How Synovial Fluid Supports Healthy Joint Function
Every bone is connected to another one. The bones in the skull do not move but the rest make a coordinated movement using the muscles. During this movement, every joint created by two or more bones will get eroded causing immense pain. This is why the synovial joints are filled with synovial fluid. It is the secretion of the membrane present inside the joints. In this article, we will analyze this fluid and find out its different functions.
What is Synovial Fluid?
The fluid present inside the joints of two or more bones protecting the connecting surfaces of the bones is called synovial fluid. It is also called a synovia and is produced by the synovial membrane or synovium. This membrane is the soft tissue lining present inside covering the internal spaces, bursae, and tendon sheaths.
This liquid is viscous and non-Newtonian in nature. The thickness of this fluid enables the joints to avoid friction and to provide smooth functioning of the associated bones. Another function of this fluid is to protect the bone heads and cartilages from wearing out and to repair them in time.
Structure of the Synovial Fluid
As mentioned earlier, the synovial fluid is found in the joints trapped by the synovium. It makes an occupied space between the bones and does not let the cartilages touch to avoid friction.
This fluid also acts as an ultra-filtration medium for the plasma of blood and contains the protein derivatives manufactured by the cells present in the joint tissues. It contains hyaluronan secreted by cells that look like fibroblasts. These cells are present in the morphology of the synovial membrane.
The prime lubricant present in this membrane is proteoglycan 4. It is produced by the interstitial fluid availed by filtering the blood plasma and the surface of the chondrocytes in the articular cartilage.
This interstitial fluid accumulates to form a layer around the cartilage inside the joint of size 50 μm. It acts as the fluid reserve for the synovial fluid present inside the joint. Its prime function is to fill the air cavities and irregularities of the articular cartilage surface. When the joints move, the synovial fluid is squeezed outside the cartilages due to mechanical pressure in order to maintain the layer inside the cartilage surface.
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As per the synovial fluid analysis, its chemical composition is made of two types of cells, Type A and B. These connective tissue cells do not have basement membranes and are derived from the blood monocytes. The prime function of these cells is to remove the debris generated in the fluid due to the movement of the joints. Type B is the one that produces hyaluronic acid, lubricin, collagenases, and other proteolytic enzymes.
The synovial fluid analysis also suggests that it is non-Newtonian in nature. It means that the viscosity of the fluid is not constant and it changes without showing any linearity when force or trauma is applied. It is a brilliant example of rheopexy characteristic where continuous stress leads to the escalation of fluid thickness and viscosity. This is one reason behind how the synovial fluid function of reducing friction can be explained.
Its chemical structure reveals that it is made of hyaluronic acid, it is a disaccharide polymer and is composed of beta-1,3 and beta-1, 4 glycosidic bonds with D-glucuronic acid and D-N-acetylglucosamine. This polymer can achieve a size ranging from 5000 to 20,000,000 Dalton.
As mentioned earlier, it is synthesized by the synovial membrane and then passed to the joint cavity in order to increase the thickness or viscosity of the fluid. It also aids in increasing the elasticity of the connected articular cartilages. The prime synovial fluid function of lubricating the joints is performed by hyaluronan or hyaluronic acid.
Functions of Synovial Fluid
The prime function of the synovial fluid is to provide lubrication to the articulating joints to avoid friction and wearing of cartilages and subsequently the heads of the bones.
Its rheopectic nature makes it a dilatant fluid between bones resulting in a more viscous fluid to protect the joints from injuries. The movement stress is removed, the viscosity returns to normal and the lubricating function is resumed.
The synovial fluid knee and in other joints also provide a complete connection between the connective tissues providing nutrition. It is also the medium for exchanging gases for the tissues present in the joints.
It also acts as the medium for eliminating metabolic wastes from the adjacent tissues constructing the bone joints in the skeletal system.
It also performs the action of a molecular sieve in producing hyaluronan and forcing it to migrate outside or inside the joints. The rate depends on the molecular weight of the polymeric hyaluronan.
Deficiency of Synovial Fluid
The synovial fluid deficiency can cause friction in the joints resulting in pain and erosion of the cartilages protecting the bone heads. The inflammation in the tissues can be reduced using anti-inflammatory drugs and antirheumatic drugs. The treatment plan depends on the cause of this deficiency.
After a comprehensive set of tests, the synovial fluid cytology report suggests the reason behind its deficiency or the inflammation in the joints. It enables the doctors to diagnose and prescribe the right medicines.
This is all about the synovial fluid structure, chemical composition, function, and deficiency. Understand the importance of this fluid in the bone joints and learn how it performs all its functions in collaboration with the adjacent membrane tissues.
FAQs on Synovial Fluid Structure Explained
1. What is the basic structure of synovial fluid?
Synovial fluid, also known as synovia, is a viscous, non-Newtonian fluid found in the cavities of synovial joints. Its structure is primarily that of an ultrafiltrate of blood plasma, but it is enriched with additional components produced by cells in the synovial membrane. It is typically clear to pale yellow and has an egg-white-like consistency, which is crucial for its function within the joint capsule.
2. What are the main components that make up synovial fluid?
The structure of synovial fluid is defined by its key components, which are essential for joint health. These include:
Hyaluronic Acid: A long-chain polysaccharide that gives the fluid its high viscosity and lubricating properties.
Lubricin: A glycoprotein responsible for cartilage-on-cartilage lubrication, reducing friction during movement.
Interstitial Fluid: The base fluid, which is filtered from blood plasma and contains proteins, electrolytes, and glucose.
Cells: A small population of cells, including phagocytes that remove debris and synoviocytes that help maintain the fluid.
3. What are the key functions of synovial fluid in a joint?
The unique structure of synovial fluid allows it to perform several critical functions within a synovial joint:
Lubrication: It drastically reduces friction between the articular cartilages of the bones, allowing for smooth, pain-free movement.
Shock Absorption: The fluid's viscosity helps it absorb shock and distribute pressure evenly across the joint during high-impact activities like running or jumping.
Nutrient and Waste Transport: It supplies oxygen and nutrients to the avascular (lacking blood vessels) articular cartilage and removes metabolic waste products like carbon dioxide.
4. How does physical activity affect the synovial fluid in our joints?
Moderate and regular physical activity has a positive effect on synovial fluid. Movement and joint loading stimulate the synovial membrane to produce more fluid, enhancing lubrication. Exercise also increases blood circulation to the joint, which improves the delivery of nutrients to the synovial membrane, helping it maintain a healthy fluid composition. This process makes the joints more resilient and efficient.
5. What role does the synovial membrane play in forming the structure of synovial fluid?
The synovial membrane is fundamental to the structure of synovial fluid as it is responsible for its production. The membrane has specialized cells called Type B synoviocytes which actively secrete hyaluronic acid and lubricin into the plasma filtrate. This changes a simple fluid into the highly specialized, viscous substance required for joint function. The membrane thus controls the fluid's volume, composition, and overall structural integrity.
6. How does the thixotropic property of synovial fluid aid in joint movement?
Synovial fluid exhibits a property called thixotropy, which means its viscosity changes with force. When a joint is at rest, the fluid is thick and gel-like, providing cushioning. During movement, the shear stress causes the fluid to become thinner and more watery. This structural change allows for minimal friction during rapid movements. When the movement stops, it returns to its thicker state, ready to provide cushioning again. This is a critical example of how its structure is adapted for dynamic function.
7. What happens to the structure of synovial fluid in joint disorders like arthritis?
In conditions like rheumatoid arthritis or osteoarthritis, the structure and composition of synovial fluid change dramatically. Inflammation causes the synovial membrane to become dysfunctional, leading to a breakdown of hyaluronic acid. This results in a decrease in the fluid's viscosity, compromising its ability to lubricate and absorb shock. The fluid volume may increase due to inflammation (an effusion), but its quality is poor, leading to increased joint friction, pain, and damage.
8. Is the structure of synovial fluid identical in all synovial joints, such as the knee versus the wrist?
While the basic structural components of synovial fluid are the same across all synovial joints, the precise concentration and volume can differ based on the joint's function. For example, large, weight-bearing joints like the knee or hip typically have a larger volume of synovial fluid with a higher concentration of hyaluronic acid to handle greater loads and provide more shock absorption. Smaller, non-weight-bearing joints like those in the wrist may have less fluid with slightly different properties tailored to their range of motion.
