Summary
Highlights
The average adult body is 60-70% water, stored in two main compartments. These are the intracellular compartment (fluid within cells) and the extracellular compartment (fluid outside cells).
The intracellular space holds two-thirds of body water. The extracellular space, accounting for one-third, includes interstitial fluid (surrounds cells), intravascular fluid (plasma in blood vessels), and transcellular fluid (in body cavities like spinal fluid, around the heart and lungs, and joints), which is the smallest compartment.
These compartments are interconnected, maintaining homeostasis by shifting water, electrolytes, and nutrients. This shifting occurs through processes like osmosis. In healthcare, IV fluids can be administered to the intravascular compartment to expand it or shift fluids between compartments to correct imbalances.
Osmosis is the passive movement of water from a higher water concentration (low solute concentration) to a lower water concentration (high solute concentration) across a semi-permeable membrane. This process is highly influenced by a fluid's solute concentration.
A solute is a solid dissolved in a liquid. Examples include sodium and chloride, which become electrolytes when dissolved. The amount of solutes in a fluid determines how osmosis affects fluid movement between intracellular and extracellular compartments.
Osmolarity is the amount of solutes within a specific fluid volume (total solute concentration per liter of solution). Fluids with high osmolarity have many solutes and less water, while fluids with low osmolarity have few solutes and more water. Healthcare professionals use different osmolarity fluids to strategically shift water in or out of compartments to treat patients.