The fluid in your joints thickens on impact, then thins again instantly
Synovial fluid looks like raw egg white, and its name probably means exactly that. It is also a clever material: when a joint takes a sudden jolt, the fluid stiffens to cushion it, then flows freely again between shocks. And the crack of a knuckle is a gas bubble forming inside it.
The fluid fills the cavities of the body's freely moving joints. It begins as a filtrate of blood plasma, enriched with two lubricants made on site: hyaluronan, a long sugar polymer produced by cells in the joint's lining membrane, and lubricin, which is secreted by cells at the cartilage surface. Normal fluid carries about 3 to 4 milligrams of hyaluronan per millilitre. It spreads into a film roughly 50 micrometres thick and soaks into pits in the cartilage, which acts as a reservoir. Movement squeezes that stored fluid back out, a process nicknamed weeping lubrication.
Its physics are unusual. Water keeps the same viscosity however hard it is stirred, but synovial fluid does not. Under a sudden rise in pressure it becomes more viscous, protecting the joint, and then returns to its normal runniness. It also feeds the cartilage cells, which have no blood supply of their own, delivering oxygen and nutrients and carrying away carbon dioxide and waste. Pressure pushes hyaluronan against the lining to form a sieve that keeps cells out and fluid in, while scavenger cells clear microbes and wear debris.
Pulling a joint's surfaces apart enlarges the capsule faster than fluid can fill it. The drop in pressure releases dissolved gases, mostly carbon dioxide, which form a bubble almost instantly. This cavitation produces the familiar high-pitched crack.
Doctors can draw the fluid with a needle, in a procedure called arthrocentesis, and read it for clues. Needle-shaped urate crystals point to gout, rod-like calcium pyrophosphate crystals to pseudogout, and very high lactate to a septic joint. An old test adds acetic acid: healthy fluid forms a mucin clot, inflamed fluid does not. The word synovia entered English around 1640, perhaps coined by Paracelsus from Greek and Latin roots for with and egg.
Source: Synovial fluid
