Fluids & Electrolytes

  1. H2O content of the Body

    1. > 50% depending on:
      1. % Body Fat
      2. Age (70% as infant -- decreases thru life)
    2. Fluid compartments:
      1. Intracellular (ICF) - ~ 2/3
      2. Extracellular (ECF) - ~ 1/3 :
        1. Plasma 20% ECF
        2. Interstitial space 80% ECF
    3. Water balance
      1. Intake:
        1. fluid ingestion 60%
        2. foods 30%
        3. metabolism 10%
      2. Output:
        1. Urine 60%
        2. Sweat 8%
        3. Feces 4%
        4. "Insensible" loss (skin, lungs) 28%
    4. Regulation of Water intake:
      1. Thirst mechanism
        1. increased plasma osmolality or decrease PV ~10% hypothalmic osmoreceptors (increase ADH)
        2. thirst inhibited by GI distension, decreased osmolality, moist mucosa of throat
        3. lag in time between dehydration and urge to drink (especially in the elderly)
      2. Disorders of H2O balance:
        1. dehydration - loss of ECF
          1. H2O loss from ECF
          2. increases osmotic pressure
          3. H2O leaves cells (ICF)
        2. hyponatremia
          1. lo blood sodium
          2. H20 Intoxication:
            • Excess H2O in blood (ECF)
            • decreases osmotic pressure
            • increases H2O entry into cells (ICF)
        3. Edema - fluid accumulation in tissues (interstitial space)

  2. Electrolytes - charged ions

    1. Dissociate in solution and exert OSMOTIC pressure gradient (most numerous solutes) - cause fluid shifts in compartments
      1. ECF:
        1. Na+ (highest cation)
        2. Cl- (highest anion)
      2. ICF:
        1. K+ (highest cation)
        2. Phosphate highest anion
        3. Proteins
    2. Fluid shifts regulion:
      1. hydrostatic pressure (fluid forced out of capillary)
      2. osmotic pressure- fluid pulled back into capillary (due to the presence of large molecules such as proteins)
      3. Rule: fluid moves freely between ECF & ICF, but solutes movement is restricted (size, active transport)
    3. Na+ - most important for homeostasis controls ECF volume & H2O distribution
    4. Na+ Balance maintained via kidney:
      1. Neural control
        1. Increase BP- pressoreceptors -constrict, dilate vessels in kidney
        2. renin release -constriction and aldosterone release
      2. Hormonal control of Na+
        1. Aldosterone (Na+ reabsorption)
        2. Sex hormones
          1. Estrogen (Na+ reabsorption) - responsible for pre-menstrual edema
          2. Progesterone (diuretic)
          3. Glucocorticoids (Na+ reabsorption)
          4. Atrial natriuretic factor - favors Na+, H2O excretion (salty urine)
    5. K+ balance
      1. required for neuromuscular, metabolic normal cardiovascular function
      2. reabsorb ~90% in prox. conv. tubule, rest is secreted--> 1-1 exchange with Na+ due to aldosterone (also excretes Mg+)

  3. Acid-Base balance

    1. physiological extremes of blood:
      1. venous pH 7.35 (carbonic, lactic acid)
        1. physiologic alkalosis pH > 7.45
      2. arterial pH 7.4
        1. physiologic acidosis pH < 7.35
      3. intracellular pH ~7.0 all reactions influenced by pH of environment
    2. H+ buffering systems:
      1. Chemical (fastest) - a. Bicarbonate (ICF & ECF) kidney b. Protein (ICF & bld. [hb]) most power c. Phosphates (intracellular not plasma)
      2. Respiration (1-3 min) - most important CO2 + H2O ---> H2CO3 ---> H+ + HCO3- hyperventilation--> alkalosis CO2 retention ---> acidosis carbonic acid <--> H+ & bicarbonate (inc) CO2 eliminated at the lungs , less H+, .. (inc) pH (more sensitive than O2)
      3. Kidneys - (slowest; hrs --> days) a. tubular secretion H+ (into urine) b. conserve bicarbonate (reabsorption) c. use of phosphates & ammonia to buffer H+