Fluid and electrolyte disorders are commonly seen in clinical practice, such as in patients with vomiting or diarrhea. In most cases, these disorders resolve on their own without intervention. However, more severe or prolonged disorders may require fluid replacement therapy. This therapy can be given orally with special electrolyte solutions or intravenously with fluids.

There are various formulations of intravenous solutions available. The choice of fluid depends on the patient's needs. For example, if there is a need to increase the patient's vascular volume, a solution containing substances that do not easily cross the capillary wall may be infused, such as 5% protein or dextran solutions. These solutions cause fluid to be retained in the vascular compartment, expanding its volume. Expansion of the extracellular fluid is usually done with isotonic saline solutions, such as 0.9% NaCl or lactated Ringer's solution. When isotonic NaCl solution is administered, there is no osmotic pressure gradient across the plasma membrane of cells, so the entire volume of the infused solution remains in the extracellular fluid.

Patients with hyperosmotic body fluids require hypotonic solutions, such as 0.45% NaCl or 5% dextrose in water (D5W). Infusing D5W is equivalent to infusing distilled water because the dextrose is metabolized to CO2 and water. Administration of these fluids increases the volumes of both the intracellular fluid and extracellular fluid. On the other hand, patients with hypotonic body fluids require hypertonic solutions, typically NaCl-containing solutions like 3% or 5% NaCl. These solutions expand the volume of the extracellular fluid but decrease the volume of the intracellular fluid. Other constituents, such as electrolytes (e.g., K+) or drugs, can be added to intravenous solutions to tailor the therapy to the patient's fluid, electrolyte, and metabolic needs.

Although there is a distribution of cations and anions between the intracellular and extracellular compartments due to the Gibbs-Donnan effect, this effect is minimal, and the ionic compositions of the interstitial fluid and plasma can be considered the same.

Maintaining cellular homeostasis is crucial for normal cellular function. The ionic composition of the intracellular fluid must be tightly regulated. This is important for pH-dependent enzyme activity, intracellular electrolyte balance, establishment of membrane potential, and intracellular signaling involving calcium ions. Additionally, maintaining the volume of cells is necessary to prevent cell damage or death. The regulation of intracellular potassium (K+) levels is around 3.5-5.0 mEq/L, and the total and ionized calcium (Ca++) levels are approximately 2.1-2.8 mmol/L and 1.1-1.4 mmol/L respectively, with an ionized concentration of approximately 10^-7 M.
Fluidandelectrolytedisordersareobservedcommonlyinclinicalpractice(e.g.,inpatientswithvomitingordiarrhea,orboth).Inmostinstances,thesedisordersareself-limited,andcorrectionofthedisorderoccurswithoutneedforintervention.However,moresevereorprolongeddisordersmaynecessitatefluidreplacementtherapy.Suchtherapymaybeadministeredorally,withspecialelectrolytesolutions,orintravenously,withfluid.
Intravenoussolutionsareavailableinmanyformulations.Thetypeoffluidadministeredtoaparticularpatientisdictatedbythepatient’sneed.Forexample,ifanincreaseinthepatient’svascularvolumeisnecessary,asolutioncontainingsubstancesthatdonotreadilycrossthecapillarywallisinfused(e.g.,5%proteinordextransolutions).Theoncoticpressuregeneratedbythealbuminmoleculescausesfluidtoberetainedinthevascularcompartment,whichexpandsitsvolume.ExpansionoftheECFisaccomplishedmostoftenwithisotonicsalinesolutions(e.g.,0.9%NaClorlactatedRingersolution).Asalreadynoted,administrationofanisotonicNaClsolutiondoesnotresultinthedevelopmentofanosmoticpressuregradientacrosstheplasmamembraneofcells.Therefore,theentirevolumeoftheinfusedsolutionremainsintheECF.
Patientswhosebodyfluidsarehyperosmoticneedhypotonicsolutions.ThesesolutionsmaybehypotonicNaCl(e.g.,0.45%NaCl)or5%dextroseinwater(D5W).AdministrationoftheD5WsolutionisequivalenttotheinfusionofdistilledwaterbecausethedextroseismetabolizedtoCO2andwater.AdministrationofthesefluidsincreasesthevolumesofboththeICFandECF.Inaddition,patientswhosebodyfluidsarehypotonicneedhypertonicsolutions.ThesearetypicallyNaCl-containingsolutions(e.g.,3%or5%NaCl).ThesesolutionsexpandthevolumeoftheECFbutdecreasethevolumeoftheICF.Otherconstituents,suchaselectrolytes(e.g.,K+)ordrugs,canbeaddedtointravenoussolutionstotailorthetherapytothepatient’sfluid,electrolyte,andmetabolicneeds.
distribution of cations and anions between these two compartments by the Gibbs-Donnan effect (see the section “ ” for details), this effect is small, and the ionic compositions of the interstitial fluid and plasma can be considered to be identical.
Maintenance of Cellular Homeostasis
Normal cellular function requires that the ionic composition of the ICF be tightly controlled. For example, the activity of some enzymes is pH dependent; therefore, intracellular pH must be regulated. In addition, the intracellular composition of other electrolytes is similarly held within a narrow range. This is necessary for the establishment of the membrane potential, a cell property especially important for the normal function of excitable cells (e.g., neurons and muscle cells) and for intracellular signaling (e.g., intracellular [Ca++]; see for details). Finally, the volume of cells must be maintained because shrinking or swelling of cells can lead to cell damage or death. The regulation of intracellular
K+ 3.5-5.0mEq/L120-150mEq/L
Ca++ 2.1-2.8(total)mmol/L 1.1-1.4(ionized)≈10−7M(ionized)