Chemical Composition and Functions of Synovial Fluid
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.
FAQs on Synovial Fluid: Definition, Features & Importance
1. What is synovial fluid and where is it found in the body?
Synovial fluid, also known as synovia, is a thick, viscous, egg-white-like fluid found in the cavities of synovial joints. These are the most common and most movable type of joint in the body, such as the knee, shoulder, and hip. The fluid is contained within the synovial capsule, which surrounds the joint.
2. What are the main components that make up synovial fluid?
Synovial fluid is essentially a dialysate (filtrate) of blood plasma, but with some key differences in composition. Its main components include:
- Hyaluronic acid: A key molecule secreted by fibroblast-like cells in the synovial membrane, which gives the fluid its high viscosity (thickness).
- Lubricin: A glycoprotein that is crucial for cartilage lubrication, reducing friction.
- Proteins: Derived from blood plasma, such as albumin and globulins.
- Water, salts, and various cells: Including phagocytic cells that help remove debris from the joint space.
3. What are the key functions of synovial fluid in a synovial joint?
The importance of synovial fluid lies in its multiple critical functions for joint health and mobility. Its primary roles are:
- Lubrication: It drastically reduces the friction between the articular cartilage of synovial joints during movement, preventing wear and tear.
- Shock Absorption: Its viscous nature allows it to act as a shock absorber, protecting the joint from impact during activities like walking or jumping.
- Nutrient and Waste Transport: Articular cartilage is avascular (lacks blood vessels). Synovial fluid is responsible for supplying it with essential nutrients and oxygen, and for removing metabolic waste products like carbon dioxide.
- Joint Stability: The fluid provides a cohesive force that helps to stabilise the joint.
4. How does the synovial membrane produce and maintain synovial fluid?
The synovial membrane (or synovium) is the soft tissue layer that lines the inner surface of the joint capsule. It is responsible for both producing and maintaining the synovial fluid. Specialized cells within this membrane, called synoviocytes, secrete hyaluronic acid and lubricin into the filtered blood plasma that enters the joint cavity. This process continuously regulates the volume, composition, and viscosity of the fluid to meet the joint's metabolic and mechanical needs.
5. What causes the amount or quality of synovial fluid to decrease?
A decrease in the quantity or a change in the quality of synovial fluid can be caused by several factors, leading to joint stiffness and pain. Common causes include:
- Ageing: The natural ageing process can lead to reduced production of hyaluronic acid, making the fluid less viscous.
- Injury or Trauma: Damage to a joint can disrupt the synovial membrane's function.
- Dehydration: Since the fluid is primarily water-based, inadequate hydration can affect its volume and effectiveness.
- Medical Conditions: Diseases like osteoarthritis involve the breakdown of cartilage and can affect the fluid's composition. Inflammatory conditions like rheumatoid arthritis can cause the fluid to become thinner and less effective as a lubricant.
6. Can synovial fluid be replenished or replaced by the body?
Yes, the body has a natural mechanism to replenish synovial fluid. The synovial membrane continuously produces and resorbs the fluid, maintaining a delicate balance. If a small amount of fluid is removed (for example, during a medical procedure called arthrocentesis), the body typically restores it within a few days. However, in chronic conditions like arthritis, the body's ability to produce high-quality fluid may be compromised.
7. How is the viscosity of synovial fluid important for its function?
The viscosity, or thickness, of synovial fluid is one of its most critical features, primarily due to hyaluronic acid. Its importance lies in its thixotropic property: the fluid becomes thinner (less viscous) under high shear stress (i.e., during rapid movement), which reduces friction. Conversely, it becomes thicker (more viscous) at low shear stress (i.e., when the joint is at rest or moving slowly), which helps in shock absorption. This dynamic viscosity allows the joint to move smoothly and be protected from impact.
8. What is the difference between synovial fluid and other body fluids like blood plasma?
While synovial fluid originates from blood plasma, there are key differences. Synovial fluid is an ultrafiltrate of plasma, meaning it has a much lower concentration of large proteins, as they are blocked from passing through the synovial membrane. The most significant difference is the presence of hyaluronic acid and lubricin in synovial fluid, which are absent in plasma. These molecules are specifically added by the synovial membrane and are responsible for the fluid's unique lubricating and viscous properties.
9. What can a medical analysis of synovial fluid reveal about a person's joint health?
A medical procedure called synovial fluid analysis (or joint fluid analysis) is a powerful diagnostic tool. By examining a sample of the fluid, doctors can determine the cause of joint swelling or arthritis. Key findings can include:
- Appearance: Healthy fluid is clear and viscous. Cloudy fluid can indicate inflammation or infection, while reddish fluid suggests bleeding in the joint.
- Cell Count: High levels of white blood cells can point to inflammatory arthritis (like rheumatoid arthritis) or septic arthritis (infection).
- Crystals: The presence of uric acid crystals confirms gout, while calcium pyrophosphate crystals indicate pseudogout.
- Culture: Testing for bacteria or fungi can diagnose an infection within the joint.
10. How do diet and hydration impact the health of synovial fluid?
Diet and hydration play a significant role in maintaining healthy synovial fluid and overall joint health. Since synovial fluid is mostly water, staying well-hydrated is essential for maintaining its volume and consistency. Additionally, a diet rich in anti-inflammatory foods like fruits, vegetables, and omega-3 fatty acids can help maintain a healthy joint environment. Nutrients like antioxidants can protect joint tissues from damage, indirectly supporting the quality of the synovial fluid.
