1. Fluid, Electrolyte and Acid-Base Balance
Chapter 27

2. Composition of the Human Body
Figure 27–1a

3. Composition of the Human Body
Figure 27–1b

4. Water content varies with age & tissue type
Fat has the lowest water content (~20%).
Bone is close behind (~22 – 25%).
Skeletal muscle is highest at ~65%.

5. Fluid Compartments
ECF (extra cellular fluid) and ICF (intracellular fluid) are called fluid compartments:
because they behave as distinct entities
are separated by cell membranes and active transport

6. Water Composition Is 60% percent of male body weight
Is 50% percent of female body weight
Mostly in intracellular fluid

7. Water Exchange
Water exchange between ICF and ECF occurs across cell membranes by:
osmosis
diffusion
carrier-mediated transport

8. Major Subdivisions of ECF
Interstitial fluid of peripheral tissues
Plasma of circulating blood

9. Minor Subdivisions of ECF
Lymph, perilymph, and endolymph
Cerebrospinal fluid (CSF)
Synovial fluid
Serous fluids (pleural, pericardial, and peritoneal)
Aqueous humor

10. Cations Body Fluids
Figure 27–2 (1 of 2)

11. Anions in Body Fluids
Figure 27–2 (2 of 2)

12. 4 Principles of Fluids and Electrolyte Regulation
All homeostatic mechanisms that monitor and adjust body fluid composition respond to changes in the ECF, not in the ICF
No receptors directly monitor fluid or electrolyte balance

13. 4 Principles of Fluids and Electrolyte Regulation
Cells cannot move water molecules by active transport
The body’s water or electrolyte content will rise if dietary gains exceed environmental losses, and will fall if losses exceed gains

14. Electrolyte concentrations are calculated in milliequivalents
mEq/L = ion concentration (mg/L) x number of charges on one ion
atomic weight
Na+ concentration in the body is 3300 mg/L
Na+ carries a single positive charge.
Its atomic weight is approximately 23.
Therefore, in a human the normal value for Na+ is:
3300 mg/L = 143 mEq/L 23
Note: One mEq of a univalent is equal to one mOsm whereas one mEq of a bivalent ion is equal to ½ mOsm. However, the reactivity of 1 mEq is equal to 1 mEq.

15. Regulation of water balance
It is not so much water that is regulated, but solutes.
osmolality is maintained at between 285 – 300 mOsm.
An increase above 300 mOsm triggers:
Thirst
Antidiuretic Hormone release

16. 3 Primary Regulatory Hormones
Affect fluid and electrolyte balance:
antidiuretic hormone
aldosterone
natriuretic peptides

17. Antidiuretic Hormone (ADH)
Stimulates water conservation at kidneys:
reducing urinary water loss
concentrating urine
Stimulates thirst center:
promoting fluid intake

18. ADH Production Osmoreceptors in hypothalamus:
monitor osmotic concentration of ECF
Change in osmotic concentration:
alters osmoreceptor activity
Osmoreceptor neurons secrete ADH

19. ADH Release (1 of 2) Axons of neurons in anterior hypothalamus:
release ADH near fenestrated capillaries
in posterior lobe of pituitary gland

20. ADH Release (2 of 2)
Rate of release varies with osmotic concentration:
higher osmotic concentration increases ADH release

21. An increase of 2 – 3% in plasma osmolality triggers the thirst center of the hypothalamus.
Secondarily, a 10 – 15% drop in blood volume also triggers thirst. This is a significantly weaker stimulus.
The Thirst Mechanism

22. Aldosterone Is secreted by adrenal cortex in response to:
rising K+ or falling Na+ levels in blood
activation of renin–angiotensin system
Determines rate of Na+ absorption and K+ loss:
along DCT and collecting system

23. Dehydration Chronic dehydration leads to oliguria.
Severe dehydration can result in hypovolemic shock.
Causes include:
Hemorrhage
Burns
Vomiting
Diarrhea
Sweating
Diuresis, which can be caused by diabetes insipidus, diabetes mellitus and hypertension (pressure diuresis).

24. Overhydration (Hypotonic hydration)
Also called water excess
Occurs when excess water shifts into ICF:
distorting cells
changing solute concentrations around enzymes
disrupting normal cell functions

25. Causes of Overhydration
Ingestion of large volume of fresh water
Injection into bloodstream of hypotonic solution
Endocrine disorders:
excessive ADH production

26. Causes of Overhydration
Inability to eliminate excess water in urine:
chronic renal failure
heart failure
cirrhosis

27. Signs of Overhydration
Abnormally low Na+ concentrations (hyponatremia)
Effects on CNS function (water intoxication)

28. Hyponatremia Hyponatremia results in:
Cerebral edema (brain swelling)
Sluggish neural activity
Convulsions, muscle spasms, deranged behavior.
Treated with I.V. hypertonic mannitol or something similar.

29. Hypotonic hydration

30. Blood pressure, sodium, and water

31. Atrial Naturetic Peptide: The heart’s own compensatory mechanism.

32. Fluid Gains and Losses
Figure 27–3

33. Water Balance
Table 27–1

34. Sources of intake & output

35. Importance of Electrolyte Balance
Electrolyte concentration directly affects water balance
Concentrations of individual electrolytes affect cell functions

36. Sodium Is the dominant cation in ECF
Sodium salts provide 90% of ECF osmotic concentration:
sodium chloride (NaCl)
sodium bicarbonate

37. Normal Sodium Concentrations
In ECF:
about 140 mEq/L
In ICF:
is 10 mEq/L or less

38. Potassium
Is the dominant cation in ICF

39. Normal Potassium Concentrations
In ICF:
about 160 mEq/L
In ECF:
is 3.8–5.0 mEq/L

40. 2 Rules of Electrolyte Balance
Most common problems with electrolyte balance are caused by imbalance between gains and losses of sodium ions
Problems with potassium balance are less common, but more dangerous than sodium imbalance

41. Sodium Balance in ECF Sodium ion uptake across digestive epithelium
Sodium ion excretion in urine and perspiration

42. Sodium Balance in ECF Typical Na+ gain and loss:
is 48–144 mEq (1.1–3.3 g) per day
If gains exceed losses:
total ECF content rises
If losses exceed gains:
ECF content declines

43. Changes in ECF Na+ Content
Do not produce change in concentration
Corresponding water gain or loss keeps concentration constant

44. Homeostatic Regulation of Na+ Concentrations in Body Fluids
Figure 27–4

45. Na+ Balance and ECF Volume
Na+ regulatory mechanism changes ECF volume:
keeps concentration stable
When Na+ losses exceed gains:
ECF volume decreases (increased water loss)
maintaining osmotic concentration

46. Large Changes in ECF Volume
Are corrected by homeostatic mechanisms that regulate blood volume and pressure
If ECF volume rises:
blood volume goes up
If ECF volume drops:
blood volume goes down

47. Fluid Volume Regulation and Na+ Concentrations
Figure 27–5 (1 of 2)

48. Fluid Volume Regulation and Na+ Concentrations

49. Homeostatic Mechanisms (1 of 2)
A rise in blood volume:
elevates blood pressure
A drop in blood volume:
lowers blood pressure

50. Homeostatic Mechanisms (2 of 2)
Monitor ECF volume indirectly by monitoring blood pressure:
baroreceptors at carotid sinus, aortic sinus, and right atrium

51. Abnormal Na+ Concentrations in ECF
Hyponatremia:
body water content rises (overhydration)
ECF Na+ concentration < 130 mEq/L
Hypernatremia:
body water content declines (dehydration)
ECF Na+ concentration > 150 mEq/L

52. ECF Volume If ECF volume is inadequate:
blood volume and blood pressure decline
renin–angiotensin system is activated
water and Na+ losses are reduced
ECF volume increases

53. Plasma Volume If plasma volume is too large: venous return increases:
stimulating natriuretic peptides (ANP and BNP)
reducing thirst
blocking secretion of ADH and aldosterone
salt and water loss at kidneys increases
ECF volume declines

54. Potassium Balance 98% of potassium in the human body is in ICF
Cells expend energy to recover potassium ions diffused from cytoplasm into ECF

55. Processes of Potassium Balance
Rate of gain across digestive epithelium
Rate of loss into urine

56. Potassium Loss in Urine
Is regulated by activities of ion pumps:
along distal portions of nephron and collecting system
Na+ from tubular fluid is exchanged for K+ in peritubular fluid

57. Potassium Loss in Urine
Are limited to amount gained by absorption across digestive epithelium (about 50–150 mEq (1.9–5.8 g)/day

58. 3 Factors in Tubular Secretion of K+
Changes in concentration of ECF:
higher ECF concentration increases rate of secretion

59. 3 Factors in Tubular Secretion of K+
Changes in pH:
low ECF pH lowers peritubular fluid pH
H+ rather than K+ is exchanged for Na+ in tubular fluid
rate of potassium secretion declines

60. 3 Factors in Tubular Secretion of K+
Aldosterone levels:
affect K+ loss in urine
ion pumps reabsorb Na+ from filtrate in exchange for K+ from peritubular fluid
High K+ plasma concentrations stimulate aldosterone

61. Electrolyte Balance

62. Electrolyte Balance
Table 27–2 (2 of 2)

63. Calcium Is most abundant mineral in the body: 1–2 kg (2.2–4.4 lb)
99% deposited in skeleton

64. Functions of Calcium Ion (Ca2+)
Muscular and neural activities
Blood clotting
Cofactors for enzymatic reactions
Second messengers

65. Hormones and Calcium Homeostasis
Parathyroid hormone (PTH) and calcitriol:
raise calcium concentrations in ECF
Calcitonin:
opposes PTH and calcitriol

66. Calcium Absorption At digestive tract and reabsorption along DCT:
are stimulated by PTH and calcitriol

67. Calcium Ion Loss In bile, urine or feces:
is very small (0.8–1.2 g/day)
about 0.03% of calcium reserve in skeleton

68. Hypercalcemia Exists if Ca2+ concentration in ECF is > 11 mEq/L
Is usually caused by hyperparathyroidism:
resulting from oversecretion of PTH
Other causes:
malignant cancers (breast, lung, kidney, bone marrow)
excessive calcium or vitamin D supplementation

69. Hypocalcemia Exists if Ca2+ concentration in ECF is < 4 mEq/L
Is much less common than hypercalcemia
Is usually caused by chronic renal failure
May be caused by hypoparathyroidism:
undersecretion of PTH
vitamin D deficiency

70. Magnesium (1 of 3) Is an important structural component of bone
The adult body contains about 29 g of magnesium
About 60% is deposited in the skeleton

71. Magnesium (2 of 3) Is a cofactor for important enzymatic reactions:
phosphorylation of glucose
use of ATP by contracting muscle fibers

72. Magnesium (3 of 3) Is effectively reabsorbed by PCT
Daily dietary requirement to balance urinary loss:
about 24–32 mEq (0.3–0.4 g)

73. Magnesium Ions (Mg2+) In body fluids are primarily in ICF:
Mg2+ concentration in ICF is about 26 mEq/L
ECF concentration is much lower

74. Phosphate Ions (1 of 3) Are required for bone mineralization
About 740 g PO43— is bound in mineral salts of the skeleton
Daily urinary and fecal losses:
about 30–45 mEq (0.8–1.2 g)

75. Phosphate Ions (2 of 3) In ICF, PO43— is required for:
formation of high-energy compounds
activation of enzymes
synthesis of nucleic acids

76. Phosphate Ions (3 of 3) In plasma, PO43—:
is reabsorbed from tubular fluid along PCT
stimulated by calcitriol
Plasma concentration is 1.8–2.6 mEq/L

77. Chloride Ions (Cl—) Are the most abundant anions in ECF
Plasma concentration is 100–108 mEq/L
ICF concentrations are usually low

78. Chloride Ions (Cl—) Are absorbed across digestive tract with Na+
Are reabsorbed with Na+ by carrier proteins along renal tubules
Daily loss is small:
48–146 mEq (1.7–5.1 g)

79. Terms Relating to Acid–Base Balance

80. Strong or Weak Strong acids and strong bases:
dissociate completely in solution
Weak acids or weak bases:
do not dissociate completely in solution
some molecules remain intact

81. Acidosis Physiological state resulting from abnormally low plasma pH
Acidemia:
plasma pH < 7.35

82. Alkalosis Physiological state resulting from abnormally high plasma pH
Alkalemia:
plasma pH > 7.45

83. Acidosis and Alkalosis
Affect all body systems:
particularly nervous and cardiovascular systems
Both are dangerous:
but acidosis is more common
because normal cellular activities generate acids

84. Relationship between PCO2 and Plasma pH
Figure 27–6

85. 3 Types of Acids in the Body
Volatile acids - Can leave solution and enter the atmosphere (ex: Carbonic acid)
Fixed acids - Are acids that do not leave solution. Once produced they remain in body fluids until eliminated by kidneys
Organic acids -

86. Sulfuric Acid and Phosphoric Acid
Are most important fixed acids in the body
Are generated during catabolism of:
amino acids
phospholipids
nucleic acids

87. Organic Acids Produced by aerobic metabolism:
are metabolized rapidly
do not accumulate
Produced by anaerobic metabolism (e.g., lactic acid)
build up rapidly

88. What’ a “buffer” Acids are proton donors Bases are proton acceptors
Strong acids & bases dissociate completely
Acid buffer systems are comprised of compounds that resist pH changes by accepting protons from solutions containing strong acids.
Base buffer systems accept OH- ions from solutions. (Not discussed in the text).

89. Buffers Are dissolved compounds that stabilize pH: Weak acids:
by providing or removing H+
Weak acids:
can donate H+
Weak bases:
can absorb H+

90. A Buffer System Consists of a combination of:
a weak acid
and the anion released by its dissociation
The anion functions as a weak base

91. Buffer Systems in Body Fluids
Figure 27–7

92. 3 Major Buffer Systems Protein buffer systems:
help regulate pH in ECF and ICF
interact extensively with other buffer systems
Carbonic acid–bicarbonate buffer system:
most important in ECF
Phosphate buffer system:
buffers pH of ICF and urine

93. Kidney Tubules and pH Regulation

94. Kidney Tubules and pH Regulation
Generation of new bicarbonate using glutamine
Figure 27–10b

95. Kidney Tubules and pH Regulation
Excretion of bicarbonate
Figure 27–10c

96. Respiratory Acid–Base Regulation

97. Respiratory Acid–Base Regulation
Figure 27–12b

98. Metabolic Alkalosis
Figure 27–14

99. Diagnostic Chart for Acid-Base Disorders
Figure 27–15 (1 of 2)

100. Diagnostic Chart for Acid–Base Disorders

101. Blood Chemistry and Acid–Base Disorders
Table 27–4

102. Enjoy