How does synovial fluid lubricate and nourish joint cartilage?

Synovial fluid is a specialized, thixotropic substance secreted by synovial lining cells that serves as both a lubricant and a nutrient source for articular cartilage.123

Lubrication

The fluid’s thixotropic properties—meaning its viscosity and elasticity change based on movement speed—allow it to form elastohydrodynamic lubricant films between the moving surfaces of a joint. By maintaining appropriate viscosity and elasticity, the fluid ensures that articulating surfaces remain separated and protected during motion. Additionally, fatty pads within the joint cavity contribute to this process by streamlining the joint space and helping to distribute the fluid effectively.13

Nutrition

Unlike most other tissues, articular cartilage lacks a direct blood supply and relies on synovial fluid for nourishment. The fluid, which is a dialyzate of blood plasma, contains essential nutrients that are distributed across the cartilage surface through joint movement. Once distributed, these nutrients slowly permeate the interior of the cartilage to support the metabolic needs of chondrocytes.41