Normal joints contain only a small amount of clear, straw-colored synovial fluid.
For example, the adult knee normally contains only 2 mL. The fluid is formed by the filtration of capillary plasma through a sieve of hyaluronate molecules. Most small molecules pass from the subsynovial capillaries through the synovium by diffusion. The concentration of electrolytes and glucose in nondiseased synovial fluid is the same as in plasma. Because synovial fluid is created by filtration, it contains proteins in concentrations that are inversely proportional to their molecular weight. Large molecules such as α-2-macroglobulin and immunoglobulins are virtually excluded. Fibrinogen is also excluded; therefore, synovial fluid will not clot. Normal synovial fluid also contains very few white blood cells and virtually no neutrophils. The viscosity of synovial fluid parallels the concentration of hyaluronic acid. Normal synovial fluid has a high concentration of hyaluronate and therefore high viscosity.