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Solute: Particles dissolved in a solvent (ie, electrolytes, glucose, urea, proteins)
Solvent: Liquid that dissolves the solute creating a solution, usually H2O
Osmolality: Number of dissolved particles in a solution, expressed as osmoles per kilogram of solvent (Osm/kg)
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Total Body Water And Distribution
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Total body water (TBW) = about 60% of total body weight.
Two-thirds of TBW is intracellular = 40% of TBW (Table 7.1).
Cell membranes are semipermeable and allow free passage of solvent, allowing water to move freely down concentration gradients between intracellular, interstitial, and plasma compartments to maintain equilibrium.
In contrast, solutes require energy to be actively pumped across membranes or need ion gradients to flow through opened channels within a membrane.
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KEY FACT
Total blood volume in adults is 70 mL/kg or about 5 L in a 70-kg person. Total blood volume is 80-90 mL/kg in children.
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Hyper- and Hypo-Osmolar States
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Serum osmolality can be measured in the laboratory using an osmometer. Normal circulating solutes include sodium salts, glucose, and urea. The normal measured serum osmolality is around 285 mOsm/kg. A rise in the measured serum osmolality can be due to an increase in a normal circulating solute (eg, hypernatremia, uremia) or the presence of additional particles (eg, alcohols, mannitol).
Osmolality can also be calculated using the following formula:
If additional particles are present, the measured osmolality will be much larger than the calculated osmolality = osmolal gap. A normal osmolal gap should be < 10 mOsm/kg. High osmolal gaps are caused by increases in measured (but not calculated) serum particles. Causes include:
Acetone
Glycerol
Ethylene glycol
Mannitol
Isopropyl alcohol
Sorbitol
Methanol
Ethanol
Formaldehyde
Paraldehyde
Severe hyperlipidemia
Severe hyperproteinemia
Account for ethanol’s contribution to the osmolal gap by dividing the blood ethanol in mg/dL by 4.6:
Hypo-osmolar states are usually caused by hyponatremia.
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KEY FACT
In patients with altered mental status and an unexplained increased anion gap, use the osmolal gap as a screening test for unmeasured osmoles, such as methanol or ethylene glycol.
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Q
An 86-year-old man with urosepsis presents dehydrated. How much intravascular fluid does 1 L of 0.9 normal saline (NS) provide? How about 1 L of 0.45 NS or D5W?