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Martini’s VisualAnatomy and Physiology

First Edition

Martini Ober

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Chapter 24

Water, Electrolytes, and Acid/Base Balance

Lecture 19

Lecture Overview

• Overview

• Fluid (water) balance– Compartments

– Body fluid composition

I l fl id hif

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– Intercompartmental fluid shifts

• Electrolyte balance

• Acid-base balance– Buffer systems

– Acidosis and alkalosis

Overview

• Our survival depends upon maintaining a normal volume and composition of– Extracellular fluid (ECF)

– Intracellular fluid (ICF)

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• Ionic concentrations and pH are critical

• Three interrelated processes– Fluid balance (How does water move?)

– Electrolyte balance (What is an electrolyte?)

– Acid-base balance (What is normal pH?)

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Water Content of the Human Body

Of the 40 liters of water in the body of an average adult male:

one third (15L) is

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- one-third (15L) is extracellular

- two-thirds (25L) is intracellular

Figure from: Hole’s Human A&P, 12th edition, 2010

Fluid CompartmentsFigure from: Hole’s Human A&P, 12th edition, 2010

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‘Compartments’ commonly behave as distinct entities in terms of ion distribution, but ICF and ECF osmotic concentrations (about 290 mOsm/L) are identical. Why?

Osmolarity and Milliequivalents (mEq)

• Recall that osmolarity expresses total solute concentration of a solution– Osmolarity (effect on H2O) of body solutions is determined

by the total number of dissolved particles (regardless of where they came from)

The term ‘osmole’ reflects the number of particles yielded

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– The term osmole reflects the number of particles yielded by a particular solute (milliosmole, mOsm, = osmole/1000)

• 1 mole of glucose (180g/mol)

• 1 mole of NaCl (58g/mol)

• An equivalent is the positive or negative charge equal to the amount of charge in one mole of H+

– A milliequivalent (mEq) is one-thousandth of an Eq

– Number of Eq = molecular wt. / valence

-> 1 osmole of particles

-> 2 osmoles of particles

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Body Fluid Ionic Composition

ECF major ions:

- sodium, chloride, and bicarbonate


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ICF major ions:

- potassium, magnesium, and phosphate (plus negatively charged proteins)

You should know these chemical symbols and charges of ions

Movement of Fluids Between Compartments

Water moves between mesothelial surfaces: peritoneal, pleural, and pericardial cavities as well as the synovial membranes. It also moves between the blood and CSF


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Net movements of fluids between compartments result fromdifferences in hydrostatic and osmotic pressures

the blood and CSF and through the fluids of the eye and ear

Fluid (Water) Balance

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* urine production is the most important regulator of water balance (water in = water out)


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Water Balance and ECF Osmolarity

• Regulation of water intake• increase in osmotic pressure of ECF → osmoreceptors in hypothalamic thirst center → stimulates thirst and drinking

• Regulation of water output

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• Obligatory water losses (must happen)• insensible water losses (lungs, skin)• water loss in feces• water loss in urine (min about 500 ml/day)

• increase in osmotic pressure of ECF → ADH is released• concentrated urine is excreted• more water is retained

• LARGE changes in blood vol/pressure → Renin and ADH release

Fluid Imbalance

12Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Dehydration and OverhydrationDehydration

• osmotic pressure increases in extracellular fluids• water moves out of cells• osmoreceptors in h th l ti l t d

Overhydration• osmotic pressure decreases in extracellular fluids• water moves into cells• osmoreceptors inhibited i h th l

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hypothalamus stimulated• hypothalamus signals posterior pituitary to release ADH• urine output decreases

in hypothalamus• hypothalamus signals posterior pituitary to decrease ADH output• urine output increases

‘Drunken’ behavior (water intoxication), confusion, hallucinations, convulsions, coma, death

Severe thirst, wrinkling of skin, fall in plasma volume and decreased blood pressure, circulatory shock, death

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Electrolyte Balance

Electrolyte balance is important:

1. Regulates fluid (water) balance


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2. Concentrations of individual electrolytes can affect cellular functions

Na+: major cation in ECF (plasma: 136-142 mEq/L; Avg ≈ 140)

K+: major cation in ICF (plasma: 3.8-5.0 mEq/L; Avg ≈ 4.0)

Regulation of Osmolarity

Osmolarity i l t d

Recall: [Na+] Osmolarity

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Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001

is regulated by altering H2O content

Fluid Volume Regulation and [Na+]Estrogens are chemically similar to aldosterone and enhance NaCl absorption by renal tubules

Glucocorticoids can also enhance tubular reabsorption of Na+

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Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001

reabsorption of Na+

Volume is regulated by altering Na+

content

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Summary Table of Fluid and Electrolyte Balance

Condition Initial Change Initial Effect Correction Result

Change in OSMOLARITY

(**Corrected by change in H2O levels)

H2O in the ECF

Na+ concentration,

ECF osmolarity

Thirst → H2O intake

ADH → H2O output H2O in the ECF

H2O in the ECF

Na+ concentration,

ECF osmolarity

Thirst → H2O intake

ADH → H2O output H2O in the ECF

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Change in VOLUME(**Corrected by change

in Na+ levels)

H2O/Na+ in the ECF volume,

BP

Renin-angiotensin: ThirstADH

aldosterone vasoconstriction

H2O intake Na+/H2O reabsorption

H2O loss

H2O/Na+ in the ECF volume,

BP

Natriuretic peptides: ThirstADH

aldosterone

H2O intake Na+/H2O reabsorption

H2O loss

You should understand this table

Potassium Balance

Potassium loss generally occurs via the urine. The rate of tubular secretion of K+ varies with:

1. Changes in the [K+]


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in the ECF

2. Changes in pH

3. Aldosterone levels

Remember that Na+ can be exchanged for H+ or K+ in the nephron tubules

Calcium Balance

[Ca2+] in ECF is about 5 mEq/L


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Strengths of Acids and Bases

• Weak acids ionize less completely and release fewer H+

(**allows them to act as buffers)

• Strong acids ionize more completely and release more H+

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• Weak bases ionize less completely and bind fewer H+

• Strong bases ionize more completely and bind more H+

( allows them to act as buffers)

Sources of Hydrogen Ions


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Some H+ is also absorbed from the digestive tract

Regulation of Hydrogen Ion Concentration

1. chemical acid-base buffer systems (physical buffers)• first line of defense• can tie-up acids or bases, but cannot eliminate them• act in seconds

2 respiratory excretion of carbon dioxide

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2. respiratory excretion of carbon dioxide• a physiological buffer (can eliminate excess acid indirectly via CO2)• minutes

3. renal excretion of hydrogen ions• a physiological buffer (can eliminate excess metabolic acids directly, e.g., keto-, uric, lactic, phosphoric)• hours to a day

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Acid-Base Buffer Systems

Bicarbonate System• the bicarbonate ion converts a strong acid to a weak acid• carbonic acid converts a strong base to a weak base• an important buffer of the ECF (~ 25 mEq/L)

H+ + HCO3- ↔ H2CO3↔ CO2 + H2O

Strong acid Weak acid

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H HCO3 H2CO3 CO2 H2O

Phosphate 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

-

Strong acid Weak acid

Acid-Base Buffer Systems

Protein Buffer SystemICF, plasma proteins, Hb

NH2 group accepts

COOH group releases

Most plentiful and powerful chemical buffer system

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phydrogen ions when pH falls

hydrogen ions when pH rises

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Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Respiratory Excretion of Carbon Dioxide

A physiologicalbuffer system


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Renal Excretion of Hydrogen Ions

*The kidney is most powerful and versatile acid-base system in the body


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Note that secretion of H+

relies on carbonic anhydrase activity within tubular cells

Net result is secretion of H+ accompanied by the

Buffering Mechanisms in the Kidney

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H accompanied by the (1)retention of HCO3

-

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Production of new HCO3

-

(2)

Summary of Acid-Base BalanceFigure from: Hole’s Human A&P, 12th edition, 2010

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Know this slide!

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Acidosis and Alkalosis

If the pH of arterial blood drops to 6.8 or rises to 8.0 for more than a few hours, survival is jeopardized

Classified according to:

1. Whether the cause is respiratory (CO2),

t b li ( th

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or metabolic (other acids, bases)

2. Whether the blood pH is acid or alkaline


Acidosis

(hypopnea)


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Respiratory acidosis Metabolic acidosis

Nervous system depression, coma, death

AlkalosisFigure from: Hole’s Human A&P, 12th edition, 2010

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Respiratory alkalosis Metabolic alkalosis

Nervousness, tetany, convulsions, death

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Acidosis and Alkalosis

• What would be the indications of acidosis and alkalosis in terms of changes in pH and PCO2? pH and HCO3

-?

• How would the body try to compensate for

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– Acidosis• Respiratory• Metabolic

– Alkalosis• Respiratory• Metabolic

See Lab Guide Handout: Marieb, Human Anatomy & Physiology, Pearson, 2004

Flow chart for Acidosis/Alkalosis

Three things to check: 1) pH – 7.35-7.452) pCO2 – 35-45 mm Hg3) HCO3

- - 22 – 26 mEq/LpH

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acidosis alkalosis

pCO2

respiratory metabolic respiratory metabolic

HCO3- pCO2 HCO3

-

pCO2

Comp Comp CompCompNo Comp

No CompNo CompNo Comp

pCO2

HCO3-

Norm HCO3

-Norm HCO3

- HCO3-

Norm pCO2

Norm pCO2

Review

• There are two major fluid compartments of the body– Intracellular

• About 2/3 of body’s fluid

l d h fl id i hi ll

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• Includes the fluid within cells

• Major ions: K+, Mg2+, PO43-, Proteins

– Extracellular• About 1/3 of body’s fluid

• Includes interstitial fluid, plasma, lymph, and transcellular fluid

• Major ions: Na+, Cl-, HCO3-

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Review

• There are two major forces affecting movement of fluid between compartments– Hydrostatic Pressure– Osmotic Pressure

• Fluid balance

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• Fluid balance– Amount of water you take in is equal to the

amount of water you lose to the environment– Intake of water in food/drink is the most

important source of fluid– Kidney regulation of water is the most

important regulator of water loss

Review

• Electrolyte balance– Balance: Gains and losses of every electrolyte are equal

– Electrolyte balance is important because• It directly affects water balance

• Electrolyte concentrations affect cell processes

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– Na+ (aldosterone, ADH, ANP)• Increased [Na+ ] in ECF -> ↑ ADH, ↑ ANP

• Decreased [Na+ ] in ECF -> ADH, ↑ aldosterone

– K+ ([K+] in plasma, aldosterone)• Increased [K+ ] in ECF -> increased secretion, ↑ aldosterone

• Decreased [K+ ] in ECF -> decreased secretion, aldosterone,

Review

• Electrolyte balance (cont’d)– Ca2+ (PTH, calcitriol, calcitonin)

• Increase in ECF -> calcitonin promotes bone deposition• Decrease in ECF -> PTH , calcitriol

– ↑ intestinal absorption– ↑ bone resorption

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– Ca2+ secretion, ↑ PO43- secretion

• Acid-base balance– Production of H+ is exactly offset by the loss of H+

– Major mechanisms of maintaining• acid-base (chemical) buffer systems: HCO3

-, PO43-, protein

• respiratory excretion of carbon dioxide• renal excretion of hydrogen ions

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Review

• Acidosis (pH < 7.35)– Excessive H+ in the plasma

– Respiratory acidosis

– Metabolic acidosis

• Alkalosis (pH > 7 45)

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• Alkalosis (pH > 7.45)– Insufficient H+ in the plasma

– Respiratory alkalosis

– Metabolic alkalosis

• Compensations

Review

Electrolyte Concentration Range (mEq/L)

Typical Value (mEq/L)

Na+

136 - 142 140

K+

3.8 - 5.0 4.0

Ca2+

4.5 – 5.8 5.0

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Cl-

96 - 106 105

HCO3-

24 - 28 25

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