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1
Hole’s Human Anatomyand Physiology
Twelfth Edition
Shier Butler Lewis
Chapter 21
Water, Electrolyte, andAcid-Base Balance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2
21.1: Introduction
• The term balance suggests a state of equilibrium• For water and electrolytes that means equal amounts enter and leave the body• Mechanisms that replace lost water and electrolytes and excrete excesses maintain this balance• This results in stability of the body at all times• Keep in mind water and electrolyte balance are interdependent
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21.2: Distribution of Body Fluids
• Body fluids are not uniformly distributed• They occupy compartments of different volumes that contain varying compositions• Water and electrolyte movement between these compartments is regulated to stabilize their distribution and the composition of body fluids
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Fluid Compartments
• Of the 40 liters of water in the body of an average adult, about two-thirds is intracellular fluid and one-third is extracellular fluid• An average adult female is about 52% water by weight, and an average male about 63% water by weight
Extracellularfluid(37%)
Intracellularfluid(63%)
40
38
36
34
32
30
28
26
24
22
20
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16
14
12
10
8
6
4
2
0
Lit
ers
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Total body water
Interstitial fluid
Plasma
Lymph
Transcellular fluid
Extracellular fluid(37%)
Membranes ofbody cells
Intracellular fluid(63%)
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Body Fluid Composition• Extracellular fluids are generally similar in composition including high concentrations of sodium, calcium, chloride and bicarbonate ions
• Intracellular fluids have high concentrations of potassium, magnesium, phosphate, and sulfate ions
Ion
co
nce
ntr
atio
n (
m E
q/L
)
20
30
50
60
70
80
90
100
110
120
130
140
Relative concentrations and ratios of ions in extracellular and intracellular fluids
150
40
10
0
14:1Ratio
(Extracellular: intracellular)
Na+
1:28
K+
5:1
Ca+2
1:19
Mg+2
26:1
Cl
3:1
HCO3
1:19
PO43
1:2
SO42
Extracellular fluid
Intracellular fluid
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Movement of Fluid Between Compartments
• Two major factors regulate the movement of water and electrolytes from one fluid compartment to another
• Hydrostatic pressure• Osmotic pressure
Interstitial fluid
Capillary wall
Transcellularfluid
Lymph
Plasma
Serousmembrane
Intracellularfluid
Cellmembrane
Lymphvessel
Fluid leaves plasmaat arteriolar end ofcapillaries becauseoutward force ofhydrostatic pressurepredominates
Fluid returns toplasma at venularends of capillariesbecause inward forceof colloid osmoticpressure predominates
Hydrostatic pressurewithin interstitialspaces forces fluidinto lymph capillaries
Interstitial fluid isin equilibrium withtranscellular andintracellular fluids
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21.3: Water Balance
• Water balance exists when water intake equals water output• Homeostasis requires control of both water intake and water output
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Water Intake• The volume of water gained each day varies among individuals averaging about 2,500 milliliters daily for an adult:
• 60% from drinking• 30% from moist foods• 10% as a bi-product ofoxidative metabolism ofnutrients called water ofmetabolism
Water inbeverages(1,500 mL or 60%)
Average daily intake of waterWater lost in sweat(150 mL or 6%)
Total output(2,500 mL)
Average daily output of water
(a) (b)
Total intake(2,500 mL)
Water inmoist food(750 mL or 30%)
Water ofmetobolism(250 mL or 10%)
Water lost in feces(150 mL or 6%)
Water lost throughskin and lungs(700 mL or 28%)
Water lost in urine(1,500 mL or 60%)
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Regulation of Water Intake
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Water Output
• Water normally enters the body only through the mouth, but it can be lost by a variety of routes including:
• Urine (60% loss)• Feces (6% loss)• Sweat (sensible perspiration) (6% loss)• Evaporation from the skin (insensible perspiration)• The lungs during breathing(Evaporation from the skin and the lungs is a 28% loss)
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Regulation of Water Output
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21.4: Electrolyte Balance• An electrolyte balance exists when the quantities of electrolytes the body gains equals those lost
Urine
Electrolyte intake
Fluids
Feces
Electrolyte output
Foods
Perspiration
Metabolicreactions
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Electrolyte Intake• The electrolytes of greatest importance to cellular functions release sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen ions• These ions are primarily obtained from foods, but some are from water and other beverages
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Regulation of Electrolyte Intake
• Ordinarily, a person obtains sufficient electrolytes by responding to hunger and thirst• A severe electrolyte deficiency may cause salt craving
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Electrolyte Output
• The body loses some electrolytes by perspiring typically on warmer days and during strenuous exercise• Some are lost in the feces• The greatest output is as a result of kidney function and urine output
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Regulation of Electrolyte Output
• The concentrations of positively charged ions, such as sodium (Na+), potassium (K+) and calcium (Ca+2) are of particular importance• These ions are vital for nerve impulse conduction, muscle fiber contraction, and maintenance of cell membrane permeability
Aldosterone is secreted
Adrenal cortex is signaled
Potassium ionconcentration increases
Renal tubulesincrease reabsorption ofsodium ions and increasesecretion of potassium ions
Sodium ions areconserved and potassiumions are excreted
Calcium ionconcentration decreases
Calcium ion concentrationreturns toward normal
Normal phosphateconcentration is maintained
Addition of phosphateto bloodstream
Renal tubules conservecalcium and increasesecretion of phosphate
Parathyroid glandsare stimulated
Parathyroid hormoneis secreted
Intestinal absorptionof calcium increases
Activity of bone-resorbingosteoclasts increases
Increased phosphateexcretion in urine
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21.5: Acid-Base Balance• Electrolytes that ionize in water and release hydrogen ions are acids• Substances that combine with hydrogen ions are bases• Acid-base balance entails regulation of the hydrogen ion concentrations of body fluids• This is important because slight changes in hydrogen ion concentrations can alter the rates of enzyme-controlled metabolic reactions, shift the distribution of other ions, or modify hormone actions
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Sources of Hydrogen Ions
Aerobicrespirationof glucose
Anaerobicrespirationof glucose
Incompleteoxidation offatty acids
Oxidation ofsulfur-containingamino acids
Hydrolysis ofphosphoproteinsand nucleic acids
Carbonicacid
Lacticacid
Acidic ketonebodies
Sulfuricacid
Phosphoricacid
H+
Internal environment
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Strengths of Acids and Bases
• Acids:• Strong acids ionize more completely and release more H+
• Weak acids ionize less completely and release fewer H+
• Bases:• Strong bases ionize more completely and release more OH-
• Weak bases ionize less completely and release fewer OH-
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Regulation of Hydrogen Ion Concentration
• Either an acid shift or an alkaline (basic) shift in the body fluids could threaten the internal environment• Normal metabolic reactions generally produce more acid than base• The reactions include cellular metabolism of glucose, fatty acids, and amino acids• Maintenance of acid-base balance usually eliminates acids in one of three ways:
• Acid-base buffer systems• Respiratory excretion of carbon dioxide• Renal excretion of hydrogen ions
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Acid-Base Buffer Systems
• Bicarbonate buffer system• The bicarbonate ion converts a strong acid to a weak acid• Carbonic acid converts a strong base to a weak base H+ + HCO3
- H2CO3 H+ + HCO3-
• Phosphate buffer system• The monohydrogen phosphate ion converts a strong acid to a weak acid• The dihydrogen phosphate ion converts a strong base to a weak base H+ + HPO4
-2 H2PO4- H+ + HPO4
-2
• Protein buffer system• NH3+ group releases a hydrogen ion in the presence of excess base• COO- group accepts a hydrogen ion in the presence of excess acid
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Respiratory Secretion of Carbon Dioxide
• The respiratory center in the brainstem helps regulate hydrogen ion concentrations in the body fluids by controlling the rate and depth of breathing• If body cells increase their production of CO2…
More CO2 is eliminated through lungs
Rate and depth of breathing increase
Respiratory center is stimulated
Cells increase production of CO2
CO2 reacts with H2O to produce H2CO3
H2CO3 releases H+
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Renal Excretion of Hydrogen Ions
• Nephrons help regulate the hydrogen ion concentration of body fluids by excreting hydrogen ions in the urine
High intake of proteins
Increased concentrationof H+ in urine
Increased secretionof H+ into fluid ofrenal tubules
Increased concentrationof H+ in body fluids
Increased metabolismof amino acids
Increased formationof sulfuric acid andphosphoric acid
Concentration of H+
in body fluids returnstoward normal
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Time Course of Hydrogen Ion Regulation
• Various regulators of hydrogen ion concentration operate at different rates• Acid-base (chemical) buffers function rapidly• Respiratory and renal (physiological buffers) mechanisms function more slowly
Phosphatebuffer system
Proteinbuffer system
First line of defenseagainst pH shift
Second line ofdefense againstpH shift
Chemicalbuffer system
Physiologicalbuffers
Bicarbonatebuffer system
Respiratorymechanism(CO2 excretion)
Renalmechanism(H+ excretion)
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21.6: Acid-Base Imbalances
• Chemical and physiological buffer systems ordinarily maintain the hydrogen ion concentration of body fluids within very narrow pH ranges• Abnormal conditions may disturb the acid-base balance
7.35
Survival range
Normal pH range
pH scale
7.45 8.07.06.8 7.8
Acidosis Alkalosis
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Acidosis
• Acidosis results from the accumulation of acids or loss of bases, both of which cause abnormal increases in the hydrogen ion concentrations of body fluids• Alkalosis results from a loss of acids or an accumulation of bases accompanied by a decrease in hydrogen ion concentrations
pH rises
pH drops
Increased concentration of H+
pH scale
7.4
Acidosis
Alkalosis
Decreased concentration of H+
Accumulationof acids
Loss ofbases
Loss ofacids
Accumulationof bases
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Acidosis• Two major types of acidosis are respiratory acidosis and metabolic acidosis
Accumulation of CO2
Respiratoryacidosis
Decreased rateand depth ofbreathing
Obstruction ofair passages
Decreasedgas exchange Accumulation of nonrespiratory acids
Metabolic acidosis
Excessive loss of bases
Kidney failureto excrete acids
Excessive production of acidicketones as in diabetes mellitus
Prolonged diarrheawith loss of alkalineintestinal secretions
Prolonged vomitingwith loss of intestinalsecretions
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Alkalosis• Respiratory alkalosis develops as a result of hyperventilation• Metabolic alkalosis results from a great loss of hydrogen ions or from a gain in bases, both accompanied by a rise in the pH of blood
Excessive loss of CO2
Hyperventilation
Decrease in concentration of H2CO3
Decrease in concentration of H+
Respiratory alkalosis
• Anxiety
• Fever
• Poisoning
• High altitude
Loss of acids
Net increase in alkaline substances
Metabolic alkalosis
Gastricdrainage
Vomiting with lossof gastric secretions
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