The volume of the extracellular fluid (ECF) increases when fluid is administered to it. The osmolality of the administered fluid is the same as that of the ECF and the intracellular fluid (ICF), resulting in no fluid movement between these compartments and no change in the volume of the ICF. The presence of the Na+,K+-ATPase in cell membranes restricts the movement of Na+ across the cell membrane, effectively confining it to the ECF. Therefore, the infused isotonic NaCl solution does not move into the cells.

When a hypotonic NaCl solution is added to the ECF, the osmolality of the ECF decreases, causing water to move into the ICF. This can occur through neurosurgical procedures, cerebrovascular accidents, or other conditions that result in the accumulation of interstitial fluid in the brain, leading to edema and neuronal swelling. Edema can raise intracranial pressure, disrupt neuronal function, and potentially result in coma and death. To remove excess fluid from brain tissue, an osmotic gradient can be imposed across the blood-brain barrier using substances like mannitol. Mannitol, which is unable to readily cross the blood-brain barrier and cell membranes, acts as an effective osmole, causing interstitial fluid to move out of the brain by osmosis.

The addition of a hypertonic NaCl solution to the ECF increases the osmolality of the ECF, leading to the movement of water out of cells. After osmotic equilibration, the osmolalities of the ECF and ICF are equal but higher than before the infusion. The volume of the ECF increases, while the volume of the ICF decreases.

The compositions of the ECF and ICF differ significantly. The ICF contains more proteins and macromolecules than the ECF. There are also differences in the concentrations of various ions. The composition of the ICF is maintained by specific cell membrane transport proteins, with the Na+,K+-ATPase being a key transporter. This transporter utilizes ATP to generate ion and electrical gradients, which can be utilized to drive the transport of other ions and molecules through ion channels and solute carriers.

The compositions of the plasma and interstitial fluid compartments of the ECF are similar, as they are separated only by the capillary endothelium, which allows ions and small molecules to permeate. The major difference between interstitial fluid and plasma is the higher protein concentration in plasma. This difference in protein concentration can affect the overall composition of the ECF compartments.
to the ECF increases the volume of this compartment by the volume of fluid administered. Because this fluid has the same osmolality as does the ECF, and therefore the ICF, there is no driving force for fluid movement between these compartments, and the volume of the ICF remains unchanged. Although Na+ can cross cell membranes, it is effectively restricted to the ECF by the activity of the Na+,K+-ATPase, which is present in the plasma membrane of all cells (see the section “
Ionic Composition of Cells”).
Therefore, there is no net movement of the infused isotonic NaCl solution into cells.
Example 2: Addition of Hypotonic Sodium Chloride to the Extracellular Fluid
Addition of a hypotonic NaCl solution to the ECF (e.g., intravenous infusion of 0.45% NaCl; osmolality ≅ 145 mOsm/kg H2O) decreases the osmolality of this fluid compartment, which results in the movement of water into
Neurosurgicalproceduresandcerebrovascularaccidents(strokes)oftenresultintheaccumulationofinterstitialfluidinthebrain(i.e.,edema)andswellingoftheneurons.Becausethebrainisenclosedwithintheskull,edemacanraiseintracranialpressureandtherebydisruptneuronalfunction,whichleadstocomaanddeath.Theblood-brainbarrier,whichseparatesthecerebrospinalfluidandbraininterstitialfluidfromblood,canbepermeatedfreelybywaterbutnotbymostothersubstances.Asaresult,excessfluidinbraintissuecanberemovedbyimposinganosmoticgradientacrosstheblood-brainbarrier.Mannitolcanbeusedforthispurpose.Mannitolisasugar(molecularweight,182g/mol)thatdoesnotreadilycrosstheblood-brainbarrierandmembranesofcells(neuronsandothercellsinthebody).Therefore,mannitolisaneffectiveosmole,andintravenousinfusionresultsinthemovementofinterstitialfluidoutofthebrainbyosmosis.
the ICF. After osmotic equilibration, the osmolalities of the ICF and ECF are again equal but lower than before the infusion, and the volume of each compartment is increased. The increase in ECF volume is greater than the increase in ICF volume.
Example 3: Addition of Hypertonic Sodium Chloride to the Extracellular Fluid
Addition of a hypertonic NaCl solution to the ECF (e.g., intravenous infusion of 3% NaCl: osmolality ≅ 1000 mOsm/kg H2O) increases the osmolality of this compartment, which results in the movement of water out of cells. After osmotic equilibration, the osmolalities of the ECF and ICF are again equal but higher than before the infusion. The volume of the ECF is increased, whereas that of the ICF is decreased.
Composition of Body Fluid Compartments
The compositions of the ECF and ICF differ considerably. The ICF has significantly more proteins and macromolecules than the ECF. There are also differences in the concentrations of many ions. The composition of the ICF is maintained by the action of a number of specific cell membrane transport proteins. Principal among these transporters is the Na+,K+adenosine triphosphatase (Na+,K+-ATPase), which converts the energy in ATP into ion and electrical gradients, which can in turn be used to drive the transport of other ions and molecules by means of ion channels and solute carriers (e.g., symporters and antiporters).
The compositions of the plasma and interstitial fluid compartments of the ECF are similar because those compartments are separated only by the capillary endothelium, a barrier that ions and small molecules can permeate. The major difference between the interstitial fluid and plasma is that the latter contains significantly more protein. Although this differential concentration of protein can affect the