The Body’s Fluid Compartments & Water Movement

The Story

Most of the human body is waterThe universal solvent essential for life., and that water is parceled into compartments separated by membranes. The largest share sits inside cellsThe basic structural and functional units of life. as intracellular fluidThe fluid inside a cell, primarily composed of cytosol. (ICF). The rest is extracellular fluid(ECF) Fluid outside cells, including plasma and interstitial fluid. (ECF), itself divided into the interstitial fluidThe fluid surrounding cells within tissues. that bathes the tissues and the plasmaThe liquid component of blood. that travels inside blood vessels. These compartments are not sealed tanks; they are in constant conversation. Water crosses freely through cell membranes and capillary walls, and where water goes is decided by the pull of dissolved particles – by osmosis. Solutes that cannot cross a membrane create an osmotic pull, drawing water toward the more concentrated side until the concentrations balance.

This is why a swollen ankle is a compartment problem. Stina’s edema is water that has moved out of the plasma compartment and into the interstitial space, accumulating where it should not. To understand why it moved we need the rules of water movementA fundamental property of life involving motion of the body or its parts.: water follows solute, osmosis equalizes concentrations across a semipermeable membrane, and the body works constantly to keep each compartment’s volume and concentration in a narrow range. Sodium, because it is the dominant ECF solute, largely sets how much water the extracellular space holds – which is exactly why ‘just cut the salt’ sounds reasonable. But salt is only one lever. The pressures inside the capillary are another, and in Stina’s case they are the lever that actually slipped.

From Stina’s chart: Stina’s edema means fluid has shifted out of her vessels and into the spaces between her cells – a shift this page maps compartment by compartment.