Fluid Lubricates Joints

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

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Image: Dr. Manuel González Reyes / Pixabay / Public Domain - No restrictions

Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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Joints of vertebrates are protected by use of synovial fluid as a lubricant.

“Whereas technology tries to polish the hard metal of bearings to as fine a finish as possible, nature covers the touching surfaces with a spongelike substance which is comparatively stiff yet quite elastic: cartilaginous tissue, which differs from hard bone tissue essentially in that it lacks deposits of calcium crystals. One could compare this tissue to fiberglass from which the fibers have been removed. The fine pores of the cartilaginous sliding layer are soaked through with lubricating synovial fluid. When the joint is subjected to pressure, the layer compresses and the fluid is pushed out of the thin ducts. The gliding principle is the same as the one used for air-cushion vehicles, with the difference that in bone bearings the cushion (of fluid) is produced on the spot…Synovial fluid might also compete in the market with modern lubricants. It contains slightly less than blood serum, but on the other hand it carries an organic acid with very long molecules which are probably linked to proteins. The more the gliding speeds vary in the lubrication layer, the lower the viscosity of the fluid.” (Tributsch 1984:41-42)

A diagram with parts labled showing the joint cavity and membranes between two bones

Image: OpenStax College / Wikipedia / CC BY - Creative Commons Attribution alone

Structure of synovial joint.

Last Updated September 14, 2016

References

Book

How Life Learned to Live: Adaptation in Nature

The MIT Press | 26/02/1984 | Helmut Tributsch

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