Synovial Fluid in Human Joints

Synovial fluids are joint fluids, they are also termed synovia. It is a viscous, non-newtonian fluid present in the cavity of synovial joints between the bones. This fluid looks like egg white fluid with a transcellular fluid component of extracellular fluid. The primary synovial fluid function is to reduce friction between the articular cartilages during the movement of synovial joints. 

Structure of Synovial Fluid 

The synovial membrane present in the inner membrane of synovial joints will secrete the synovial fluid into the joint cavity. This synovial fluid acts as an ultrafiltration membrane for blood plasma, which contains the protein derivative produced by the cells present inside the joint tissues. Synovial fluid contains hyaluronan, which is secreted by the fibroblast-like cells present in the synovial membrane. Synovial fluid contains proteoglycan 4, the lubricant secreted by the chondrocytes surface present in articular cartilage and interstitial fluid filtered from the blood plasma. 

The interstitial fluid forms a thin layer of around 50 μm on the surface of the cartilage. Further, it seeps into the microcavities and irregularities in the articular cartilage surface for filling all spaces between the fluid. The fluid present in the articular cartilage serves as a synovial fluid reserve.  During the movement of joints, the synovial fluid present in the cartilages is squeezed out mechanically to maintain the fluid layer on the cartilage surface. 

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Chemical Composition of Synovial Fluid

Synovial tissues are the vascularized connective tissue, which lacks the basement membrane. The synovial tissues consist of two types of cells. They are Type A and Type B. Type A cells are derived from blood monocytes. Further, it removes the wear and tears debris from the synovial fluid.  Type B cells produce hyaluronan, which is made up of hyaluronic acid and proteinases, lubricin, and collagenases. The synovial fluid shows the non-newtonian flow characteristic: The viscosity coefficient is not constant and the fluid is not linearly viscous. The synovial fluid shows the rheopexy characteristics, which increases the viscosity and fluid thickness over the period of continuous stress. Usually, the synovial fluid contains 3-4mg/ml of hyaluronic acid. It is a polymer of disaccharides and composed of  D-N-acetylglucosamine and D-glucuronic acid combined by alternating beta-1,4 and beta-1,3 glycosidic bonds. 

Hyaluronan is synthesized by the synovial membrane, which is secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages, and it lubricates the surface between synovium and cartilage. It also contains phagocytic cells, which remove microbes and debris and helps protect joints from normal wear and tears. 

Features of Synovial Joints

The synovial fluids present between the joints of the bone will act as the chemical dialyzer of blood plasma. This means it purifies and filters the portion of the blood plasma through the membrane which contains a larger amount of hyaluronic acid than the other plasma dialyzates. Since the synovial joints have both viscous and elastic characteristics, they are marked as the thixotropic fluid. 

Here, the viscosity of the synovial fluid between bones will decrease with an increase in the speed of the fluid during its motion. On the other hand, the elasticity of the fluid will increase with an increase in the speed of the fluid. Thixotropy features of synovial joints are mainly due to the presence of hyaluronic acid present in the synovia. The main source of the hyaluronic acid present in synovial fluid is the synovial lining cells. Further, the thixotropic properties help to make the elastohydrodynamic lubricant films between the movement and the fixed conarticular surfaces between the bone pairs.  

The primary functions of the synovial fluids are two parts. They are nutrition and lubrication. As the synovial fluid acts as a blood-plasma dialyzate, it can nourish the articulating parts of the articular cartilages. The motion of the synovial fluid completely depends on the connections with the fatty pads. It helps for the distribution of nutritions over the articular surfaces and it slowly passes through the interior layer of cartilage. 

Synovial Fluid Function

• The synovial fluid lubricates the articulating joints and reduces friction. 

• As the synovial fluid acts as a dilatant fluid, it possesses rheopectic properties. Synovial fluid has a more vicious capacity when the pressure is applied. The synovial fluid present in diarthrotic joints will become thick the moment shear in order to protect the joint and subsequently. Normal viscosity of the synovial fluid instantaneously resumes its lubricating function between shocks. 

• Synovial fluid usually supplies oxygen and nutrients, further they remove carbon dioxide and metabolic waste from the chondrocytes present in the surrounding cartilage tissues. 

• The pressure acts on the joints forces to secrete hyaluronan in the fluid acts on the synovial membrane, which acts as a barrier against cells and helps to migrate into or out through the joint space. The molecular sieving is completely dependent on the molecular weight of the hyaluronan.