Fluids and Electrolytes

Fluids and Electrolytes

The body is mostly liquid—various electrolytes dissolved in water. Electrolytes are ions (electrically charged versions) of essential elements—predominantly sodium (Na+), chloride (Cl), oxygen (O2), hydrogen (H+), bicarbonate (HCO3), calcium (Ca2+), potassium (K+), sulfate (SO42-), and phosphate (PO43-). Only ionic forms of elements can dissolve or combine with other elements. Electrolyte balance must remain in a narrow range for the body to function. The kidneys maintain chemical balance throughout the body by producing and eliminating urine. They regulate the volume, electrolyte concentration, and acid-base balance of body fluids; detoxify and eliminate wastes; and regulate blood pressure by regulating fluid volume. The skin and lungs also play a vital role in fluid and electrolyte balance. Sweating results in loss of sodium and water, and every breath contains water vapor.

Fluid balance

The kidneys maintain fluid balance in the body by regulating the amount and components of fluid inside and around the cells.

Intracellular fluid

The fluid inside each cell is called intracellular fluid. Each cell has its own mixture of components in the intracellular fluid, but the amounts of these substances are similar in every cell. Intracellular fluid contains large amounts of potassium, magnesium, and phosphate ions.

Extracellular fluid

The fluid in the spaces outside the cells, called extracellular fluid, is constantly moving. Normally, extracellular fluid includes blood plasma and interstitial fluid. In some pathologic states, it accumulates in a so-called third space, the space around organs in the chest or abdomen.

Extracellular fluid is rapidly transported through the body by circulating blood and between blood and tissue fluids by fluid and electrolyte exchange across the capillary walls. It contains large amounts of sodium, chloride, and bicarbonate ions, plus such cell nutrients as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide (CO2), transported from the cells to the lungs for excretion, and other cellular products, transported from the cells to the kidneys for excretion.

The kidneys maintain the volume and composition of extracellular fluid and, to a lesser extent, intracellular fluid by continually exchanging water and ionic solutes, such as hydrogen, sodium, potassium, chloride, bicarbonate, sulfate, and phosphate ions, across the cell membranes of the renal tubules.

Fluid exchange

Four forces act to equalize concentrations of fluids, electrolytes, and proteins on both sides of the capillary wall by moving fluid between the vessels and the interstitial fluid. Forces that move fluid out of blood vessels are:

  • hydrostatic pressure of blood

  • osmotic pressure of tissue fluid.

Forces that move fluid into blood vessels are:

Hydrostatic pressure is higher at the arteriolar end of the capillary bed than at the venular end. Oncotic pressure of plasma increases slightly at the venular end as fluid is drawn into the blood vessel. When the endothelial barrier (capillary wall) is normal and intact, fluid escapes at the arteriolar end of the capillary bed and is returned at the venular end. The small amount of fluid lost from the capillaries into the interstitial tissue spaces is drained off through the lymphatic system and returned to the bloodstream.

Acid-base balance

Regulation of the extracellular fluid environment involves the ratio of acid to base, measured clinically as pH. In physiology, all positively charged ions are acids and all negatively charged ions are bases. To regulate acid-base balance, the kidneys secrete hydrogen ions (acid), reabsorb sodium (acid) and bicarbonate
ions (base), acidify phosphate salts, and produce ammonium ions (acid). This keeps the blood at its normal pH of 7.35 to 7.45. Important pH boundaries include:

  • < 6.8 incompatible with life

  • < 7.2 cell function seriously impaired

  • < 7.35 acidosis

  • 7.35 to 7.45 normal

  • > 7.45 alkalosis

  • > 7.55 cell function seriously impaired

  • > 7.8 incompatible with life.

Pathophysiologic concepts

The regulation of intracellular and extracellular electrolyte concentrations depends on these factors:

  • balance between intake of substances containing electrolytes and output of electrolytes in urine, feces, and sweat

  • transport of fluid and electrolytes between extracellular and intracellular fluid.

Fluid imbalance occurs when regulatory mechanisms can’t compensate for abnormal intake and output at any level from the cell to the organism. Fluid and electrolyte imbalances include edema, isotonic alterations, hypertonic alterations, hypotonic alterations, and electrolyte imbalances. Disorders of fluid volume or osmolarity result. Many conditions also affect capillary exchange, resulting in fluid shifts.


Despite almost constant interchange through the endothelial barrier, the body maintains a steady state of extracellular water balance between the plasma and interstitial fluid. Increased fluid volume in the interstitial spaces is called edema. It’s classified as localized or systemic. Obstruction of the veins or lymphatic system or increased vascular permeability usually causes localized edema in the affected area, such as the swelling around an injury. Systemic, or generalized, edema may be due to heart failure or renal disease. Massive systemic edema is called anasarca.

Edema results from abnormal expansion of the interstitial fluid or the accumulation of fluid in a third space, such as the peritoneum (ascites), pleural cavity (hydrothorax), or pericardial sac (pericardial effusion).


Many fluid and electrolyte disorders are classified according to how they affect osmotic pressure, or tonicity. (See Alterations in tonicity.) Tonicity describes the relative concentrations of electrolytes (osmotic pressure)
on both sides of a semipermeable membrane (the cell wall or the capillary wall). The word normal in this context refers to the usual electrolyte concentration of physiologic fluids. Normal saline solution has a sodium chloride concentration of 0.9%.