Where is Hyaluronic Acid located in the body?
Hyaluronic Acid is found naturally in most every cell in the body and occurs in high concentrations in specific body locations. In each body location, it serves a different function. Unfortunately, HA also has a half-life ( the time it takes for the molecule to get broken down and excreted from the body) of less than 3 days and possibly even as little as one day in the skin. For this reason, it is imperative that the body continually replenish itself with HA. Below are some of the areas in the human body where it is present and critical to anatomical function.
Hyaluronic Acid in Bones and Cartilage
Hyaluronic Acid is found in all bones and cartilage structures throughout the body. 
Both of these structures provide a resilient rigidity to the structure of the human body. HA is especially found in various forms of cartilage but none more than the hyaline cartilage. As you've probably guessed it, hyaline is short for hyaluronic acid. Hyaline cartilage covers the ends of the long bones where articulation (bending) occurs and provides a cushioning effect for the bones. The hyaline cartilage has been called the "gristle cartilage" because its resistance to wear and tear. Hyaline cartilage also supports the tip of the nose, connects the ribs to the sternum and forms most of the larynx and supporting cartilage of the trachea and bronchial tubes in the lungs.
Hyaluronic Acid in Synovial fluid
Our joints (like the elbows and knees) are surrounded by a membrane called the synovial membrane which forms a capsule around the ends of the two articulating bones. This membrane secretes a liquid called the synovial fluid. Synovial fluid is a viscous fluid with the consistency of motor oil. It has many functions, but none more than providing the elastic shock absorbing properties of the joint. Its second most important function in the joint is to carry nutrients to the cartilage and to also remove waste from the joint capsule.
Hyaluronic Acid in Tendons and Ligaments/Connective tissue
Connective tissue is found everywhere in the body. It does much more than connect body parts; it has many forms and functions. Its major functions include binding, support, protection, and insulation. One such example of connective tissue is the cordlike structures that connect muscle to bone (tendons) and bone to bone (ligaments). In all connective tissue there are three structural elements. They are ground substance (hyaluronic acid), stretchy fibers (collagen and elastin) and a fundamental cell type. Whereas all other primary tissues in the body are composed mainly of living cells, connective tissues are composed largely of a nonliving ground substance, the hyaluronic acid, which separates and cushions the living cells of the connective tissue. The separation and cushioning allow the tissue to bear weight, withstand great tension and endure abuse that no other body tissue could. All of this is made possible because of the presence of the HA and its ability to form the gelatinous ground substance fluid.
Hyaluronic Acid in Scalp Tissue and Hair Follicles
Structurally the scalp is identical to the skin tissue located throughout the body except it also contains about 100,000 hair follicles that give rise to hair. Actually the hair and the hair follicle are a derivative of skin tissue. There are two distinctive skin layers, one, the epidermis (outer layer) which gives rise to the protective shield of the body and the other, the dermal layer (deep layer) which makes up the bulk of the skin and is where the hair follicle is located. This dermal layer is composed of connective tissue and the connective tissue, with its gelatinous fluid like characteristics provides support, nourishes and hydrates the deep layers of the scalp. The result is healthy lustrous hair and a moisturized scalp. Again, all of this is made possible because of the presence of HA in the scalp.
Hyaluronic Acid in Lips
The lips are a core of skeletal muscle covered by skin tissue. The dermal layer of the lips is composed primarily of connective tissue and its components hyaluronic acid and collagen that give the structure (shape) and plumpness to the lips. The HA binds to water creating a gelatinous fluid that hydrates the surrounding tissue and keeps the collagen (responsible for keeping the skin tight) nourished and healthy. The result is healthy well hydrated and plump lips that are well protected from the environment.
Hyaluronic Acid in Eyes
Hyaluronic acid is highly concentrated inside the eyeball. The fluid inside the eye called the vitreous humor is composed almost completely of hyaluronic acid. The HA gives the fluid inside the eye a viscous gel like property. This gel acts as a shock absorber for the eye and also serves to transport nutrients into the eye. HA has been directly injected into the eye during procedures to help maintain the shape of the eye during surgery. It has been said that after the 5th decade of life, our eyes stop producing the much needed hyaluronic acid resulting in various eye needs.
Hyaluronic Acid in Gum Tissue
The Gums (gingivoe) are composed of dense fibrous connective tissue (ligaments) which secure the teeth to the aveloar bone (jaw bone). Once again, connective tissue is composed of a fibrous tissue surrounded by hyaluronic acid (extra-cellular matrix). Without the presence of HA, the gum tissue becomes unhealthy. If it is present it helps to provide the tensile strength of the ligaments that secure the tooth in place by providing hydration and nourishment. The result is a healthy set of gums.
Skin
Although Hyaluronic Acid (HA) can be found naturally in most every cell in the body, it is found in the greatest concentrations in the skin tissue. Almost 50% of the bodies HA is found here. It is found in both the deep underlying dermal areas as well as the visible epidermal top layers. Young skin is smooth and elastic and contains large amounts of HA that helps keep the skin stay young and healthy. The HA provides continuous moisture to the skin by binding up to 1000 times its weight in water. With age, the ability of the skin to produce HA decreases.
ECM (ground substance)
The extracellular matrix (ECM) is a gelatinous (gel-like) fluid that surrounds almost all living cells and is essential to life. It gives structure and support to the body and without it, we would just be a trillions cells without a shape or function. It is essentially the mortar between the bricks. The skin, bones, cartilage, tendons and ligaments are examples where the ECM is located in the body. The ECM is composed of material (fibrous elements) called elastin and collagen surrounded by a gelatinous substance (Hyaluronic Acid). HA's roles in the ECM is to help the stretchy fibers in the body from overstretching and drying out by continually bathing them in this nutritious water base gelatinous fluid. It also serves as a wonderful medium through which nutrients and waste are transported to and from the cells of these structures. This fluid would not exist if it was not for the ability of the HA molecule to bind up to 1000 times its weight in water.
Hyaluronic Acid in the skin
The skin is the largest organ in the body comprising about 15% of the body weight. Roughly 50% of the Hyaluronic Acid in our body is found in the skin. HA and Collagen are vital to maintaining the skin’s layers and structure. It is the collagen that gives the skin its firmness but it is the HA that nourishes and hydrates the collagen. Imagine the collagen as the stretchy fibers that restore the skin back to shape when stretched. Collagen is like a rubber band but stretch that rubber band a million times, like what we do with our skin and without any moisture. Eventually that rubber band gets overstretched (saggy) and dried out and will most likely break. This is much the same way the collagen in our skin reacts leaving our skin in need of moisture. Now imagine that same rubber band stretched a million 
times while under water the whole time. Chances of that rubber band drying out and breaking are minimal. Consider the Hyaluronic Acid as the water that keeps the collagen moist and elastic. Collagen is continuously surrounded and nourished by the gelatinous HA substance. Young skin is smooth and highly elastic because it contains high concentrations of Hyaluronic Acid, which helps skin stay healthy. As we grow older, the body loses its ability to maintain this same concentration in the skin. With decreasing levels of HA in the skin, so goes the ability of the skin to hold water. The result, the skin becomes drier and loses its ability to maintain it's hydration. Hyaluronic acid acts as a space filler by binding to water and thus keeping the skin wrinkle-free.