Page 306 - Fundamentals of anatomy physiology

Synovial Fluid
What is synovial fluid? It is a clear, viscous solution with the
consistency of egg yolk or heavy molasses. Synovial fluid
resembles interstitial fluid, but contains proteoglycans with a
high concentration of hyaluronan (hyaluronic acid) secreted
by fibroblasts of the synovial membrane. p. 165 Even in a
large joint such as the knee, the total quantity of synovial fluid
in a joint is small—normally less than 3 mL. The synovial fluid
within a joint has three primary functions:
1. Lubrication. Articular cartilages act like sponges filled with
synovial fluid. When part of an articular cartilage is com-
pressed, some of the synovial fluid is squeezed out of the
cartilage and into the space between the opposing surfaces.
This thin layer of fluid greatly reduces friction between
moving surfaces, just as a thin film of water makes a high-
way slick by reducing friction between a car’s tires and the
road. When the compression stops, synovial fluid is pulled
back into the articular cartilage.
2. Nutrient Distribution. The synovial fluid in a joint must
circulate continuously to provide nutrients and a waste dis-
posal route for the chondrocytes of the avascular articular
cartilage. It circulates whenever the joint moves. The com-
pression and re-expansion of the articular cartilage pumps
synovial fluid into and out of the cartilage matrix. As the sy- stretched so much that some of the collagen fibers are torn, but
novial fluid flows through the areolar tissue of the synovial
membrane, waste products are absorbed and additional
nutrients are delivered by diffusion across capillary walls.
3. Shock Absorption. Synovial fluid cushions joints that are
subjected to compression from shocks and sudden im-
pacts. This occurs because the viscosity of synovial fluid
increases with increased pressure. For example, your hip,
knee, and ankle joints are compressed as you walk. They
are even more severely compressed when you jog or run.
When the pressure across a joint suddenly increases,
the resulting shock is lessened as the viscosity of syno-
vial fluid increases. As the pressure lessens, viscosity de-
creases and it regains its lubricating function across the
articular surfaces.
Accessory Structures
Synovial joints may have a variety of accessory structures, in-
cluding pads of cartilage or fat, ligaments, tendons, and bursae
(Figure 9–1b).
Cartilages and Fat Pads
In several joints, including the knee (Figure 9–1b), menisci
and fat pads may lie between the opposing articular surfaces. A
meniscus (me-NIS-kus; a crescent; plural, menisci), also called
an articular disc, is a pad of fibrocartilage located between op-
posing bones within a synovial joint. Menisci may subdivide a
Chapter 9 Joints 293
synovial cavity, channel the flow of synovial fluid, or allow for
variations in the shapes of the articular surfaces.
Fat pads are localized masses of adipose tissue covered by
a layer of synovial membrane. They are commonly superficial
to the joint capsule (Figure 9–1b). Fat pads protect the articular
cartilages and act as packing material for the joint. When the
bones move, the fat pads fill in the spaces created as the joint
cavity changes shape.
Ligaments
The capsule that surrounds the entire joint is continuous with
9the periostea of the articulating bones. Accessory ligaments
support, strengthen, and reinforce synovial joints. Capsular
ligaments, or intrinsic ligaments, are localized thickenings of the
joint capsule. Extrinsic ligaments are separate from the joint cap-
sule. These ligaments may be located either inside or outside
the joint capsule, and are called intracapsular or extracapsular
ligaments, respectively.
Ligaments are very strong. In a sprain, a ligament is
the ligament as a whole survives and the joint is not damaged.
With excessive force, one of the attached bones often breaks
before the ligament tears. In general, a broken bone heals much
more quickly and effectively than does a torn ligament. The
reason is that ligaments have no direct blood supply and thus
must obtain essential substances by diffusion.
Tendons
Tendons are not part of the joint itself, but tendons passing
across or around a joint may limit the joint’s range of motion
and provide mechanical support for it. For example, tendons
associated with the muscles of the arm help brace the shoulder
joint.
Bursae
Bursae (BUR-se. ; singular, bursa, a pouch) are small, thin, fluid-
filled pockets in connective tissue. They contain synovial fluid
and are lined by a synovial membrane. Bursae may be con-
nected to the joint cavity or separate from it. They form where
a tendon or ligament rubs against other tissues. Located around
most synovial joints, including the shoulder joint, bursae re-
duce friction and act as shock absorbers. Synovial tendon sheaths
are tubular bursae that surround tendons where they cross
bony surfaces. Bursae may also appear deep to the skin, cover-
ing a bone or lying within other connective tissues exposed to
friction or pressure. Bursae that develop in abnormal locations,
or because of abnormal stresses, are called adventitious bursae.
Factors That Stabilize Synovial Joints
A joint cannot be both highly mobile and very strong. The
greater the range of motion at a joint, the weaker it becomes.