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Water and Electrolyte balanceV. Sutarmo Setiadji3/5/20121VSS

Body composition: 18 %-protein, 15 %-fat, 7 %-mineral, 60% - waterWater : 2/3 intracellular fluid 1/3 extracellular fluidPlasma (intravascular space)Lymph, CSF, synovial fluidinterstitial fluidgut3/5/20122VSS

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Kation:Na+, K+, Ca2+, Mg2+

Anion:Cl-, HCO3-, HPO42- , A-Electrolytes:3/5/20128VSS

Someions in the bodyKationAntarselIntraselNa+14512K+4150Ca2+1 - 210-7Mg2+142AnionCl-1134HCO3-2712A-1375

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Units for measuring concentrationMoles: a mole (mol) is the gram molecular weight of a substance the molecular weight of the substance in grams.1 mol of NaCl = 23 g + 35.5 g =58.5 g 1 mmol of NaCl = 58.5 mg1 mol subst contain 6.02 X 1023 particles(Avogadro number)The mole: the standard unit for expressing the amount of a substance in SI system Molarity: number of moles of solute in 1 L of solution.VSS3/5/201210

Tubular ReabsorptionWater reabsorption: About 180 L per day is filtered by glomerulus, and only 2 L urine is produced per day 178 L is reabsorbed.Aquaporin-1 (water channels) naturally installed (inserted) in proximal tubules 60 70 % of water and solute are reabsorbed here (Obligatory water reabsorbtion).3/5/201211VSS

About 30 40% of water is reabsorb facultatively, which is regulated by ADH (arginine vasopressin = AVP), a hormone secreted by posterior hypophysis = neurohypophysis.The increase of plasma osmolality will increase the secretion of ADH which function in installing the aquaporin-2 in the collecting ducts more water will be reabsorbed.3/5/201212VSS

Units for measuring concentrationMoles: a mole (mol) is the gram molecular weight of a substance the molecular weight of the substance in grams.1 mol of NaCl = 23 g + 35.5 g =58.5 g 1 mmol of NaCl = 58.5 mg1 mol subst contain 6.02 X 1023 particles(Avogadro number)The mole: the standard unit for expressing the amount of a substance in SI system Molarity: number of moles of solute in 1 L of solution.3/5/201213VSS

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When loss of water is greaterthan loss ofelectrolytes(salt), theosmotic pressureof the extracellular fluidsbecomes higher than in the cells. Since water passes from a region of lower to a region of higher osmoticpressure, water flows out of the cells into theextracellular fluid, tending to lower its osmoticpressureand increase its volume toward normal. As a result of the flow of water out of the cells,3/5/201216VSS

Dehydration describes a state of negative fluid balance that may be caused by numerous disease entities. Diarrheal illnesses are the most common etiologies. Worldwide, dehydration secondary to diarrheal illness is the leading cause of infant and child mortality.3/5/201217VSS

PathophysiologyThe negative fluid balance that causes dehydration results from decreased intake, increased output (renal, GI, or insensible losses), or fluid shift (ascites, effusions, and capillary leak states such as burns andsepsis). The decrease in total body water causes reductions in both the intracellular and extracellular fluid volumes. Clinical manifestations of dehydration are most closely related to intravascular volume depletion. As dehydration progresses, hypovolemic shock ultimately ensues, resulting in end organ failure and death.Young children are more susceptible to dehydration due to larger body water content, renal immaturity, and inability to meet their own needs independently. Older children show signs of dehydration sooner than infants due to lower levels of extracellular fluid (ECF).Dehydration can be categorized according to osmolarity and severity. Serum sodium is a good surrogate marker of osmolarity assuming the patient has a normal serum glucose. Dehydration may be isonatremic (130-150 mEq/L), hyponatremic (< 130 mEq/L), or hypernatremic (>150 mEq/L). Isonatremic dehydration is the most common (80%). Hypernatremic and hyponatremic dehydration each comprise 5-10% of cases. Variations in serum sodium reflect the composition of the fluids lost and have different pathophysiologic effects.Isonatremic (isotonic) dehydration occurs when the lost fluid is similar in sodium concentration to the blood. Sodium and water losses are of the same relative magnitude in both the intravascular and extravascular fluid compartments.Hyponatremic (hypotonic) dehydration occurs when the lost fluid contains more sodium than the blood (loss of hypertonic fluid). Relatively more sodium than water is lost. Because the serum sodium is low, intravascular water shifts to the extravascular space, exaggerating intravascular volume depletion for a given amount of total body water loss.Hypernatremic (hypertonic) dehydration occurs when the lost fluid contains less sodium than the blood (loss of hypotonic fluid). Relatively less sodium than water is lost. Because the serum sodium is high, extravascular water shifts to the intravascular space, minimizing intravascular volume depletion for a given amount of total body water loss.Neurologic complications can occur in hyponatremic and hypernatremic states. Severe hyponatremia may lead to intractable seizures, whereas rapid correction of chronic hyponatremia (>2 mEq/L/h) has been associated with central pontine myelinolysis. During hypernatremic dehydration, water is osmotically pulled from cells into the extracellular space. To compensate, cells can generate osmotically active particles (idiogenic osmoles) that pull water back into the cell and maintain cellular fluid volume. During rapid rehydration of hypernatremia, the increased osmotic activity of these cells can result in a large influx of water, causing cellular swelling and rupture; cerebral edema is the most devastating consequence. Slow rehydration over 48 hours generally minimizes this risk.

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