Joints are made of bones, cartilage, ligaments, tendons, a lubricating fluid, and a protective membrane that lines the space between bones. The exact combination depends on the type of joint. Your skull bones, for example, are fused together with almost no movement, while your knee is a complex assembly of moving parts designed to bend, twist, and absorb impact thousands of times a day. Most people asking this question are thinking about the movable joints, so that’s where we’ll spend most of our time.
Three Types of Joints, Three Different Builds
Not every joint in your body works the same way, and they’re not all built from the same materials. Joints fall into three broad categories based on how much they move and what holds them together.
Fibrous joints are locked in place by tough connective tissue. The joints between your skull bones are the classic example. They allow almost no movement and have no internal cavity or fluid.
Cartilaginous joints are connected by cartilage and allow limited motion. The discs between your vertebrae and the joint at the front of your pelvis are cartilaginous. They flex just enough to absorb shock and let you bend your spine.
Synovial joints are the most complex and the most mobile. Your knees, hips, shoulders, elbows, and fingers are all synovial joints. They have a fluid-filled cavity, a smooth cartilage surface, and a surrounding capsule. Because these joints carry the heaviest loads and move the most, they have the most components and are the most prone to injury and arthritis.
Bone: The Framework
Every joint starts with at least two bones meeting. In a synovial joint, one bone typically has a rounded end that fits into a shaped cavity on the other, like a ball in a socket (your hip) or a hinge (your elbow). The bone tissue right at the joint surface, called subchondral bone, is denser than the spongy bone deeper inside. It acts as a firm foundation beneath the cartilage layer, distributing the forces of movement into the skeleton.
Bone is roughly two-thirds mineral (mostly calcium and phosphorus crystals) and one-third protein (primarily collagen). That mineral content is what makes it rigid enough to support your weight, while the collagen gives it just enough flexibility to resist cracking under stress.
Articular Cartilage: The Shock Absorber
The ends of bones inside a synovial joint are coated with a smooth, glassy layer called articular cartilage. This is the tissue that lets your bones glide against each other without grinding. It’s slippery, resilient, and surprisingly simple in composition: 60 to 80 percent water by weight, 15 to 22 percent collagen fibers, and 4 to 7 percent proteoglycans, which are large molecules that trap water like a sponge.
The collagen in articular cartilage is almost entirely type II, a variety that forms a dense, interwoven mesh. That mesh gives the tissue its shape and tensile strength. The proteoglycans sit within the mesh and attract water, which is what gives cartilage its ability to compress under load and spring back afterward. When you step down from a curb, your knee cartilage briefly flattens, squeezes out a tiny amount of water, then re-absorbs it once the load passes. This cycle happens with every step you take.
Articular cartilage has no blood supply, no nerves, and very few cells. That’s why it’s painless when healthy but also why it heals so poorly when damaged. Once cartilage wears thin, the underlying bone starts absorbing forces it was never designed to handle directly, which is the basic mechanism behind osteoarthritis.
Fibrocartilage: The Tough Cousin
Some joints contain a second type of cartilage called fibrocartilage. Your knee meniscus and the labrum in your hip and shoulder are made of this material. Fibrocartilage has a higher proportion of collagen and less water than articular cartilage, making it stiffer and better at resisting tearing forces. It acts as a gasket, deepening the joint socket and distributing load more evenly across the articular surface. When people tear a meniscus or a labrum, they’re damaging this fibrocartilage layer.
Synovial Fluid: The Lubricant
The space inside a synovial joint is filled with a small amount of clear, viscous fluid. Synovial fluid is mostly water, but its slippery quality comes from two key ingredients: hyaluronic acid (present at roughly 3 to 4 milligrams per milliliter) and a glycoprotein called lubricin. Hyaluronic acid is a long chain molecule that makes the fluid thick and elastic, similar in consistency to egg white. Lubricin coats the cartilage surfaces directly and prevents them from sticking together.
This fluid does more than just reduce friction. It also delivers oxygen and nutrients to the cartilage, which has no blood vessels of its own. The nutrients diffuse from the fluid into the cartilage tissue, and waste products diffuse back out. Movement helps this exchange. When you compress and release cartilage during walking or bending, you’re essentially pumping fresh fluid through the tissue. That’s one reason prolonged inactivity can degrade joint health over time.
How Joint Lubrication Actually Works
Joint lubrication isn’t a single mechanism. Your joints use several strategies depending on how fast you’re moving and how much weight you’re carrying. During quick movements, a thin film of synovial fluid (less than 20 micrometers thick) forms between the cartilage surfaces and keeps them physically separated. The pressure in this fluid film supports the load, similar to how a car tire hydroplanes on a wet road.
When you’re standing still or moving slowly under heavy load, a different system takes over. The cartilage itself deforms slightly, squeezing fluid out of its surface to create a self-renewing lubricating layer. This is called squeeze-film lubrication, and it allows joints to carry high loads for short periods without damage. At the same time, lubricin molecules form a protective coating just one to 100 nanometers thick on each cartilage surface, preventing direct contact even when the fluid film runs thin. In practice, your joints use a combination of all these methods simultaneously.
The Synovial Membrane
Lining the inside of the joint capsule is a thin tissue called the synovial membrane. This is the tissue responsible for manufacturing synovial fluid. It contains two types of specialized cells. Type B synoviocytes are fibroblast-like cells that produce the hyaluronic acid and other components of the fluid. Type A synoviocytes function more like immune cells, clearing debris and pathogens from the joint space. When this membrane becomes inflamed, as in rheumatoid arthritis, it thickens, produces excess fluid, and can eventually erode the cartilage and bone beneath it.
Ligaments and Tendons
Ligaments connect bone to bone and hold the joint together. They’re made primarily of type I collagen fibers arranged in parallel bundles, which gives them enormous tensile strength in one direction. Think of them as high-strength straps that prevent bones from sliding or rotating beyond their intended range. The ligaments in your knee (like the ACL and MCL) are familiar examples, but every synovial joint has its own set.
Tendons connect muscle to bone and cross the joint to produce movement. They share a similar collagen composition with ligaments but are structured to transmit pulling force from a contracting muscle to the skeleton. Both tissues have limited blood supply compared to muscle, which is why sprains and tendon injuries can take weeks or months to heal fully.
The Joint Capsule
Surrounding the entire synovial joint is a tough, fibrous sleeve called the joint capsule. Its outer layer is dense connective tissue that provides structural support and helps prevent dislocation. The inner layer is the synovial membrane described above. Together, they form a sealed environment that keeps synovial fluid in and contaminants out. In some joints, the capsule is reinforced by thickened bands that function as additional ligaments. In others, like the shoulder, the capsule is relatively loose to allow a wider range of motion, which also makes those joints more vulnerable to dislocation.
Other Structures Inside and Around Joints
Several additional components round out the picture. Bursae are small, fluid-filled sacs positioned near joints (but outside the capsule) to reduce friction between tendons, ligaments, and bone. Your knee alone has more than a dozen. Nerves thread through and around the joint, providing your brain with constant feedback about position, pressure, and pain. Blood vessels supply the ligaments, capsule, and synovial membrane, even though the cartilage itself goes without.
Fat pads sit inside some joints, particularly the knee, acting as additional shock absorbers and helping to distribute synovial fluid during movement. These are sometimes overlooked, but they contain nerve endings and can become a source of pain when irritated or compressed.