Chapter 025

The Human Body in Health and Illness, 4th edition

Barbara Herlihy

Chapter 25:Water, Electrolyte,

and Acid-Base Balance

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Lesson 25-1 Objectives

• Describe the two main fluid compartments.• Define intake and output.• List factors that affect electrolyte balance.• Describe the most common ions found in the

intracellular and extracellular compartments.• List three mechanisms that regulate pH in the

body.• Discuss acid-base imbalances, acidosis and

alkalosis.

Copyright © 2011, 2007 by Saunders, an imprint of Elsevier Inc. All rights

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Body Fluids: Distribution


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• Extracellular compartmentwater outside cells– Interstitial fluid– Plasma– Lymph – Transcellular fluid

• Intracellular compartmentwater inside cells

Composition of Body Fluid

Extracellular• Na+

• Cl-

• HCO3-

• Plasma contains more protein than other extracellular compartments.

Intracellular• K+

• Phosphate• Mg2+


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Water Balance: Intake = Output

•Intake Average 2500 mL/24 hours Regulated by thirst

•Output Average 2500 mL/24 hours Leaves via kidneys, skin, lungs, digestive tract Regulated primarily by kidneys


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Water Imbalances

• Dehydration– Deficiency of body water in interstitium– Poor skin turgor and “tenting”– Untreated hypovolemia, shock

• Edema– Excess body water, usually in interstitium – Clinical assessmentdaily weights– Sudden weight gain suggests water retention. 2.2 lb = 1 liter of water


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Fluid Spacing: Where the Water Is

• First spacing: Normal distribution• Second spacing: Interstitial edema

– Responds to treatment with diuretics, excreted

• Third spacing: Excess water in poorly accessible spaces– Examples: Ascites, water in digestive tract

(paralytic ileus)


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Electrolyte Balance: Review of Terms

• Examples of ions: Na+, Cl-, K+, HCO3-

Cation examples: Na+, K+, Ca2+, Mg2+

Anion examples: Cl-, HCO3-

• Examples of electrolytes: NaCl, KCl, NaHCO3

• Example of ionization: NaCl Na+ + Cl-


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Major Ions: Functions

Sodium (Na+)– Chief extracellular cation– Regulates extracellular volume– Hypernatremia, hyponatremia

Potassium (K+)– Chief intracellular cation– Participates in nerve and muscle function– Hyperkalemia, hypokalemia


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Major Ions: Functions (cont’d.)

• Calcium (Ca2+)– Strengthens bone and teeth– Participates in muscle contraction– Hypocalcemic tetany, hypercalcemia

• Magnesium (Mg2+)– Participates in nerve and muscle function


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Major Ions: Functions (cont’d.)

• Chloride (Cl−)– Chief extracellular anion– Involved in extracellular volume control

• Bicarbonate (HCO3−)

– Part of bicarbonate buffer system– Participates in acid-base balance– Alterations in plasma Cl- affect HCO3

− levels.


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Acid-Base: Review of Terms

• Acid: Dissociates into H+ (the acid) and an anion

• Base: Removes H+ from solution

• pH: Unit of measurement indicating [H+] in solution


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pH Scale


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• Normal blood pH: 7.35-7.45

• Acid [H+], pH, pink scale

Acidosis: pH < 7.35

• Base [H+], pH, blue scale

Alkalosis: pH > 7.45

Regulation of pH

• Buffers: First line of defense in regulating pH

• Lungs: Second line of defense

• Kidneys: Third line of defense


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Acid-Base Balance: Buffers

• Buffers maintain normal pH.• Always work in pairs

– One part of a buffer (base) removes H+.

– The other part (acid) donates H+.

• Buffer systems– Bicarbonate-carbonic acid buffer pair– Other buffer pairs

15Copyright © 2011, 2007 by Saunders,

an imprint of Elsevier Inc. All rights reserved.

Renal Control of Acid-Base

• Reabsorb or excrete H+, as needed.• Reabsorb or excrete bicarbonate, as needed.

– Kidneys reabsorb filtered bicarbonate.– Kidneys also make bicarbonate.

17Copyright © 2011, 2007 by Saunders,


Causes of ImbalancesAcid-Base Imbalances

Acidosis Alkalosis

Respiratory Metabolic Respiratory Metabolic


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Compensations (C) for Imbalances Acidosis Alkalosis

Respiratory Metabolic Respiratory Metabolic

C C C C

Kidneys Lungs Kidneys LungsCopyright © 2011, 2007 by Saunders,


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Respiratory Acidosis


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• Hypoventilation (B)• CO2 retention,

emphysema – H+

– pH• Renal compensation

Kidneys excrete H+. They make and reabsorb HCO3

-.

Metabolic Acidosis

• Caused by nonrespiratory conditions• Example: Diabetic ketoacidosis (DKA)

H+ H+ H+ H+ H+

• CO2 + H2O ⇄ H2CO3 ⇄ H+ + HCO3

-

Respiratory compensation (Kussmaul respirations)Copyright © 2011, 2007 by Saunders,


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Respiratory Alkalosis


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• Hyperventilation (C) (example is a panic attack)– CO2 excretion– H+

– pH • Renal compensation

– Kidneys retain H+.– They excrete HCO3

-.

Metabolic Alkalosis

• Caused by nonrespiratory conditions– Example: Persistent vomiting (loss of H+)

• Loss of H+ drives the reaction to the right to produce more H+

CO2 + H2O H2CO3 ⇄ H+ + HCO3-

• Compensation: Hypoventilation – Increased [CO2] triggers production of H+Copyright © 2011, 2007 by Saunders,


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Chapter 025

Health & Medicine

Transcript of Chapter 025