Definition
The synovial fluid, called “synovial fluid” in medical synovia and in colloquial speech, is a viscous and clear fluid that is present in the joint cavities. It is formed by the mucosa of the joint capsule and serves to reduce the frictional forces during joint movements and to supply the joint cartilage with nutrients. In addition, the synovia is also found in bursae and tendon sheaths.
Formation of the gel liquid
The joint fluid (synovia) is formed by the so-called synoviocytes. They are also called synovial cells and line the synovial membrane, also called synovialis or synovial membrane. Synoviocytes differ greatly in their appearance and there are basically two types: Type A and type B.
The cells of type A have more of a decomposing function by absorbing and dissolving cell debris and other remnants. Those of type B are the actual and producing synoviocytes. They produce a mixture of hyaluronic acid, collagen and fibronectin, the latter two being important components of the synovial membrane itself. Hyaluronic acid, also called hyaluronan, together with water and other mucus-forming products of the synoviocytes, is the main component of the synovial fluid and ensures its viscosity. In addition, the synovia also contains electrolytes and enzymes which, together with the water, come from the blood plasma.
Function of the synovial fluid
As already mentioned, the synovial fluid has two important functions. On the one hand, it serves to reduce friction during joint stress and, in addition, to supply the joint cartilage with nutrients and oxygen. Hyaluronan is primarily responsible for the viscosity of the fluid.
It binds water and transforms it into a more viscous mass that is not simply pressed out of the joint space under stress but remains there. This largely prevents direct contact between the two joint surfaces. Interestingly, the synovia also has the property that its viscosity decreases during shear movements, so that it can serve as a lubricant, for example, during rapid movements that trigger high shear forces.
Its second important function is to feed the joint cartilage. Articular cartilage is not permeated by vessels and is therefore not supplied with blood. Therefore, both nutrients and oxygen can only reach the cartilage by diffusion from the joint fluid. This is only possible because tissues such as cartilage or ligaments have a very slow metabolism and therefore do not have a high demand for these substances. The property of slow metabolism is also known as bradytrophic.
