1. Fluid, Electrolyte, & Acid-Base Balance
Saladin Ch. 24
Fluid, Electrolyte, & Acid-Base Balance

2. Fluid Compartments
Fluid occupies two main compartments. [55-60% body weight]
Intracellular fluid (ICF) – about two thirds by volume, contained in cells = 65% of body fluids
Extracellular fluid (ECF) – consists of two major subdivisions = 35%
Plasma – the fluid portion of the blood.
Interstitial fluid (IF) – fluid in spaces between cells.

3. Fluid Compartments
Other fluids. [lymph, CSF, GI fluids, synovial fluid, ocular humors, pleural, pericardial and peritoneal fluids, glomerular filtrate]

4. Figure 26.1

5. Fluid Movement Among Compartments
Water moves by osmosis which is determined by electrolyte movement & balance

6. Water Balance & ECF Osmolality
For proper hydration, water intake must equal water output.
Total ingested water [2500 mL/day]
Metabolic [from aerobic resp. and dehydration synthesis] [200mL/day]
Food & drink – [2300 mL]

7. Water Balance & ECF Osmolality
Average outputs – increase with activity, environment, etc.
Total output [2500mL]
Kidneys – urine [1500mL/day]
Skin & sweat – 500mL/day
Lungs – esp. [300mL/day]
GI tract – feces [200mL/day]

8. Figure 26.4

9. REGULATION OF WATER UPTAKE
Regulated by adjusting water ingestion – thirst
Local responses – decreased saliva  dry mouth
Increased blood osmotic pressure  hypothalamus  thirst [may get ADH]
Decreased blood volume  release of renin  angiotensin II  thirst

10. REGULATION OF WATER OUTPUT
Regulated by urinary water and NaCl loss – kidneys can’t replace loss, just reduce further loss.
ADH increases water reabsorption in renal collecting ducts  aquaporins inserted into principal cell membranes  water reabsorption.

12. Disorders of Water Balance: Dehydration
Water loss exceeds water intake & the body is in negative fluid balance.
Causes include: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, & diuretic abuse.

13. Disorders of Water Balance: Dehydration
Dehydration – get increased osmolarity; Diabetes mellitus & insipidus, overuse of diuretics, diarrhea; [can lead to shock] Use isotonic salts solutions for replacement of extreme fluid loss – prevents hypotonic swelling.

14. Excessive loss of H2O from ECF 2 ECF osmotic pressure rises 3 1
Excessive loss of H2O from ECF 2
ECF osmotic pressure rises 3
Cells lose H2O to ECF by osmosis; cells shrink
(a) Mechanism of dehydration
Figure 26.7a

15. Disorders of Water Balance: Fluid Excess
Water intoxication – take in faster than can remove – hypotonic ECF
Hyponatremia]  cell swelling and cerebral edema

16. Excessive H2O enters the ECF 2 ECF osmotic pressure falls 3 1
Excessive H2O enters the ECF 2
ECF osmotic pressure falls 3
H2O moves into cells by osmosis; cells swell
(b) Mechanism of hypotonic hydration
Figure 26.7b

17. Disorders of Water Balance: Edema
Atypical accumulation of fluid in interstitial spaces, leading to tissue swelling.
Caused by anything that increases flow of fluids out of the bloodstream or hinders their return.
Increases blood pressure.
May involve vessel blockage, CHF.

19. Electrolyte Balance
Electrolyte - substance that dissociates into cations & anions in water & conducts electricity in solution.

20. Functions of Electrolytes
Essential minerals - enzyme cofactors.
Control of osmosis.
Maintenance of acid/base balance.
Conduction of electrical current.

21. Extracellular & Intracellular Fluids
Each fluid compartment of the body has a distinctive pattern of electrolytes.
Extracellular fluids are similar (except for the high protein content of plasma).
Na+ is the chief cation.
Cl- is the major anion.

22. Extracellular & Intracellular Fluids
Intracellular fluids have low Na+and Cl- .
K+ is chief cation.
Proteins & phosphate are chief anions.

23. Figure 26.2

24. Sodium & Electrolyte Balance
Na+ holds central position in fluid & electrolyte balance. [Plasma levels = 142 mEq/L]
Sodium salts account for 90 of all solutes in the ECF.
Na+ is the only cation exerting significant osmotic pressure - most important solute in determining water distribution.

25. Sodium & Electrolyte Balance
Adult need 0.5 g/day – in America always have EXCESS – problem is getting rid of it
Concentrations are maintained by maintaining water levels – “water follows salt”
65% of Na+ in renal filtrate is automatically reabsorbed.

26. Regulation of Sodium Balance:
Levels regulated by:
Aldosterone - [increases reabsorption of Na+ and excretion of K+], ascending loop, DCT & collecting ducts
ADH [increases water reabsorption in response to increases in Na]
ANP [increases Na+ excretion by kidneys by inhibing ADH & aldosterone]

27. Regulation of Sodium Balance:
Estrogens – enhances Na reabsortion and water retention
Progesterone may decrease Na+ reabsorption by blocking aldosterone.
Glucocorticoids – can cause edema

28. Na+ Imbalances Hypernatremia  water retention, hypertension & edema
Hyponatremai hypotonic hydration if not corrected

29. K+ and Electrolyte Balance
Most abundant intracellular cation.
Functions in impulse conduction, OP regulation, protein synthesis & pH.
Levels regulated in the kidneys by aldosterone.

30. Influence of Aldosterone
Aldosterone stimulates potassium ion secretion by “principal cells.”
In collecting ducts, for each Na+ reabsorbed, a K+ is secreted.
Increased K+ in ECF around adrenal cortex causes release of aldosterone & potassium secretion.

31. K+ Imbalances
Hyperkalmia – get different responses if fast or slow onset. Fast - The heart is really sensitive to too much [can cause membrane depolarization - hyperexcitability]; Slow – cells become LESS excitable
Hypokalmia - too little [causes hyperpolarization and non-responsiveness].

32. Regulation of Chloride
Cl- is the major anion accompanying Na+ in the ECF.
Functions in OP regulation, HCl formation.
Levels regulated by Na+ movements.

33. Regulation of Calcium & Phosphate
Calcium-most abundant mineral.
Abundant extracellular cation.
Functions in bone, teeth, blood clotting, impulse conduction, muscle contraction.

34. Regulation of Calcium & Phosphate
Levels controlled by:
PTH
Calcitrol
Calcitonin

35. Regulation of Calcium & Phosphate
Hypercalcemia – from alkalosis or hypothyroidism – inhibits depolarization  weakness, arrhythmia
Hypocalcemia – Vit. D deficiency, lactation, pregnancy, etc. Too little can increase excitability & may lead to tetanus;

36. Regulation of Calcium & Phosphate
Functions: Nucleic acids, ATP, phospholipids, bone, etc.
Control: Kidneys reabsorb if levels drop,
Parathyroid hormone stimulates excretion

37. Acid-Base Balance Normal pH of blood is 7.35 – 7.45.
3 mechanisms to maintain proper pH:
Buffer systems
Changes in respiration
Excretion by kidney

38. Acid-Base Balance Acid = proton [H+] donor;.
Most H+ originates as metabolic products.
Strong dissociates completely
Weak acid only dissociates a little
Base = proton acceptor
Strong – binds a lot,
Weak binds less

39. Chemical Buffer Systems
Buffers resist changes in pH when strong acid or strong base are added.
Weak acid + salt of that acid.
Works by taking up or releasing H+ ions to maintain pH at a given level.
Three major chemical buffer systems:
Bicarbonate/carbonic acid buffer system

40. Chemical Buffer Systems
Phosphate buffer system
Protein buffer system
Any changes in pH are resisted by the entire chemical buffering system.

41. Bicarbonate Buffer System
A mixture of carbonic acid (H2CO3) & its salt, usually sodium bicarbonate (NaHCO3) (potassium or magnesium bicarbonates work as well).
This system is the only important ECF buffer.
H++ HCO3¯  H2CO3  CO2 + H2O

42. Phosphate Buffer System
This system is an effective buffer in urine & intracellular fluid.
OH ¯ + H2PO4-  H2O +CO2
H++ HPO42-  H2PO4-

43. Protein Buffer System
Plasma & intracellular proteins are the body’s most plentiful & powerful buffers - does ¾ of all.
Protein molecules are amphoteric and can function as both weak acids & a weak bases.
The acid group (COOH) can give up an H+ to neutralize a base.
The amine group can accept an H+, raising pH.

44. Respiration
The respiratory system regulation of acid-base balance is a physiological buffering system.
Exhalation of CO2 lowers H+ by lowering carbonic acid
With alkalosis – get shallow slow breathing to help ACCUMULATE CO2

45. Kidneys
Kidneys can expel H’s by secretion – neutralize it in tubules with bicarb, phosphate, etc.
Only way to remove non-carbonic acid H+ [phosphoric uric, lactic acid, ketone bodies]
Also only way to regulate alkaline substances – can break down bicarb to CO2 and reabsorb that.

47. Blood pH Acidosis: pH below 7.35 Alkalosis: pH above 7.45
Compensation-physiological changes that occur to bring pH back to normal.

48. Blood pH Normal Blood Values pH 7.35 – 7.45 PCO2 35 – 45 mm Hg
HCO3- 22 – 26 mEq/L

49. Blood pH Blood Acidosis pH below 7.35 depression of CNS  coma
Blood Alkalosis
pH above 7.45
Over excitability of CNS and PNS  nervousness  spasms  convulsions

50. Respiratory Acidosis and Alkalosis
Respiratory acidosis is the most common cause of acid-base imbalance.
Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema.

51. Respiratory Acidosis and Alkalosis
Elevated pCO2, low pH
Compensation – renal H+ secretion, increased bicarbonate reabsorption
Treatment – respiratory and IV bicarbonate

53. Respiratory Acidosis and Alkalosis
Low pCO2, high pH
Caused by hyperventilation [e.g. From oxygen deficiency]
Compensation – renal – decrease H+ secretion, bicarbonate elimination
Treatment – breathe into a bag

54. Metabolic Acidosis Low systemic and arterial bicarbonate.
Caused by loss of bicarbonate, accumulation of another acid or kidney failure.
Compensation – respiratory hyperventilation [increased rate and depth]. PCO2 falls.
Treatment – NaHCO3 IV

55. Metabolic Alkalosis
Rising blood pH and bicarbonate levels indicate metabolic alkalosis.
Typical causes are loss of acid (e.g vomiting), intake of excess base (e.g., from antacids),
Compensation – respiratory – hypoventilation [slow, shallow]. PCO2 rises.
Treatment – electrolyte/fluid therapy