This process is what scientists refer to as osmosis. Osmosis is one of the main ways in which our body keeps or retains moisture. In fact, our body will actually move minerals around itself in order to get water to move more efficiently. Unfortunately, all of this changes when we get into a bath tub. Before taking a bath, water levels in our body are balanced.
Every cell has the right amount.
However, bath water is not pure or balanced. In fact, bath water is filled with minerals, ions, and other chemicals of its own. Once in this new environment, your body responds by trying to regain the balance. And just like a balloon that's lost all its air, your fingertips wrinkle up as they loose water. But don't worry, when you leave the tub, your body works rapidly to replace the water it's lost.