D’YOUVILLE COLLEGE
BIOLOGY 307/607 - PATHOPHYSIOLOGY
Lecture 17 - FLUID & ELECTROLYTE IMBALANCES
Chapter 16
1.
Body Water:
• compartments (fig. 16 - 1 & ppt. 1):
- 65% of body water is intracellular (= ICF)
- remaining body water is extracellular (= ECF) mainly distributed through
plasma (8%), interstitial fluid (25%), and transcellular water (2%); exchanges
between compartments occur via osmosis & bulk flow (fig. 16 - 2 & ppt. 2)
• water balance:
- neutral: intake = output; positive: intake exceeds output; negative: output
exceeds intake
- daily outputs: urination (1500 ml.) + fecal (300 ml.) + exhalation (300 ml.)
+ insensible loss (evaporation through skin and sweating -- 400 ml.)
- daily intakes: drinking (1300 ml.) + ingested food (900 ml.) + metabolic
water (formed during metabolic reactions -- 300 ml.) (fig. 16 - 3, table 16 - 1 & ppts. 3 & 4)
- physiological maintenance via ADH & thirst mechanisms (fig. 16 - 8 &
ppt. 5)
2.
Electrolytes:
• important cations & anions: charged particles (ions) that conduct currents
through fluids are electrolytes; important cations (positively charged): sodium,
potassium, & calcium; important anions (negatively charged): chloride, bicarbonate,
& phosphate (proteins are mostly confined to cytoplasm & plasma and usually are
anionic)
- distribution: (figs. 16 - 4, 16 - 5 & ppts. 6 to 8) results from diffusion
between body fluid compartments, with important contributions from transport
Bio 307/607 lec 17
- p. 2 -
systems (active transport systems = 'pumps' establish asymmetries of distribution (table
16 - 2), e.g., extracellular sodium (approx. 140 milliosmoles/liter) exceeds
intracellular by factor of 10 (14 mOsm./l.) & intracellular potassium is about 35x
more concentrated than extracellular (140 mOsm./l. vs. 4 mOsm./l.)
Bio 307/607 lec 17
- p. 3 -
• osmotic balance (fig. 16 - 6 & ppt. 8): any solution that has higher solute
concentration than cytoplasm is hypertonic; water will leave cells and cause
shrinkage of cells
- solution that has identical solute concentration with cytoplasm is isotonic
- solution that has lower solute concentration than cytoplasm is hypotonic;
water will enter cells and cause swelling of cells
• balance of electrolytes: input through dietary intake; output through sweating,
feces, & urine
- sodium has such a high concentration in ECF, its processing by the kidney
exerts major influence upon processing of bicarbonate and chloride, and especially water (fig.
16 -7 & ppt. 9), e.g., renin-angiotensin system stimulates aldosterone release, which
stimulates sodium retention & also water retention (fig. 16 - 9 & ppt. 10)
3.
Fluid Imbalance:
• dehydration: excessive water loss through vomiting, diarrhea, profuse
sweating, prolonged inadequate intake, diabetes insipidus (failing ADH mechanism) or
diabetes mellitus (failing insulin mechanism) (fig. 16 - 10 & ppt. 11)
• water intoxication: increased body water unaccompanied by sodium
increase; likely causes are excessive water drinking (polydipsia) or excessive ADH
secretion (SIADH), latter caused by tumors, neural disorders, and drugs
• fluid imbalances: loss and/or gain of sodium accompanies water
imbalance; these are called volume depletions or expansions (table 16 - 3); some of the
major consequences involve changes in skin turgor, cellular shrinkage or swelling
(especially harmful in cerebrum), hypovolemic shock, and increased blood viscosity
due to increased hematocrit (figs. 16 - 11, 16 - 12 & ppts. 12 & 13)
Bio 307/607 lec 17
4.
- p. 4 -
Electrolyte Imbalance:
• sodium imbalances:
- deficiency (hyponatremia) is seldom due to inadequate intake; losses may
occur through failing aldosterone reflex, failing kidney retention, or excessive water levels (=
relative hyponatremia)
- excess (hypernatremia) usually results from water loss outpacing sodium loss
(relative hypernatremia); usually water retention mechanisms (ADH & thirst) correct
imbalance
• potassium imbalances: mainly cause disruptions of electrical properties of
neural and muscular tissues, e.g., cardiac function (figs. 16 - 14, 16 - 15 & ppt. 14)
- hypokalemia (deficiency) may arise from factors that shift potassium into
cells or instigate excessive losses or impaired intake (figs. 16 - 13, 16 - 20, 16 - 21 & ppts. 15
& 16)
- hyperkalemia (excess) may arise from factors that produce effects
opposite to those instigating a deficiency -- factors that cause potassium release from
cytoplasm, or impair excretion; more likely to be life-threatening than hypokalemia
5.
Acid-Base Imbalance:
• Acids: hydrogen ion donors; concentration: # of gram-molecular weights
(moles)/liter; strength: degree of dissociation
- strong acids typically are mostly ionized yielding the maximum # of H+:
HCl <- ---------> H+ + Cl
+
- weak acids will be mostly non-ionized yielding small # of H :
-
HA <-------- --> H+ + A
- volatile acid: carbon dioxide forms weak acid in water (carbonic acid)
- all other sources of hydrogen ions (e.g. lactic acid, hydrochloric acid,
ketone bodies) are fixed acids; a loss of bicarbonate ion produces an acidosis also
Bio 307/607 lec 17
- p. 5 -
•. Bases: consume hydrogen ions from solution; the anion portion of a
dissociated acid is known as conjugate base
• Body Fluid pH:
- intracellular fluid (cytoplasm) - normal pH = 7.0
- extracellular fluid (plasma, interstitial fluid) has normal pH = 7.4 (range
7.35 - 7.45)
- pHECF > 7.45 constitutes alkalosis; pHECF < 7.35 constitutes acidosis
- alkalosis causes hyperactivity of nervous system (convulsions)
- acidosis causes depression of nervous system (coma)
- changes in PCO2 (elevation -> resp. acidosis; depression -> respiratory
alkalosis)
- changes in other acids (elevation = metabolic acidosis; depression = metabolic
alkalosis)
- pH changes caused by bicarbonate ion (depression = metabolic acidosis;
elevation = metabolic alkalosis) (fig. 16 - 23 & ppt. 17)
• Buffers: systems of weak acid + conjugate base which react to offset changes
in hydrogen ion concentration of a solution; increasing acidity causes buffer system to
behave as a base; decreasing acidity causes buffer system to behave as an acid (fig. 16
- 17 & ppt. 18); systems interact to maintain pH between 7.35 - 7.45
- bicarbonate system: mostly associated with extracellular fluid; important
because of linkage to respiratory and renal influences on pH; ratio of bicarbonate to
carbonic acid (normally 20:1) provides powerful influence over acid-base balance
H2O + CO2 <-- * --> H2CO3 <-------- ---> H+ + HCO3* = carbonic anhydrase enzyme
- protein system(s): highest concentration; mostly intracellular (e.g.
hemoglobin oxygenation reaction)
- phosphate system: mostly intracellular, also important in renal tubules
- ammonia: mainly renal tubules
Bio 307/607 lec 17
- p. 6 -
• Compensation of Acid-Base Imbalance (fig. 16 - 24 & ppt. 21):
- respiratory adjustment:
- hyperventilation blows off CO2 causing shift in bicarbonate system
+
which consumes more H (alkalinizing effect, higher pH)
- hypoventilation accumulates CO2 causing shift in bicarbonate system
+
which elevates H (acidifying effect, lower pH)
- renal adjustment (figs. 16 - 18, 16 - 19 & ppts. 19 & 20):
- correcting acidosis: kidneys secrete hydrogen ion (tubular secretion) in
conjunction with bicarbonate absorption, strengthening buffer system
- correcting alkalosis: kidneys retain hydrogen ion in conjunction with
bicarbonate excretion
- other buffers in renal tubules extend H+ load that can be excreted