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Abstract Fluid and electrolyte physiology is central to the clinical management of critically ill patients. Total body water (TBW) is defined as the total volume of water within the body. TBW is comprised of the intracellular and the extracellular compartments. Intracellular fluid (ICF) makes up two thirds of TBW, and extracellular fluid (ECF) accounts for the remaining one third. The ECF also contains small quantities of other cations, including potassium, calcium, and magnesium. The cations are electrochemically balanced principally by chloride and lactate anions Sensible water losses can be measured and include urine (800–1,500 mL/24 h), stool (0–250 mL/24 h) and sweat (minimal). Sweat is a hypotonic mixture of electrolytes and water and does not contribute significantly to daily water loss except in very arid and hot climates. Insensible water losses are unmeasurable and include loss from the skin and lungs. This accounts for 600–900 mL/24 h (8–12 mL/kg/day). Insensible water loss increases 10 % for each degree of body temperature >37.2 °C. |