They decide what passes in and out through your skin. They prevent salts, minerals, and other important body chemicals from leaving, but under special circumstances they let water go in and out under controlled amounts. As I said previously, nature likes to keep things balanced. If the amount of minerals on one side of the epithelial layer is more than on the other side, water will cross over to the side with more minerals. 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.