1. Chapter 26 Fluid, Electrolytes, and Acid/Base Balance Lecture 17
Marieb’s Human
Anatomy and Physiology
Marieb w Hoehn
Chapter 26
Fluid, Electrolytes, and Acid/Base Balance
Lecture 17
Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1]
Slides 16 – 38; 50 min [Lecture 2]
118 min (38 slides plus review of course Web sites and syllabus)

2. Lecture Overview Overview Fluid (water) balance Electrolyte 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
Because of decreased bone mass and decreased fat mass, infants are up to about 73% water by weight. The elderly have about 45% body water. In a normal adult male (70Kg), there is about 60% body water, while the normal adult female has about 50% body water (due to increased % of body fat).
Figure from: Hole’s Human A&P, 12th edition, 2010

5. Fluid Compartments
Figure from: Hole’s Human A&P, 12th edition, 2010
‘Compartments’ commonly behave as distinct entities in terms of ion distribution, but ICF and ECF osmotic concentrations are identical (about mOsm/L). Why?

6. Osmolarity
Amount of solute
Osmolarity =
Volume of H2O

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)
Figure from: Hole’s Human A&P, 12th edition, 2010
You should know these chemical symbols and charges (valences) of ions

9. Important Principles of Fluids and Electrolytes
Homeostatic mechanisms respond to changes in the ECF, not the ICF
No receptors directly monitor total fluid or electrolyte balance, but do this indirectly through plasma volume and osmotic concentrations
Cells cannot move water molecules by active transport (“Water follows salt”)
Content of water or electrolytes
Will rise if dietary gains exceed losses
Will fall if dietary losses exceed gains

10. Movement of Fluids Between Compartments
Figure from: Hole’s Human A&P, 12th edition, 2010
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
Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressures

11. Fluid (Water) Balance
Figure from: Hole’s Human A&P, 12th edition, 2010
* 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 Overhydration
Dehydration (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
Severe thirst, wrinkling of skin, fall in plasma volume and decreased blood pressure, circulatory shock, death
‘Drunken’ behavior (water intoxication), confusion, hallucinations, convulsions, coma, death

15. Electrolyte Balance Electrolyte balance is important since:
Figure from: Hole’s Human A&P, 12th edition, 2010
Electrolyte balance is important since:
It regulates fluid (water) balance
Concentrations of individual electrolytes can affect cellular functions
Na+: major cation in ECF (plasma: mEq/L; Avg ≈ 140)
K+: major cation in ICF (plasma: mEq/L; Avg ≈ 4.0)

16. Regulation of Osmolarity
Figures from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Recall: [Na+]  Osmolarity
Osmolarity is regulated by altering H2O content
** Osmolarity = Amt of solute / volume of H2O

17. Fluid Volume Regulation and [Na+]
Volume is regulated by altering Na+ content
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+
Primary mechanisms of Na+ regulation: intake, aldosterone, adh, natriuretic peptides, and steroids (estrogen, glucocorticoids).

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
 Na+ concentration,
 ECF osmolarity
 Thirst →  H2O intake
 ADH →  H2O output
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
Figure from: Hole’s Human A&P, 12th edition, 2010
Potassium loss generally occurs via the urine. The rate of tubular secretion of K+ varies with:
Changes in the [K+] in the ECF
Changes in pH
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
Figure from: Hole’s Human A&P, 12th edition, 2010
[Ca2+] in ECF is about 5 mEq/L
Recall that calcitonin is more important in children whose osteoclasts release approx. 5 mg per day whereas in adults it is only about 0.8 gm.

21. Strengths of Acids and Bases
Strong acids ionize more completely and release more H+
Weak acids ionize less completely and release fewer H+ (**allows them to act as buffers)
Strong bases ionize more completely and bind more H+
Weak bases ionize less completely and bind fewer H+

22. Sources of Hydrogen Ions
Figure from: Hole’s Human A&P, 12th edition, 2010
Volatile acids – can leave solution and enter the atmosphere, e.g., H2CO3
Fixed acids – do not leave solution; once produced they remain in body fluids until eliminated at the kidneys, e.g., Sulfuric and phosphoric acids
Organic acids – participants in, or by-products of, aerobic metabolism. E.G., lactic acid, ketone bodies
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
Strong acid
Weak acid
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

25. Acid-Base Buffer Systems
Protein Buffer System
ICF, plasma proteins, Hb
Most plentiful and powerful chemical buffer system
COOH group releases hydrogen ions when pH rises
NH2 group accepts hydrogen ions when pH falls -
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

26. Respiratory Excretion of Carbon Dioxide
Figure from: Hole’s Human A&P, 12th edition, 2010
A physiological buffer system

27. Renal Excretion of Hydrogen Ions
Figure from: Hole’s Human A&P, 12th edition, 2010
*The kidney is most powerful and versatile acid-base regulating system in the body

28. Buffering Mechanisms in the Kidney
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-
(2)
Ability of kidneys to eliminate H+ is dependent upon buffers in the urine. If these buffers were not present, the kidneys could secrete less than 1% of the 100 mEq/L of H+ produced every day. The secretion of H+ can continue only down to a pH of tubular fluid of about 4.0 – After this, H+ would flow back into the tubule cells as fast as they are pumped out. Buffers (mainly carbonic acid-bicarbonate, ammonia, and phosphate) help keep the pH of the tubular fluid above this pH ‘cutoff’.
Production of new HCO3-
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

29. Summary of Acid-Base Balance
Figure from: Hole’s Human A&P, 12th edition, 2010
(Seconds)
Know this slide!
(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:
Whether the cause is respiratory (CO2), or metabolic (other acids, bases)
Whether the blood pH is acid or alkaline
Figure from: Hole’s Human A&P, 12th edition, 2010

31. Acidosis Nervous system depression, coma, death Respiratory acidosis
Figure from: Hole’s Human A&P, 12th edition, 2010
(hypopnea)
Respiratory acidosis
Metabolic acidosis
Nervous system depression, coma, death

32. Alkalosis Nervousness, tetany, convulsions, death
Figure from: Hole’s Human A&P, 12th edition, 2010
The expected change in pH with respiratory alkalosis can be estimated with the following equations:
Acute respiratory alkalosis: Change in pH = X (40 – PCO2)
Chronic respiratory alkalosis: Change in pH = X (40 – PCO2)
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
See Handout: Marieb, Human Anatomy & Physiology, Pearson, 2004

34. Flow chart for Acidosis/Alkalosis
Three things to check: 1) pH – ) pCO2 – mm Hg 3) HCO – 26 mEq/L pH  
acidosis
alkalosis
 pCO2
 HCO3-
 pCO2
HCO3-
respiratory
metabolic
respiratory
metabolic
Norm HCO3-
Norm pCO2
Norm HCO3-
Norm pCO2
HCO3-
 pCO2
 HCO3-
 pCO2
No Comp
Comp
No Comp
Comp
No Comp
Comp
No Comp
Comp

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) Acid-base balance
Ca2+ (PTH, calcitriol, calcitonin)
Increase in ECF -> calcitonin promotes bone deposition
Decrease in ECF -> PTH , calcitriol
↑ intestinal absorption
↑ bone resorption
 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

39. Review Acidosis (pH < 7.35) Alkalosis (pH > 7.45) Compensations
Excessive H+ in the plasma
Respiratory acidosis
Metabolic acidosis
Alkalosis (pH > 7.45)
Insufficient H+ in the plasma
Respiratory alkalosis
Metabolic alkalosis
Compensations

40. Concentration Range (mEq/L)
Review
Electrolyte
Concentration Range (mEq/L)
Typical Value (mEq/L)
Na+
140 K+
4.0
Ca2+
4.5 – 5.8
5.0
Cl-
105
HCO3- 25