Download Ch 26 summary and more Essays (university) Advanced Physics in PDF only on Docsity!
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. 2.Indicate the relative fluid volume and solute composition of the fluid compartments of the body. 3.Contrast the overall osmotic effects of electrolytes and nonelectrolytes. 4.Describe factors that determine fluid shifts in the body.
Water Balance and ECF Osmolality
5.List the routes by which water enters and leaves the body. 6.Describe feedback mechanisms that regulate water intake and hormonal controls of water output in urine. 7.Explain the importance of obligatory water losses. 8.Describe possible causes and consequences of dehydration, hypotonic hydration, and edema.
Electrolyte Balance
9.Indicate routes of electrolyte entry and loss from the body.
10.Describe the importance of ionic sodium in fluid and electrolyte balance of the body, and indicate its relationship to normal cardiovascular system functioning.
11.Describe mechanisms involved in regulating sodium balance, blood volume, and blood pressure.
12.Explain how potassium, calcium, and anion balances in plasma are regulated.
Acid-Base Balance
13.List important sources of acids in the body.
14.Name the three major chemical buffer systems of the body and describe how they resist pH changes.
15.Describe the influence of the respiratory system on acid-base balance. 16.Describe how the kidneys regulate hydrogen and bicarbonate ion concentrations in the blood.
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
18.Explain why infants and the aged are at greater risk for fluid and electrolyte imbalances than are young adults.
Chapter Outline
I.Body Fluids (pp. 996–998; Figs. 26.1–26.3)
A. Body Water Content (p. 996)
- Total body water is a function of age, body mass, and body fat. a. Due to their low body fat and bone mass, infants are about 73% water. b. The body water content of men is about 60%, but because women have relatively more body fat and less skeletal muscle than men, theirs is about 50%.
- Body water declines throughout life, ultimately comprising about 45% of total body mass in old age. B. Fluid Compartments (p. 996; Fig. 26.1)
- There are two main fluid compartments of the body: The intracellular compartment contains slightly less than two-thirds by volume; the remaining third is distributed in the extracellular fluid.
- There are two subcompartments of the extracellular fluid: blood plasma and interstitial fluid. C. Composition of Body Fluids (pp. 996–998)
- Nonelectrolytes include most organic molecules, do not dissociate in water, and carry no net electrical charge.
- Electrolytes dissociate in water to ions, and include inorganic salts, acids and bases, and some proteins.
- Electrolytes have greater osmotic power because they dissociate in water and contribute at least two particles to solution.
- The major cation in extracellular fluids is sodium, and the major anion is chloride; in intracellular fluid the major cation is potassium, and the major anion is phosphate.
- Electrolytes are the most abundant solutes in body fluids, but proteins and some nonelectrolytes account for 60–97% of dissolved solutes. D. Fluid Movement Among Compartments (p. 998; Figs. 26.2–26.3)
- Anything that changes solute concentration in any compartment leads to net water flows.
- 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.
- Nearly protein-free plasma is forced out of the blood by hydrostatic pressure, and almost completely reabsorbed due to colloid osmotic (oncotic) pressure of plasma proteins.
- Movement of water between the interstitial fluid and intracellular fluid involves substantial two-way osmotic flow that is equal in both directions.
- Ion fluxes between the interstitial and intracellular compartments are restricted; but movement of nutrients, respiratory gases, and wastes typically occur in one direction.
II.Water Balance and ECF Osmolality (pp. 998–1002; Figs. 26.4–26.7)
A. For the body to remain properly hydrated, water intake must equal water output (pp. 998–999).
- Most water enters the body through ingested liquids and food, but is also produced by cellular metabolism.
- Water output is due to evaporative loss from lungs and skin (insensible water loss), sweating, defecation, and urination. B. Regulation of Water Intake (pp. 999–1000; Figs. 26.4–26.5)
- Atrial natriuretic peptide reduces blood pressure and blood volume by inhibiting release of ADH, renin, and aldosterone, and directly causing vasodilation.
- Estrogens are chemically similar to aldosterone, and enhance reabsorption of salt by the renal tubules.
- Glucocorticoids enhance tubular reabsorption of sodium, but increase glomerular filtration. C. Regulation of Potassium Balance (pp. 1006–1007)
- 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 the cell.
- Potassium balance is chiefly regulated by renal mechanisms, which control the amount of potassium secreted into the filtrate.
- Blood plasma levels of potassium are the most important factor regulating potassium secretion.
- Aldosterone influences potassium secretion, because potassium secretion is simultaneously enhanced when sodium reabsorption increases. D. Regulation of Calcium and Phosphate Balance (p. 1008)
- Calcium ion levels are closely regulated by parathyroid hormone and calcitonin; about 98% is reabsorbed. a. Parathyroid hormone is released when blood calcium levels decline, and targets the bones, small intestine, and kidneys. b. Calcitonin is an antagonist to parathyroid hormone, and is released when blood calcium rises, targeting bone. E. Regulation of Anions (p. 1008)
- Chloride is the major anion reabsorbed with sodium, and helps maintain the osmotic pressure of the blood.
IV.Acid-Base Balance (pp. 1008–1015; Figs. 26.11–26.14; Table 26.2)
A. Because of the abundance of hydrogen bonds in the body’s functional proteins, they are strongly influenced by hydrogen ion concentration (pp. 1008–1009).
- 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.
- Most hydrogen ions originate as metabolic by-products, although they can also enter the body via ingested foods. B. Chemical Buffer Systems (pp. 1009–1010; Fig. 26.11)
- 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.
- 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 bicarbonate ions of the salt bind excess H+^ , forming more carbonic acid. b. When a strong base is added to solution, the sodium bicarbonate remains relatively unaffected, but carbonic acid dissociates further, donating more H +^ to bind the excess hydroxide.
c. Bicarbonate concentration of the extracellular fluid is closely regulated by the kidneys, and plasma bicarbonate concentrations are controlled by the respiratory system.
- 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.
- 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+^ (pp. 1010–1011)
- Carbon dioxide from cellular metabolism enters erythrocytes and is converted to bicarbonate ions for transport in the plasma.
- When hypercapnia occurs, blood pH drops, activating medullary respiratory centers, resulting in increased rate and depth of breathing and increased unloading of CO 2 in the lungs.
- When blood pH rises, the respiratory center is depressed, allowing CO 2 to accumulate in the blood, lowering pH. D. Renal Mechanisms of Acid-Base Balance (pp. 1011–1014; Figs. 26.12–26.14)
- 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. a. Bicarbonate ions can be conserved from filtrate when depleted, and their reabsorption is dependent on H+^ secretion. b. Type A intercalated cells of the renal tubules can synthesize new bicarbonate ions while excreting more hydrogen ions. c. Ammonium ions are weak acids that are excreted and lost in urine, replenishing the alkaline reserve of the blood. d. When the body is in alkalosis, type B intercalated cells excrete bicarbonate, and reclaim hydrogen ions. E. Abnormalities of Acid-Base Balance (pp. 1014–1015; Table 26.2)
- Respiratory acidosis is characterized by falling blood pH and rising PCO 2 , which can result from shallow breathing or some respiratory diseases.
- Respiratory alkalosis results when carbon dioxide is eliminated from the body faster than it is produced, such as during hyperventilation.
- 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.
- Metabolic alkalosis is indicated by rising blood pH and bicarbonate levels, and is the result of vomiting or excessive base intake.
- Respiratory rate and depth increase during metabolic acidosis, and decrease during metabolic alkalosis.
- In renal compensation for respiratory acidosis, blood P (^) CO2 and bicarbonate ion concentrations are high; in respiratory alkalosis, blood pH is high, but P (^) CO2 is low.
V. Developmental Aspects of Fluid, Electrolyte, and Acid-Base Balance 0 0 0 7(p.
Interactive Physiology ®^ 10-System Suite: Water Homeostasis Activity: Mechanisms and Consequences of ADH Release Section 26.3 Electrolyte Balance (pp. 1002–1008) MP3 Tutor Session: Regulation of Blood Volume and Blood Pressure Interactive Physiology ®^ 10-System Suite: Electrolyte Homeostasis Case Study: Fluids and Electrolytes Section 26.4 Acid-Base Balance (pp. 1008–1015) Interactive Physiology ®^ 10-System Suite: Acid/Base Homeostasis PhysioEx™ 8.0: Acid/Base Balance Case Study: Renal Failure Section 26.5 Developmental Aspects of Fluid, Electrolyte, and Acid-Base Balance (p. 1015) Chapter Summary Crossword Puzzle 26. Web Links Chapter Quizzes Art Labeling Quiz Matching Quiz Multiple-Choice Quiz True-False Quiz Chapter Practice Test Study Tools Histology Atlas myeBook Flashcards Glossary
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix G of the main text.
Short Answer Essay Questions
14.The body fluid compartments include the intracellular fluid compartment, located inside the cells with fluid volume of approximately 25 liters, and the extracellular fluid compartment (plasma and interstitial fluid), located in the external environment of each cell with fluid volume of approximately 15 liters. (p. 996)
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 irritable and depolarize as water, driven by the hypertonic ECF, moves out of them by osmosis. Collectively, these events cause a subjective sensation of thirst. The quenching of thirst begins as the mucosa of the mouth and throat are moistened and continues as stretch receptors in the stomach and intestine are activated, providing feedback signals that inhibit the hypothalamic thirst center. (p. 999)
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 ECF is maintained by both thirst and the antidiuretic hormone (ADH). A rise in plasma osmolality triggers thirst and the release of ADH; a drop in plasma osmolality inhibits thirst and ADH. (pp. 999–1000)
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 adjustments in water volume. The regulation of the sodium-water balance is inseparably linked to blood pressure and entails a variety of neural and hormonal controls: (1) aldosterone—increases the reabsorption of sodium from the filtrate; water follows passively by osmosis, increasing blood volume (and pressure). The renin-angiotensin mechanism is an important control of aldosterone release; the juxtaglomerular apparatus responds to: (a) decreased stretch (due to decreased blood pressure), (b) decreased filtrate osmolality, or (c) sympathetic nervous system stimulation, resulting ultimately in aldosterone release from the adrenal cortex. (2) ADH—osmoreceptors in the hypothalamus sense solute concentration in the ECF: increases in sodium content stimulate ADH release, resulting in increased water retention by the kidney (and increasing blood pressure). (3) Atrial natriuretic peptide—released by cells in the atria during high-pressure situations, it has potent diuretic and natriuretic (sodium- excreting) effects; the kidneys do not reabsorb as much sodium (therefore water) and blood pressure drops. (pp. 1002–1007)
18.Respiratory system regulation of acid-base balance provides a physiological buffering system. Falling pH, due to rising hydrogen ion concentration or P (^) co 2 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. (pp. 1010–1011)
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.
20.(a) The rate of H+^ secretion rises and falls directly with CO 2 levels in the ECF. The higher the
content of CO 2 in the peritubular capillary blood, the faster the rate of H +^ secretion. (b) Type A intercalated cells secrete H +^ actively via a H+^ -ATPase pump and via a K +^ -H+ antiporter. The secreted H+^ combines with HPO 4 2–^ , forming H 2 PO 4 –^ , which then flows out in urine. (c) The dissociation of carbonic acid in the tubule cells liberates HCO 3 –^ as well as H +^. HCO 3 –^ is shunted into the peritubular capillary blood. The rate of reabsorption of bicarbonate depends on the rate of secretion or excretion of H +^ in the filtrate. (p. 1012)
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 metabolic rate, high rate of insensible water loss, and inefficiency of the kidneys. (p. 1015)
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 hypothalamus or the hypothalamohypophyseal tract leading to the posterior pituitary. Because of the lack of ADH, the collecting tubules and possibly the convoluted part of the distal convoluted tubule are not absorbing water from the glomerular filtrate. The large
