Total body water (TBW) accounts for 60% of the body weight in kilograms (kg) in men and 50% in women.
TBW distribution
Approximately 66% of TBW is in the intracellular fluid (ICF) compartment.
Approximately 33% of TBW is in the extracellular fluid (ECF) compartment.
The ECF compartment is subdivided into the interstitial fluid (ISF) compartment
(75% of the ECF compartment; fluid around the cells) and the intravascular fluid (IVF) compartment (25% of the ECF compartment).
IVF compartment subdivisions include the blood (specifically the plasma component) within the heart, aorta, pulmonary artery, muscular arteries, arterioles, capillaries, venules, and veins.
Because fat contains less water than lean muscle, water accounts for a lower percentage of total body weight in women, older adults, and obese individuals.
Water losses occur via the skin, kidneys, gastrointestinal (GI) tract, and respiratory tract.
Renal excretion of water is regulated via concentration (retain water) or dilution (lose water).
Insensible water loss refers to the total loss of water via the skin (predominantly) and the respiratory tract via evaporation (normal loss ranges from 500 to 1000 mL/day). Fever markedly increases insensible water loss and is often not considered in proper fluid replacement in a hospitalized person.
Thirst is essential for preventing and correcting a water deficit. Increase in plasma osmolality (POsm) and hypovolemia stimulate thirst.
Sodium (Na+) is the major ECF cation (+ charge). Chloride (Cl−) is the major ECF anion (− charge).
Potassium (K+) is the major ICF cation. Phosphate (PO43−) is the major ICF anion.
Osmolarity
a. Definition: The number of particles (osmoles) dissolved in plasma in mOsm/kg
b. Three different tonicity states may be present in a person.
(1) Isotonic state = normal POsm
(2) Hypotonic state = decreased POsm
(3) Hypertonic state = increased POsm
blood osmolarity = 2 (serum Na+) + serum glucose/18 + serum blood urea nitrogen (BUN)/2.8 =
275–295 mOsm/kg