Body Water and Electrolytes

Keeping body water and dissolved substances (solutes) balanced is essential for all body functions. Because all body processes rely on molecular movements and chemical reactions, deviations in water levels and concentrations of dissolved substances can affect muscle and nerve function, change protein structures making them nonfunctional, affect blood pressure, cause edema (swelling), and result in cellular damage.

Approximately 60 percent of the mass of an average adult human is from water. Of this water, known as total body water, about two-thirds is found inside cells, and one-third is outside the cells. Water that is contained within the cell membrane (inside the cell) is referred to as intracellular fluid (ICF). Water found outside the cells is called extracellular fluid (ECF). Twenty percent of the ECF is found in the blood plasma (fluid, noncellular portion of the blood), five percent is transcellular fluid, and the remaining 75 percent of ECF is referred to as interstitial fluid, which is the fluid found outside of body cells the closely resembles plasma, but has less protein. Transcellular fluid is the fluid found in epithelial-lined spaces, such as cerebrospinal fluid, fluid in joint cavities, and fluid found inside the eye. In order to maintain normal fluid volume in the body, an individual's water input must match the water output. Water input occurs through ingestion and to a much lesser extent by the production of water through metabolic reactions. Water output results from urination, defecation, sweat, and evaporation from the skin (not including from sweat) and the respiratory surface during ventilation (specifically during exhalation).
Solutes are substances that dissolve in a fluid medium. They are found in body water as electrolytes, proteins and other nutrients, hormones, and various waste products and toxins. An electrolyte is a chemical that dissociates (separate) in water and becomes positively or negatively charged. For example, when NaCl dissolves in water, it dissociates to Na⁺ and Cl^–. Common electrolytes found in the body include sodium ions (Na⁺), potassium ions (K⁺), chloride ions (Cl^–), bicarbonate ions (HCO₃^–), calcium ions (Ca⁺⁺), magnesium ions (Mg⁺⁺), phosphate ions (PO₄^–3), acids (chemicals that release hydrogen ions [H⁺] in a solution), bases (chemicals that release hydroxide ions [OH^–] in a solution or accept a H⁺), and some proteins. Nonelectrolyte solutes do not dissociate in water and carry no charge. These include glucose sugar, lipids (fats), and urea (a toxic nitrogen-containing chemical generated during protein breakdown). The distribution of a solute depends on its size, its electrical charge, and how it is transported across a cell membrane.

Although the overall solute concentration is the same in the ECF and ICF because water moves freely between the body water compartments, the compositions of the solutes may differ. Cells have semipermeable membranes, which allow some particles to move between compartments but not others. Water moves freely in and out of cells, so if the solute concentration increases in one compartment, water will flow from the more dilute area to the area with a higher concentration, keeping the fluids in equilibrium, or at the same concentration. One major difference between intracellular and extracellular fluids is that ICF is high in K+ and low in Na+ and ECF is high in Na+ and low in K+⁺. This difference is maintained by the constant actions of Na⁺/K⁺ ATPase pumps in the cell membranes. These protein pumps use ATP molecules for energy while actively pumping sodium and potassium ions against their concentration gradients. Another difference is in the distribution of negatively charged ions. The ECF contains high amounts of Cl^– while ICF has significantly more proteins and phosphates, both carrying negative charges. The negatively charged proteins are too large to cross the cell membrane and therefore stay contained in the cell. The negative charges of the proteins are counterbalanced by the negatively charged chloride ions in the extracellular fluid. Within the extracellular fluid compartment, blood plasma differs from interstitial fluid in that it also contains negatively charged blood proteins. Blood proteins act as carrier molecules, aid in blood clotting, serve as nutrients for the tissues, and play an important role in maintaining water balance between the blood and the interstitial fluids. The presence of blood proteins prevents too much fluid from leaking out of the blood vessels during gas and nutrient exchange in the leaky capillary blood vessels.

Under certain conditions toxic solutes may be present in the body fluids, as a result of normal metabolic processes (protein breakdown) or from ingestion. If these substances are not eliminated from the body, they can disrupt cellular functions. For example, ammonia is a nitrogen-containing by-product of protein metabolism, and if allowed to accumulate in the blood, the blood would become very acidic. Most ammonia is converted to the less toxic nitrogenous waste product urea.

Water and solute balance and elimination of toxins from the body are largely dependent on the urinary system. The volume and concentration of the urine can be modified by the urinary system to keep water and solute concentrations stable. For example, if large amounts of water are consumed, the urinary system will produce a copious dilute urine, thereby conserving solutes while voiding water. The urinary system also plays an important role in eliminating toxins from the body. The urine-producing component of the urinary system is nephrons located in the kidneys. They produce a filtrate by filtering out substances from the blood and then modifying the filtrate to adjust the final urine composition, as needed.