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Every person has hundreds of bursae scattered throughout the body.
The function of a bursa is to decrease friction between two surfaces that move in different directions. Therefore, you tend to find bursae at points where muscles, ligaments, and tendons glide over bones. The bursae can be thought of as a ziplock bag with a small amount of oil and no air inside. In the normal state, this would provide a slippery surface that would have almost no friction. A problem arises when a bursa becomes inflamed.