Synovial fluid is a fluid-like material that is present in many of the joints of the body. It serves the purpose of lubricating and nourishing certain parts of the joint. The joints in which this type of fluid is present are known as synovial joints, and these include the elbow, knee, shoulder, and hip joints, among others.
Synovial fluid has a thick consistency, somewhat like an egg. It is not like most other fluids present in the body and elsewhere, partly because it does not flow like a liquid. It may be more accurate to think of this fluid as a type of connective tissue, because of its composition and because of the work it performs.
Each synovial joint in the body is somewhat like its own organ, with needs and nutritional requirements that differ from other areas of bone. Synovial fluid performs certain mechanical functions, such as cushioning joints and making it easy for bones and cartilage to move past each other. It also has the job of bringing oxygen and other nutrients to the cartilage and other areas of the joint. In addition to providing nutrients, it also removes carbon dioxide and other waste products from the cartilage, and takes these back into the bloodstream to be removed from the body.
To keep the synovial fluid in the same place around a joint, it is contained within the synovial membrane. The way in which the fluid is contained in the joint may be responsible for a phenomenon quite familiar to most of us, namely the "cracking" of joints. When someone produces a cracking or popping sound from one of the synovial joints, whether intentionally or not, it is popularly theorized that synovial fluid plays a role in this. When the two bones of a joint are pulled away from each other, the synovial membrane expands, but the fluid volume does not. In order to fill the empty space, gases dissolved in the fluid are pulled out, and when they fill this new empty space, a popping sound is made.
It is common in the medical field to remove a sample of synovial fluid for testing. There are various parameters and attributes which are analyzed in such a test, such as color, clarity, and white blood cell count. The observation and testing of this fluid can aid in the diagnosis of dozens of different ailments, from rheumatic fever to scurvy. The fluid is obtained by inserting a syringe needle into the area of the joint where the fluid is, and extracting a small amount into the syringe. Needles used in this procedure can be somewhat large, so the area is usually anesthetized beforehand.