Membranes and compartments

Membranes divide your body into compartments with different contents: the fluid inside cells, the fluid around them, and the blood plasma. The plasma membrane keeps potassium high inside a cell and sodium high outside; the capillary wall keeps most plasma proteins in the blood.

Life depends on these differences, and on controlling what crosses between compartments. Water follows solute across membranes, so a change in one compartment's salt concentration moves water in or out of cells. When fluid collects in the wrong compartment, as in swollen ankles, ask which barrier or which force changed.