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ROUTES OF GAINS AND LOSSES
Water and electrolytes are gained in various ways. A healthy per-son gains fluids by drinking and eating. In patients with some disorders, fluids may be provided by the parenteral route (intra-venously or subcutaneously) or by means of an enteral feeding tube in the stomach or intestine.
The usual daily urine volume in the adult is 1 to 2 L. A general rule is that the output is approximately 1 mL of urine per kilo-gram of body weight per hour (1 mL/kg/h) in all age groups.
Sensible perspiration refers to visible water and electrolyte loss through the skin (sweating). The chief solutes in sweat are sodium, chloride, and potassium. Actual sweat losses can vary from 0 to 1,000 mL or more every hour, depending on the environmental temperature. Continuous water loss by evaporation (approxi-mately 600 mL/day) occurs through the skin as insensible per-spiration, a nonvisible form of water loss. Fever greatly increases insensible water loss through the lungs and the skin, as does loss of the natural skin barrier (through major burns, for example).
The lungs normally eliminate water vapor (insensible loss) at a rate of approximately 400 mL every day. The loss is much greater with increased respiratory rate or depth, or in a dry climate.
The usual loss through the GI tract is only 100 to 200 mL daily, even though approximately 8 L of fluid circulates through the GI system every 24 hours (called the GI circulation). Because the bulk of fluid is reabsorbed in the small intestine, diarrhea and fistulas cause large losses. In healthy people, the daily av-erage intake and output of water are approximately equal (Table 14-2).
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