Intravenous infusion of a 0.9% NaCl solution, which has an osmolality of approximately 290 mOsm/kg H2O, is commonly used to supplement the body's fluid volume. It should be noted that there are some exceptions to the osmolality of certain body fluids. For example, the cerebrospinal fluid has a slightly higher osmolality compared to the extracellular fluid (ECF) found elsewhere in the body. Additionally, certain regions within the kidney may have osmolalities that differ from the ECF. However, these variations are relatively small in comparison to the total volume of the ECF, which is greater than 12 L. 

Serum, derived from clotted blood, differs from plasma in that it lacks clotting factors. However, the osmolality and concentrations of other molecules and ions in plasma and serum are nearly identical. When electrolyte solutions are administered intravenously, such as via the infusion of NaCl solutions, they rapidly equilibrate between the plasma and interstitial fluid due to the high permeability of capillary walls to water and electrolytes. As a result, these fluids effectively distribute throughout the entire ECF. 

A 0.9% NaCl solution, which contains 0.9 g of NaCl per 100 mL, has a NaCl concentration of 154 mmol/L. Since NaCl does not dissociate completely in solution, resulting in an osmolality of 1.88 Osm/mol, the osmolality of this solution is approximately 290 mOsm/kg H2O, which closely resembles the normal osmolality of the ECF. 

In certain clinical scenarios, a more accurate estimate of serum osmolality, as well as the osmolalities of the ECF and intracellular fluid (ICF), can be obtained by considering the osmoles contributed by glucose and urea. These two solutes are the next most abundant after NaCl in the ECF, with the other components contributing only a few additional milliosmoles. Estimating serum osmolality involves expressing glucose and urea concentrations in milligrams per deciliter, dividing by 18 for glucose and 2.8 for urea to convert to millimoles per liter, and then converting to milliosmoles per kilogram of H2O. This estimation is particularly useful in managing patients with elevated serum glucose concentrations due to diabetes mellitus and in patients with chronic renal failure whose serum urea concentration is elevated due to reduced renal excretion. 

The ability of a substance to induce water movement across the plasma membrane of a cell depends on whether the substance itself can cross the membrane. Glucose and urea are able to cross the cell membrane, but although they contribute to serum osmolality as measured by a laboratory osmometer (where all molecules are considered "effective osmoles"), they are ineffective osmoles for water movement across many, but not all, cell membranes. In contrast, Na+ is an effective osmole for water movement across the plasma membrane of virtually all cells. Equation 2.2 provides the best estimate of the effective osmolality of the serum. The concentration of urea in plasma is measured as the nitrogen content in the urea molecule, also known as blood urea nitrogen (BUN).
Addition of an isotonic NaCl solution (e.g., intravenous infusion of 0.9% NaCl: osmolality ≈ 290 mOsm/kg H2O) bSome exceptions do exist. The cerebrospinal fluid is part of the ECF, but its osmolality is slightly higher than that of the ECF elsewhere in the body. Also, regions within the kidney can have osmolalities that are either less than or greater than that of the ECF. However, these volumes are small (≈150 mL) in comparison with the total volume of the ECF (≥12 L). cSerum is derived from clotted blood. Thus serum differs from plasma by the absence of clotting factors. With regard to osmolality and the concentrations of other molecules and ions, the osmolality and concentrations in plasma and serum are virtually identical. dFluids are usually administered intravenously. When electrolyte solutions are infused by this route, equilibration between plasma and interstitial fluid is rapid (i.e., minutes) because of the high permeability of many capillary walls for water and electrolytes. Thus these fluids are essentially added to the entire ECF. eA 0.9% NaCl solution (0.9 g NaCl/100 mL) contains 154 mmol/L of NaCl. Because NaCl does not dissociate completely in solution (i.e., 1.88 Osm/mol), the osmolality of this solution is 290 mOsm/kg H2O, which is very similar to that of normal ECF.
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments
Insomeclinicalsituations,itispossibletoobtainamoreaccurateestimateoftheserumosmolality,andthustheosmolalitiesoftheECFandICF,byalsoconsideringtheosmolescontributedbyglucoseandurea,asthesearethenextmostabundantsolutesintheECF(theothercomponentsoftheECFcontributeonlyafewadditionalmilliosmoles).Accordingly,serumosmolalitycanbeestimatedasfollows: 18 28.
Theglucoseandureaconcentrationsareexpressedinunitsofmilligramsperdeciliter(dividingby18forglucoseand2.8for allowsconversionfromtheunitsofmilligramsperdecilitertomillimolesperliterandthustomilliosmolesperkilogramofH2O).Thisestimationofserumosmolalityisespeciallyusefulintreatingpatientswhohaveanelevatedserumglucoseconcentrationsecondarytodiabetesmellitus,andinpatientswithchronicrenalfailure,whoseserumureaconcentrationiselevatedbecauseofreducedrenalexcretion.
,theabilityofasubstancetocausewatertomoveacrosstheplasmamembraneofacelldependsonwhetherthesubstanceitselfcrossesthemembrane.Recall
Eq.1.9 whereΠe= theeffectiveosmoticpressureandσ= thereflectioncoefficientforthesubstance.Formanycells,glucoseandureacrossthecellmembrane.Althoughtheycontributetoserumosmolality,asmeasuredbyalaboratoryosmometerwhereallmoleculesare“effectiveosmoles,”theyareineffectiveosmolesforwatermovementacrossmany,butnotall,cellmembranes.Incontrast,Na+ isan“effectiveosmole”forwatermovementacrosstheplasmamembraneofvirtuallyallcells.
Eq.2.2 givesthebestestimateoftheeffectiveosmolalityoftheserum.
*Theureaconcentrationinplasmaismeasuredasthenitrogenintheureamolecule,orbloodureanitrogen(BUN).