Introduction
Approximately 60% of body weight is fluid, commonly referred to as total body water, which is primarily composed of water containing sodium, potassium, and chloride. The remaining 40% is solid tissue, primarily bone.
Two-thirds of our total body water is located in the intracellular space and one-third in the extracellular space. The extracellular space is further divided into the interstitial space, containing three-fourths of the extracellular volume, and plasma or intravascular space, containing the remaining one-fourth of extracellular volume.
Total Body Water (60%)
Intracellular (40% of body weight)
Extracellular (20% of body weight)
Extracellular Space (20%)
Interstitial (15% of body weight)
Plasma (5% of body weight)
Key Points:
- • Intracellular compartment has high K+ (140 mEq/L) and low Na+ (10 mEq/L)
- • Extracellular compartment has low K+ (4 mEq/L) and high Na+ (140 mEq/L)
- • This difference is maintained by Na+/K+ ATPase pump
- • Cell membranes are permeable to water but require transporters for ions