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, nutri-
ents, 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 extra-
cellular.
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 tis-
sues. 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.Electrolytes
An electrolyte is a substance that splits into ions when placed into water. An ion is
an electrically charged particle that is either positively or negatively charged. A pos-
itively charged ion is called a cation and a negatively charged ion is called an anion.- Sodium (Na+), potassium (K+), calcium (Ca2+) , and Magnesium (Mg++) are
electrolytes that are cations. - Chloride (Cl−), Bicarbonate (HCO 3 −), Phosphate (PO 4 −), and Sulfate (SO 4 −)
are electrolytes that are anions.
An electrolyte is measured as a millimole per liter (mmol/L). A millimole is
the atomic weight of the electrolyte in milligrams. For example, the atomic(^158) CHAPTER 10 Fluid and Electrolyte Therapy