Most of the chemical levels and chemical reactions in the human body are in a constant state of flux. When the various components of a particular system are in a relatively constant, unchanging state, we say that the system is in equilibrium. An example of chemical equilibrium in the body that can become upset or stressed is the level of water both inside and outside of your body's cells. When the water levels are in equilibrium, we say that the cells are isotonic. But if you exercise heavily and consequently sweat a lot, your body loses water via perspiration. This causes the water level in your cells to decrease to compensate, thus the cells shrink slightly and become hypertonic. Drinking some fresh water helps replace the lost water in your body, thus putting your cells back into an isotonic state of equilibrium.