You and your body maintain homeostasis by

The body maintains homeostasis by maintaining a relatively constant environment within the body. The organ systems help control the internal environment so that it remains relatively constant. Most cells of the body are surrounded by a small amount of fluid, and normal cell functions depend on the maintenance of the cells' fluid environment within a naroow range of conditions including temperature, volume, and chemical content. For example: the digestive, respiratory, circulatory and urinary systems function together so that each cell in the body receives adequate oxygen and nutrients and so that waste products do not accumulate to a toxic level.

That would be a working theory.