Disorders of Water Balance
The human body at birth is approximately 75% water by weight. By 1 month this value decreases to 65%, and by adulthood to 60% for males and 50% for females. The higher fat content in females decreases water content. For the same reason, obesity and advanced age further decrease water content.
The normal adult daily water intake averages 2500 mL, which includes approximately 300 mL as a byproduct of the metabolism of energy substrates. Daily water loss averages 2500 mL and is typically accounted for by 1500 mL in urine, 400 mL in respi-ratory tract evaporation, 400 mL in skin evaporation, 100 mL in sweat, and 100 mL in feces. Evaporative loss is very important in thermoregulation because this mechanism normally accounts for 20–25% of heat loss.
Both ICF and ECF osmolalities are tightly regulated to maintain normal water content in tis-sues. Changes in water content and cell volume can induce serious impairment of function, particularly in the brain .
The osmolality of ECF is equal to the sum of the concentrations of all dissolved solutes. Because Na + and its anions account for nearly 90% of these sol-utes, the following approximation is valid:
Plasma osmolality = 2 × Plasma sodium concentration
Moreover, because ICF and ECF are in osmotic equilibrium, plasma sodium concentration [Na+]plasma generally reflects total body osmolality:
Because sodium and potassium are the major intra- and extracellular solutes, respectively:
Combining the two approximations:
Using these principles, the effect of isotonic, hypotonic, and hypertonic fluid loads on compart-mental water content and plasma osmolality can be calculated ( Table 49–3). The potential importance of intracellular potassium concentration is readily apparent from this equation. Thus significant potas-sium losses may contribute to hyponatremia.
In pathological states, glucose and—to a much lesser extent—urea can contribute significantly to extracellular osmolality. A more accurate approxi-mation of plasma osmolality is therefore given by the following equation:
where [Na+] is expressed as mEq/L and blood urea nitrogen (BUN) and glucose as mg/dL. Urea is an ineffective osmole because it readily permeates cell membranes and is therefore frequently omitted from this calculation:
Plasma osmolality normally varies between 280 and 290 mOsm/L. Plasma sodium concentra-tion decreases approximately 1 mEq/L for every 62 mg/dL increase in glucose concentration. A discrepancy between the measured and calculated osmolality is referred to as an osmolal gap. Signifi-cant osmolal gaps indicate a high concentration of an abnormal osmotically active molecule in plasma such as ethanol, mannitol, methanol, ethylene gly-col, or isopropyl alcohol. Osmolal gaps may also be seen in patients with chronic kidney failure (attrib-uted to retention of small solutes), patients with ketoacidosis (as a result of a high concentration of ketone bodies), and those receiving large amounts of glycine (as during transurethral resection of the prostate). Lastly, osmolal gaps may also be present in patients with marked hyperlipidemia or hyper-proteinemia. In such instances, the protein or lipid part of plasma contributes significantly to plasma volume; although plasma [Na+] is decreased, [Na+] in the water phase of plasma (true plasma osmolal-ity) remains normal. The water phase of plasma is normally only 93% of its volume; the remaining 7% consists of plasma lipids and proteins.
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