chapter 26 fluid, electrolytes, and acid/base balance lecture 17 marieb’s human anatomy and...

Chapter 26

Fluid, Electrolytes, and Acid/Base Balance

Lecture 17

Marieb’s HumanAnatomy and

Physiology

Marieb Hoehn

2

Lecture Overview

• Overview

• Fluid (water) balance– Compartments– Body fluid composition– Intercompartmental fluid shifts

• Electrolyte balance

• Acid-base balance– Buffer systems– Acidosis and alkalosis

3

Overview

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

• Ionic concentrations and pH are critical• Three interrelated processes

– Fluid balance (How does water move from one place to the other? )– Electrolyte balance (What is an electrolyte?)– Acid-base balance (What is normal pH?)

4

Water Content of the Human Body

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

- one-third (15L) is extracellular

- two-thirds (25L) is intracellular

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

5

Fluid Compartments

‘Compartments’ commonly behave as distinct entities in terms of ion distribution, but ICF and ECF osmotic concentrations are identical (about 290-300 mOsm/L). Why?


7

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 by a particular solute (milliosmole, mOsm, = osmole/1000)

• 1 mole of glucose (180g/mol)

• 1 mole of NaCl (58g/mol)

• Osmolarity = #moles/L X # particles yielded

• 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

8

Body Fluid Ionic Composition

ECF major ions:

- sodium, chloride, and bicarbonate

ICF major ions:

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

You should know these chemical symbols and charges (valences) of ions


10

Movement of Fluids Between Compartments

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

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


11

Fluid (Water) Balance

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


12

Water Balance and ECF Osmolarity

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

• Regulation of water output• 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

13

Fluid Imbalance

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

14

Dehydration and OverhydrationDehydration (removing only H2O)

• osmotic pressure increases in extracellular fluids• water moves out of cells• osmoreceptors in hypothalamus stimulated• hypothalamus signals posterior pituitary to release ADH• urine output decreases

Overhydration (adding only H2O)

• osmotic pressure decreases in extracellular fluids• water moves into cells• osmoreceptors inhibited 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

15

Electrolyte Balance

Electrolyte balance is important since:

1.It regulates fluid (water) balance

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)


16

Regulation of OsmolarityFigures from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Osmolarity is regulated by altering H2O content

Recall: [Na+] Osmolarity

** Osmolarity = Amt of solute / volume of H2O

17

Fluid Volume Regulation and [Na+]

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

chemically similar to aldosterone and enhance NaCl absorption by renal tubules

Glucocorticoids can also enhance tubular reabsorption of Na+

Volume is regulated by altering Na+ content

18

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

Change in VOLUME(**Corrected by change

in Na+ levels)

H2O/Na+ in the ECF volume,

BP

Renin-angiotensin: Thirst ADH

aldosterone vasoconstriction

H2O intake

Na+/H2O reabsorption

H2O loss

H2O/Na+ in the ECF volume,

BP

Natriuretic peptides: Thirst ADH

aldosterone

H2O intake

Na+/H2O reabsorption

H2O loss

You should understand this table

19

Potassium Balance

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

1. Changes in the [K+] in the ECF

2. Changes in pH

3. Aldosterone levels

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


20

Calcium Balance

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


21

Strengths of Acids and Bases

• Weak bases ionize less completely and bind fewer H+

• Strong bases ionize more completely and bind more H+

• Weak acids ionize less completely and release fewer H+ (**allows them to act as buffers)

• Strong acids ionize more completely and release more H+

22

Sources of Hydrogen Ions

Some H+ is also absorbed from the digestive tract


23

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• 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

24

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

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

Strong acid Weak acid

25

Acid-Base Buffer Systems

Protein Buffer SystemICF, plasma proteins, Hb

NH2 group accepts hydrogen ions when pH falls

COOH group releases hydrogen ions when pH rises

-

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

Most plentiful and powerful chemical buffer system

26

Respiratory Excretion of Carbon Dioxide

A physiological buffer system


27

Renal Excretion of Hydrogen Ions

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


28

Note that secretion of H+ relies on carbonic anhydrase activity within tubular cells

Net result is secretion of H+ accompanied by the (1)retention of HCO3

-

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

Buffering Mechanisms in the Kidney

Production of new HCO3

-

(2)

29

Summary of Acid-Base Balance

Know this slide!


(Seconds)

(Minutes)

(Hours-Days)

30

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), or metabolic (other acids, bases)

2. Whether the blood pH is acid or alkaline


31

Acidosis

Respiratory acidosis Metabolic acidosis

Nervous system depression, coma, death

(hypopnea)


32

Alkalosis

Respiratory alkalosis Metabolic alkalosis

Nervousness, tetany, convulsions, death


33

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 – Acidosis

• Respiratory• Metabolic

– Alkalosis• Respiratory• Metabolic

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

34

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

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

35

Review

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

• About 2/3 of body’s fluid

• 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-

36

Review

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

• 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

37

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

– 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,

38

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 Ca2+ secretion, ↑ PO4

3- 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

39

Review

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

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

• Compensations

40

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

Cl- 96 - 106 105

HCO3- 24 - 28 25

chapter 26 fluid, electrolytes, and acid/base balance lecture 17 marieb’s human anatomy and...

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