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A bursa is a fluid-filled sac that cushions an area of friction between tissues, such as tendon and bone. Bursae reduce friction between moving parts of the body, such as in the shoulder, elbow, hip, knee, and heel.
The number varies, but most people have about 160 bursae throughout the body. Bursae are lined with special cells, called synovial cells, which secrete a fluid rich in collagen and proteins. This synovial fluid acts as a lubricant when parts of the body move. Inflammation of a bursa is referred to as bursitis.