滑液, or synovial fluid, is a thick, clear liquid found in the cavities of joints, such as the knees and elbows. Its primary function is to lubricate the joints, reducing friction between the cartilage surfaces during movement. This helps to protect the joints and allows for smooth motion. In addition to lubrication, 滑液 also provides nourishment to the cartilage and acts as a shock absorber during physical activities. It is produced by the synovial membrane, which lines the joint capsule. Proper levels of 滑液 are essential for joint health and mobility, making it crucial for activities like walking and running.