Vacuoles are membrane-enclosed spaces. They are found in both plants and animal cells. Protozoans like the paramecium have many different types of vacuoles that have specific functions. One particular vacuole involved in water regulation is the contractile vacuole.
How it works
Along the edges of the vacuole are mitochondria that produce ATP. The ATP is the energy needed to pump ions from the cytoplasm into the vacuole. As the ions in the vacuole increase in concentration, water flows into the vacuole. As more and more water enters the vacuole it expands.
The contractile vacuole is used to expel excess water and some wastes from the cell. However, its main function is to remove water.