Which fluid lubricates joints and nourishes articular cartilage?

The correct answer is synovial fluid, as it plays a vital role in the health and function of joints. Synovial fluid is a thick, viscous liquid produced within the synovial membranes that line the cavities of synovial joints. Its primary functions include lubricating the joint surfaces, allowing for smooth movement during physical activities, and serving as a shock absorber to prevent damage to bones during rigorous movements.

Additionally, synovial fluid contains nutrients that are essential for the maintenance of articular cartilage—the smooth tissue that covers the ends of bones in synovial joints. This nourishing aspect is crucial because articular cartilage does not have its own blood supply; therefore, it relies on synovial fluid for the delivery of nutrients and the removal of waste products.

In contrast, the other options do not serve the same specific functions in joints. Serous fluid is found in serous membranes that line body cavities, interstitial fluid is the fluid surrounding cells in tissues, and plasma is the liquid component of blood that carries cells and various substances throughout the body. Each of these fluids has important roles in different physiological contexts but does not fulfill the specific lubricating and nourishing roles of synovial fluid in joint function.