Body Fluids

Water is 60% of adult body weight. However, water is 45% to 55% of an older adult's body weight and as much as 70% to 80% of an infant's weight is water. This makes older adults and infants at high risk for fluid imbalance. Lean adults have more water than heavy adults because adipose cells (cells containing fat) contain less water than other cells. Water is the solvent that contain salts, nutrients, and wastes that are solutes dissolved in the water and transported by the water throughout the body. Salts are electrolytes.

Body fluids are stored in compartments. These are intracellular and extracellular.

Intracellular fluid (ICF) is inside the cell and consists of 40% of body weight.

Extracellular fluid (ECF) is divided into smaller compartments. These spaces between the cells are called the interstitial space. The space is occupied by plasma and lymph, transcellular fluid, and fluid in the bone and connective tissues. This makes up 20% of body weight. About a third is plasma and two thirds of extracellular fluid is in the space between the cells. Transcellular fluid is also ECF but is found in the gastrointestinal (GI) tract, cerebrospinal space, aqueous humor, pleural space, synovial space, and the peritoneal space. Although fluid in the transcellular space is a small volume when compared with intracellular and extracellular compartments, the increase or decrease in volumes in transcellular spaces can have a dramatic effect on the fluid-electrolyte balance.

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