Composition of Body Fluids

Water functions as a solvent accommodating various solutes, which can be categorized under electrolytes and non-electrolytes. Non-electrolytes are usually held together by covalent bonds, restricting them from dissociating in solution, thereby leading to a lack of electrically charged components upon dissolving in water. They are predominantly organic molecules, such as glucose, creatinine, and urea. Electrolytes, on the other hand, are compounds that can break down into ions in water. Electrolytes are usually inorganic salts, acids, and bases, including both organic and inorganic, along with some proteins.

Water's movement adheres to osmotic gradients, moving from an area with lower osmolality to an area with higher osmolality, and as such, electrolytes are most effective in causing fluid shifts. Electrolyte concentrations in body fluids are typically expressed in milliequivalents per liter (mEq/L), indicating the number of electric charges in a liter of solution.

There is a distinctive electrolyte composition patternin each fluid compartment. Sodium is the main cation, and chloride is the major anion in the extracellular fluids. However, plasma contains slightly fewer chloride ions than interstitial fluid due to the presence of non-penetrating anionic plasma proteins. Contrastingly, intracellular fluids (ICF) contain less Na⁺ and Cl⁻, with potassium being the most abundant cation and HPO₄ ²⁻; being the major anion. Cells also have substantial soluble proteins. Sodium and potassium ion concentrations in ECF and ICF are almost inverse, reflecting the activity of cellular ATP-dependent sodium-potassium pumps that maintain low Na⁺ concentrations and high K⁺ concentrations within cells.

While electrolytes are the most plentiful solutes in body fluids and are responsible for their chemical and physical reactions, they do not constitute the majority of dissolved solutes in these fluids. Proteins and other large non-electrolytes comprise a significant portion of the dissolved solutes in body fluids. In plasma, proteins alone account for approximately 7% of the plasma's volume and contribute substantially to its osmotic concentration. These large molecules are crucial in maintaining fluid balance across different compartments, as they are largely retained in the plasma. In comparison, the interstitial fluid (IF) contains a smaller proportion of proteins and other large molecules, while the intracellular fluid (ICF) has a high concentration of proteins, essential for cellular functions and maintaining the osmotic balance within cells.