Water, Electrolyte and Acid-Base Balance

Water, Electrolyte and Acid-Base Balance

I. Topics
A. Fluid balance
B. Electrolyte balance
C. Acid-base balance

II. Body systems involved
A. Urinary: water, electrolyte, pH
B. Respiratory: CO2, water loss
C. Digestive: water, electrolyte absorption
D. Integumentary: water, electrolyte loss
E. Endocrine: hormone control of electrolyte & water
F. Nervous: electrolyte & water
G. Cardiovascular: hormone control of electrolyte; bicarbonate buffer system in blood
H. Lymphatic: interstitial fluid return to cardiovascular

III. Fluid content
A. Baby: 75%; amniotic fluid consumption & absorption
B. Man: 55-60%; muscle tissue has lots of water; women less due to higher fat (low water tissue)
C. Elderly & obese: 45%

IV. Fluid compartments
A. Separated by selectively permeable membranes
B. Different chemical compositions
C. Intracellular fluid (ICF)
a. 65%
b. Major anions & buffers
i. Protein
ii. Phosphate
c. Major cations
i. K+
ii. Mg++
d. Low Ca++ usually
D. Extracellular fluid (ECF)
a. Total 35%
b. Major anions
i. Cl-
ii. HCO3-: buffer component
c. Major cations
i. Na+
ii. Some Ca++, K+, Mg++
d. Significant difference: plasma has more proteins than does any other in this group of
e. Interstitial: 25%
f. Blood plasma & lymph: 8%
g. Transcellular: 2%; cerebrospinal, bile, synovial, peritoneal, pleural, eye fluids, digestive, urinary, respiratory
E. Exchange among compartments
a. Across capillary walls
b. Across cell membranes
c. Water by osmosis: requires water channels in cell membrane
d. Water & small solutes by filtration: arterial ends of capillaries
e. Osmotic gradients between ICF & ECF do not last long (seconds) because of rapid water osmotic movement
f. Electrolytes (Na+, K+) most important in establishing osmotic gradients since so numerous
g. Proteins very important where osmotic gradient is ?manipulated? by filtration
F. Water flux possible among compartments (normal)
Digestive tract ? arteries ? interstitial fluid ?? intracellular fluid ??
interstitial fluid ? veins & lymph ? general circulation
G. Content of bloodstream will affect interstitial fluid that will affect intercellular fluid; site of regulation for balance in body = blood

V. Fluid balance
A. Normal gain/loss ? 2.5 L/day
B. Gains
a. Metabolic water: aerobic respiration & dehydration anabolic reactions
b. Preformed water: food (700 ml) & liquids (1600 ml)
C. Losses
a. Urine: 1.5 L; somewhat variable; minimal = 400ml
b. Feces: 0.2 L
c. Expired breath: 0.3 L, variable, more loss in cold (0.65 L)
d. Sweat: 0.1 L; extremely variable; up to 5L
e. Cutaneous transpiration: 0.4 L
D. Categories of losses
a. Insensible: breath & transpiration
b. Obligatory: breath, transpiration, sweat, feces, minimal urine output
c. Even in dehydration obligatory losses continue, so dehydration worsens if not treated
E. Water intake & output
a. Thirst center
i. Hypothalamus: responds to increased levels of angiotensin II, ADH & osmotic shrinkage of cells in thirst center
ii. Salivation inhibited
iii. Mouth is dry
iv. Other clues as well = ?; pharyngeal area
v. Increase of blood osmolarity by 2-3% or blood loss of 10-15% strongly stimulates hypothalamus
b. Quenching thirst
i. Immediate
1. Cooling & moistening mouth & pharynx
2. Distension of stomach & small intestine
3. Prevents overhydration
ii. Later: reduction of blood osmolarity
c. Regulation of water output
i. Vary urine volume
ii. Usually involves changes in Na+ urine content too (aldosterone/ANF)
iii. Hormones:
1. ADH: independent control of water output; aquaporins in collecting ducts
2. Present: more concentrated urine since Na+ is lost as usual; blood volume increases
3. Absent: less concentrated urine since more water is lost; blood volume decrease
4. Aldosterone /ANF and Na+
F. Disorders of water balance
a. Involve volume, concentration or distribution among compartments
b. Volume & concentration
i. Deficiency
1. Two types
a. Hypovolemia: volume depletion: loss of water and electrolytes
b. Dehydration: loss of water
2. Hypovolemia
a. Osmolarity of blood unchanged; volume decreased
b. Hemorrhage, severe burns, chronic v/d
3. Dehydration
a. Osmolarity if blood increased; volume decreased
b. Lack of access: young/old/exposure
c. The diabetes: mellitus & insipidus; 1st sign = PU; why?
d. Profuse sweating: sweat is hypotonic
e. Inappropriate diuretics
f. Secretory & osmotic diarrheas
4. Infants most vulnerable
a. High metabolic rate; more toxins need more water for excretion
b. Kidneys can?t concentrate urine very well
c. Greater body surface to volume ratio: loose more water to evaporation
d. Higher water content
5. Fluid recall with dehydration
a. Dehydration affects all fluid compartments
b. Blood osmolarity higher pulls more water from interstitial fluids
c. Higher osmolarity of interstitial fluids pulls water out of cells
d. Neurons susceptible: shrink & don?t function as well
e. Does this occur with hypovolemia?
6. Serious effects of fluid deficiency of any type
a. Circulatory shock due to volume: ischemia, etc, etc
b. What is source of neuron damage here?
ii. Excess
1. Volume excess:
a. ECF isotonic
b. Renal failure or excess aldosterone
2. Water intoxication:
a. ECF is hypotonic; cells swell
b. Psychogenic or intentional overhydration
c. Excess ADH
d. Hyponatremia = hallmark sign of water intoxication
3. Serious effects of overhydration
a. Pulmonary hypertension
b. With intoxication: cerebral edema; why?
c. Distribution
i. Fluid sequestration
ii. Edema: fluid retained in interstitial spaces
1. Local: inflammation
2. Generalized: CHF, liver failure, etc
3. Increased filtration pressure on arterial end: hypertension
4. Decreased osmotic pressure on venous end: low blood osmolarity
5. Blocked lymphatic return: surgery
iii. Third-spacing
1. Internal hemorrhage
2. Ascites in abdomen
3. Pleural effusion
4. Surgical ?dead space? & seromas
5. Intestinal sequestration of fluids
6. Period of development will govern effect on body?s fluid content
a. Peracute: hypovolemic hypotension
b. Chronic: total body water may increase; weight gain & fluid retention

VI. Electrolyte balance
A. Role of electrolytes
a. Chemical reactants
b. Generate electrical potential across cell membranes
c. Essential in creating osmotic gradients & causing water flux into fluid compartments
B. Cations: Na+. K+, Ca++, H+
C. Anions: Cl-, HCO3-, PO4 ---
D. Sodium
a. Functions
i. Responsible for 90% of osmolarity of ECF
ii. NaHCO3 buffer for ECF
iii. Na+ gradient for depolarization & cotransport (glucose)
b. Control
i. Retention: aldosterone (steroid) causes more Na+/K+ pumps to be installed in cell membranes of DCT & cortical portion of collecting duct
ii. Three effects: lowers blood K+, raises blood Na+ & water follows Na+ into blood (volume increases)
iii. Three stimuli for aldosterone release: hypovolemia (via rennin & angiotensin II), hyponatremia (low Na+) & hyperkalemia (high K+)
iv. No changes in Na+ concentration in blood although blood volume may increase
v. Atrial natriuretic factor ANF: opposite effects of aldosterone
vi. Estrogen & glucocorticoids both retain Na+; progesterone encourages it?s loss
c. Imbalances
i. Hypernatermia:
1. > 145 meq/L
2. Overzealous IV fluid administration (Nabicarboante, NaCl), aldosteronism
ii. Hyponatremia:
1. < 130 meq/L
2. Water intoxication, excess ADH
E. Potassium
a. Functions
i. Resting membrane potential: repolarization
ii. Intracellular osmolarity
b. Linked to Na+ homeostasis
c. Intracellular cation: blood levels don?t reflect total body K+
d. 90% reabsorbed in PCT of nephron
e. Adjustments made in DCT & cortical portion of collecting duct
i. Intercalated cells: reabsorb K+ when blood levels low
ii. Aldosterone increases K+ loss to urine
f. Levels out of range of normal hurt excitable tissues, especially the heart
i. Hypokalemia: < 3.5 meq/L
1. Excitable membranes: greater diffusion out of cell; hyperpolarized
2. Cells harder to excite: muscle weakness, loss of muscle tone & depressed reflexes, irregular electrical activity of heart (U waves, flattened T wave); heart may stop in diastole
3. Causes:
a. Aldosteronism
b. Diuretics
c. Intracellular shift: insulin, alkalosis
d. Diet contributing factor
ii. Hyperkalemia: > 5 meq/L
1. Excitable membranes: less diffusion out of cell; facilitated
2. Cells easier to depolarize but harder to repolarize; muscle cramping & cardiac standstill (heart may stop in systole); ECG has wide QRS & tall T waves
3. Causes
a. Decreased excretion: renal dz, Addison?s dz
b. Extracellular shift: crushing/destruction of cells, acidosis, insulin deficiency
g. Extracellular measurements of an intracellular cation; shifts in/out of cell important
F. Calcium
a. Functions
i. Essential for intracellular activity: muscle contraction,
neurotransmitter exocytosis, etc
ii. Blood clotting
iii. Ca++ levels in cytoplasm kept low to prevent precipitation of
calcium phosphate crystals
b. Homeostasis
i. Tightly regulated blood level
ii. PTH, calcitonin & metabolites of vitamin D
iii. Bone resoprtion/deposition, gut absorption & kidney
loss/retention
iv. Three forms in blood
1. Ionic: physiologically active form
2. Bound to albumin (for each change in 1g/dL of albumin = change of 1mg/dL Ca++ in blood)
3. With nonprotein anions
c. Imbalances affect mainly skeletal muscle tissue
a. Hypercalcemia (>5.8 mEq/L)
i. Inhibits depolarization of nerve & muscle cells
ii. Effect on RMP: greater; hyperpolarized (more + charges on outside of membrane)
iii. Heart: - chronotrope, + inotrope
iv. If occurs with normal [PO4], can cause precipitation of calcium phosphate
crystals in soft tissues
v. Causes can include cancer, primary hyperPTHism and secondary hyperPTHism in renal dz
b. Hypocalcemia (<4.5 mEq/L)
i. Facilitates excitable membranes because RMP is reduced (less + outside membrane)
ii. Convulsions & muscle tetany (role of SR with hypocalcemia ?)
iii. + Trousseau (carpal ischemic spasm) & Chvestock (facial nerve percussion & unilateral facial muscle spasm)
iv. Heart: + chronotrope, - inotrope
v. Causes can include: vitamin D deficiency, diarrhea, pregnancy & lactation,
vi. At very low levels laryngospasm & suffocation can occur
3. Other electrolytes that may also be of concern include PO4 and Mg.
a. Phosphate:
i. Intracellular anion
ii. Decreased levels affect RBC function: lowered AATP reduces 2,3,DPG
Which reduces myocardial oxygenation
which reduces CO
iii. Increased levels in renal failure: calcification of soft tissues
b. Magnesium
i. Intracellular cation
ii. Important coenzymes

VII. Acid-Base Balance
A. Stabilization of blood pH is critical: 7.35-7.45
B. Three systems help regulate the [H+]
a. Chemical buffers
i. Weak acid + weak base
ii. Each buffer system works best at an optimum pH
iii. Limited by the amounts of the components of the buffer system available for reaction
iv. Bicarbonate, phosphate (intracellular & urine) & protein (intracellular)
v. Works immediately
b. Lung
i. Regulates CO2 portion of bicarbonate buffer
ii. More is lost as [H+] rises; increased CO2 increases RR
iii. Less is lost as [H+] drops; decreased CO2 decreases RR
iv. Works in minutes
c. Kidney
i. Regulates HCO3- portion of bicarbonate buffer
ii. Regulates [H+] directly
iii. Works in days
C. Bicarbonate buffer
a. Balance of CO2:HCO3- should be 1:20
b. CO2 + H2O ?? H2CO3 ?? H+ and HCO3-
c. Main ECF buffer; unlimited amounts of reactants CO2 & H2O
d. Only buffer that can be adjusted by the kidney (HCO3- & H+)
i. Renal tubules excrete H+ onto series of bases in urine in exchange for Na+
1. NaHCO3 first ? CO2 + H2O
2. Na2HPO4 second ? NaH2PO4
3. Third, deamination of amino acids in tubules produce NH3 & NH3 combines with H+ & Cl- ? NH4Cl
4. 1 conserves and 2-3 creates more HCO3-: the alkaline reserve of the bicarbonate system
ii. At and above pH of 4.5, H+ excretion stops
D. Phosphate buffer
a. H2PO4- ?? HPO4-- and H+
b. Intracellular and urine buffer
c. Urine buffer helps ?keep? H+ in the urine
E. Protein buffer
a. 3/4ths of all buffering capacity of the body
b. Amphoteric molecules
i. R-COOH ?? R-COO- and H+
ii. R-NH2 and H+ ?? R-NH3+
iii. R = rest of protein
c. Intracellular and in blood plasma
i. Hemoglobin in RBCs
ii. Proteins found inside all cells
d. Amount of this buffer doesn?t change normally
F. Imbalances
a. Acidosis: pH < 7.35; pH below 6.8 incompatible with life
b. Alkalosis: pH > 7.45; pH above 8 incompatible with life
c. Basic cause of imbalance
i. Respiratory
1. CO2 retention = respiratory acidosis
2. CO2 low = respiratory alkalosis
ii. Metabolic
1. Excess H+ or low HCO3- = metabolic acidosis
2. Low H+ or high HCO3- = metabolic alkalosis
d. Compensation: non-causative system adjusts to bring pH closer to normal
i. Total compensation: pH normal; H+ and HCO3- values abnormal
ii. Partial compensation: pH, H+ and HCO3- values abnormal
e. Blood-gas analysis (ABGs)
i. Arterial sample: radial artery
ii. Measures: pH, pP of CO2/O2 & HCO3-
iii. Normal values
1. pH 4.35-7.45
2. pP CO2 35-45 mmHg; >45 = acid value
3. pP O2 80-95 mmHg or 95-99% saturation
4. HCO3- 22-26 mEq/L; >26 = alkaline value
iv. Use tic-tac-toe for ABG analysis
v. Finally, ask if compensated
G. Anion gap: cations should about equal anions; when don?t = a gap; gap indicates presence
of other unmeasured anions such as organic acids; with a gap there are more cations than anions.

VIII. Types of acid-base imbalances
A. Respiratory acidosis: acute; CO2 elevated with low pH and normal range HCO3-
a. Pulmonary/thoracic: ARDS, pneumothorax, hemothorax
b. Increased resistance to air flow: upper airway obstruction, aspiration, severe bronchospasm and/or swelling of mucosal lining of bronchial tree
c. CNS depression
d. Neuromuscular: Guillain-Barre syndrome, MG crisis, high-cervical cordotomy
e. Cardiovascular: cardiac arrest, pulmonary edema
f. Mechanical ventilation: excessive dead space & insufficient ventilation
B. Respiratory acidosis: chronic, CO2 elevated, pH near normal, HCO3- increased
a. Obstructive disease: COPD (emphysema, bronchitis), sleep apnea, cystic fibrosis
b. Restriction of ventilation: Pickwickian syndrome, kyphoscoliosis
c. Neuromuscular:
d. CNS depression: tumor, bulbarpoliomyelitis, chronic sedative overdose
e. Decompensation and pH drop likely with added problems
C. Respiratory alkalosis: acute, CO2 low, pH elevated and HCO3- normal
a. Hypoxemia: increased respiratory effort due to stimulation of secondary respiratory drive; pneumonia, high altitude, CHF, hypotension & anemia
b. Direct stimulation of CNS: fear, anxiety, CVA
D. Metabolic acidosis: acute, CO2 variable, HCO3- and pH (H+ high) low
a. Loss of HCO3-: diarrhea, drainage of pancreas, bile or gut contents
b. Renal: inability to excrete H+ due to tubular disease
c. Excess acid
1. Production: ketones; lactic acid
2. Ingestion: alcohols, salicylates
E. Metabolic acidosis: chronic: CO2 low, HCO3 and pH (H+ high) low
a. Renal failure
F. Metabolic alkalosis: acute; CO2 variable, HCO3 and pH (H+ low) high
a. H+ loss: gastric suction, vomiting, loop diuretics, excess mineralocorticoid
b. H+ shift into cells; hypokalemia
G. Metabolic alkalosis: chronic,
a. Same as above

IX Intravenous fluid administration
A. Crystalloids: electrolytes & glucose
a. Substances are not restricted to the intravascular space
b. Always expand entire ECF volume
c. Isotonic NaCl solutions: expands ECF only, not ICF, usually for intravascular volume replacement
d. Saline/electrolytes (Ringer?s & Lactated Ringer?s): similar to plasma, replace ECF losses & expand intravascular volume; lactate = buffer in mild acidosis (not lactiv acidosis)
e. Hypotonic NaCl solutions or those with dextrose & water expands ECF and ICF
i. Dextrose & water: treat total body water deficits; distributed equally in ECF & ICF
ii. Hypotonic NaCl solutions expand both ICF & ECF
B. Colloids
a. Contain cells, proteins or synthetic macromolecules
b. Expand intravascular volume only
c. Can cause fluid shifts from interstitial & ICF columes