What substance is primarily stored in the sack known as a bursa?

The bursa is a small fluid-filled sac that acts as a cushion between bones and tendons or muscles around a joint. Its primary function is to reduce friction and allow for smooth movements in areas where tissues could rub against one another. The substance stored in a bursa is primarily synovial fluid, which is a thick, viscous liquid that lubricates the joint, nourishes the cartilage, and helps maintain the health of the surrounding tissues.

Synovial fluid is essential for the proper functioning of joints, especially in high-motion areas like the knees, shoulders, and elbows, where the potential for friction and irritation is greater. The presence of synovial fluid within the bursa allows for better mobility and decreases the likelihood of inflammation or injury in the joint area. In contrast, the other substances listed, such as blood, serum, and intercellular fluid, serve different functions in the body and are not primarily associated with the bursa structure.