There are three factors at work in a spa: heat, buoyancy, and massage. Together, they create a relaxing, soothing experience.
Immersion in hot water raises the body temperature and causes the blood vessels to dilate, increasing circulation. The buoyancy of the water reduces body weight by approximately 90%, which relieves pressure on joints and muscles and creates the relaxing sensation of weightlessness.
The massaging action of a hot tub is created by sending a mixture of warm water and air through jet nozzles. This "energized" stream of water loosens tight muscles and stimulates the release of endorphins, the body's natural pain killers.
Of course a hot tub offers much more than just a good hydromassage. It can also be good for your health. "Research shows that both the body and mind benefit from the simple act of immersion in warm water." The Hot & Healthy Living book summarizes the present knowledge about the many health benefits from immersion in water.