Edema

Edema is an excessive buildup of fluid in a tissue. That fluid can sit inside the cells themselves or in the intercellular tissue, the tissue that lies between one cell and the next.

To see how it collects, you have to watch two movements, not one. On one side, blood lets plasma, its liquid part, escape into the interstitium, the space between cells; that outflow is called filtration. On the other side, the lymphatic system works as a drain that carries the fluid away. As long as the drain keeps up, nothing collects. Edema shows up when more fluid comes in than that drain can pull out, like a sink that fills not because the faucet is blasting, but because the drain carries away less than what reaches it.