base imbalances include very low
residual volume of infant lungs, high rate of fluid intake and output, relatively high meta-
bolic rate, high rate of insensible water loss, and inefficiency of the kidneys. (p. 1012)
Critical Thinking and Clinical Application Questions
1. This patient has diabetes insipidus caused by insufficient production of ADH by the
hypothalamus. The operation for the removal of the cerebral tumor has damaged the
2. Problem 1: pH 7.63, 2
CO
P 19 mm Hg, HCO3
– 19.5 m Eq/L
a. The pH is elevated = alkalosis.
b. The 2
CO
P is low and is the cause of the alkalosis.
3. Emphysema impairs gas exchange or lung ventilation, leading to retention of carbon
dioxide and respiratory acidosis. Congestive heart failure produces oxygenation problems
as well as edema and causes metabolic acidosis due to an increase in lactic acid.
(pp. 1009–1010)
4. The patient has a normal sodium ion concentration; CO2 is slightly low, as is Cl–. The
potassium ion concentration is so abnormal that the patient has a medical emergency.
The greatest danger is (c) cardiac arrhythmia and cardiac arrest. (p. 998)
Suggested Readings
Adeva, M. M., and G. Souto. “Diet-Induced Metabolic Acidosis.” Clinical Nutrition 30 (4)
(Aug. 2011): 416–421.
Gennari, F. J. “Pathophysiology of Metabolic Alkalosis: A New Classification Based on the
Centrality of Stimulated Collecting Duct Ion Transport.” American Journal of Kidney
Diseases 58 (4) (Oct. 2011): 626–636.
Hunt, Curtiss D., and L. K. Johnson. “Calcium Requirements: New Estimations for Men
and Women by Cross-Sectional Statistical Analyses of Calcium Balance Data from
Metabolic Studies.” The American Journal of Clinical Nutrition 86 (4) (Oct. 2007):
1054–1063.
Karet, F. E. “Disorders of Water and Acid-Base Homeostasis.” Nephron. Physiology 118 (1)
(2011): 28–34.
Koeppen, B. M. “The Kidney and Acid-Base Regulation.” Advances in Physiology Education
33 (4) (Dec. 2009): 275–281.
Shirreffs, Susan M., et al. “Rehydration After Exercise in the Heat: A Comparison of 4
Commonly Used Drinks.” International Journal of Sport Nutrition & Exercise
Metabolism 17 (3) (June 2007): 244–258.
Vestergaard, Peter. “Skeletal Effects of Systemic and Topical Corticosteroids.” Current Drug
Safety 3 (3) (Sept. 2008): 190–193.
