Composition of Body Fluids

Electrolytes are compounds that dissociate into ions in water, including inorganic salts, acids, and bases. Non-electrolytes are organic molecules like glucose, lipids, creatinine, and urea held together by covalent bonds that do not dissociate into ions. Electrolytes are most effective at causing fluid shifts because they influence osmotic gradients in body fluids.

Sodium and potassium ion concentrations in extracellular fluid and intracellular fluid are nearly inverse due to ATP-dependent sodium-potassium pumps. These cellular pumps actively maintain low sodium and high potassium concentrations inside cells while keeping sodium high and potassium low outside cells, enabling proper cellular function and fluid balance.

Sodium and chloride are the principal ions in plasma and interstitial fluid, with smaller amounts of phosphate, bicarbonate, and other cations and anions. Plasma contains slightly fewer chloride ions than interstitial fluid due to the presence of non-penetrating anionic plasma proteins that occupy space in the solution.

Proteins and other large non-electrolytes comprise a significant portion of dissolved solutes in body fluids. In plasma, proteins alone account for approximately 7% of volume and contribute substantially to osmotic concentration. These large molecules are largely retained in plasma and crucial for maintaining fluid balance across different compartments.

Intracellular fluid contains lower levels of sodium and chloride ions compared to extracellular fluid, with potassium as the primary cation and phosphate as the major anion. It also contains a significant amount of soluble proteins essential for cellular functions and maintaining osmotic balance within cells.

Extracellular fluid, which includes plasma and interstitial fluid, is clinically vital because it enables rapid solute and fluid exchange between blood plasma and interstitial fluid. During hemorrhage or dehydration, blood or saline is administered primarily to maintain extracellular fluid volume and preserve this critical exchange function.

Electrolyte concentrations in body fluids are typically expressed in milliequivalents per liter (mEq/L), which indicates the number of electric charges in a liter of solution. This measurement is important because electrolytes are the most plentiful solutes in body fluids and are responsible for their chemical and physical reactions.