GLOSSARY

Interstitial Fluid is extracellular fluid surrounding tissue cells, distinguished from plasma inside blood vessels.

Total body water is distributed within cells and outside them. Extracellular fluid includes plasma and interstitial fluid, whereas whole blood also contains cells. Barriers between compartments differ in permeability, and water and solute movements change distribution. Drinking a glass does not permanently add that entire volume exclusively to blood. Compartment terminology identifies location rather than a separate dietary kind of water.

Sodium and potassium are distributed differently across cell membranes and, with other particles, influence water movement. Visible swelling or one weighing cannot precisely quantify intracellular water. Body-composition measurements depend on method, conditions and hydration. A rapid change in reported lean mass can therefore reflect water shifts rather than muscle growth or loss, especially when measurements are made under different conditions.

Osmosis is net water movement across a water-permeable barrier in response to differences in solute effects. Particle count alone is insufficient: membrane permeability to each solute matters. Freely crossing particles have different sustained effects on cell volume from confined particles. This explains why measuring a solution's total particle concentration is not always enough to predict cellular swelling or shrinking.