Body Fluids
- Total body water is the water contained in all body-fluid compartments and is approximately 60% of adult body weight.
- Newborns and adult males generally have a higher body-water percentage because they contain more lean tissue.
- Adult females and people with more adipose tissue generally have a lower body-water percentage because adipose tissue contains relatively little water.
- Intracellular fluid is the fluid inside cells and contains about two-thirds of total body water.
- The dominant intracellular positive ion is potassium ion (K⁺).
- Magnesium ion (Mg²⁺) is another important intracellular positive ion.
- Intracellular negative charges come mainly from proteins and organic phosphates.
- ATP, ADP, and AMP are organic phosphates that contribute to intracellular negative charge.
- Extracellular fluid is the fluid outside cells and contains about one-third of total body water.
- Plasma and interstitial fluid are the two major extracellular-fluid compartments.
- The dominant extracellular positive ion is sodium ion (Na⁺).
- The major extracellular negative ions are chloride ion (Cl⁻) and bicarbonate ion (HCO₃⁻).
- A cation is a positively charged ion, an anion is a negatively charged ion, and organic phosphates are phosphate-containing carbon-based molecules inside cells.
- Plasma makes up approximately one-fourth of extracellular fluid.
- Plasma therefore represents approximately one-twelfth of total body water.
- Interstitial fluid surrounds cells and makes up approximately three-fourths of extracellular fluid.
- Interstitial fluid therefore represents approximately one-fourth of total body water.
- Interstitial fluid resembles plasma but contains much less protein because most plasma proteins remain inside capillaries.
- Plasma is the fluid portion of blood inside vessels, while interstitial fluid is the extracellular fluid between tissue cells outside blood vessels.
- Interstitial fluid is an ultrafiltrate of plasma, meaning it is plasma-derived fluid that has passed through a filtration barrier.
- The 60-40-20 rule estimates total body water as 60%, intracellular fluid as 40%, and extracellular fluid as 20% of body weight.
- A 70-kg adult therefore has approximately 42 L of total body water.
- The 60-40-20 rule is an estimate, so body composition can shift the actual values.
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