Calcium is present in large quantities in the human body, constituting the skeleton as a scaffolding to support the body, and also plays an important role in secretion, transportation, muscle contraction, and nerve conduction. Calcium exists in two forms in the body, one is ionized calcium, which exists within the blood circulation and is known as blood calcium, and the other is bound calcium, which binds to proteins, carbonic acid, or phosphoric acid and is deposited in the tissues. Calcium normally permeates all tissues and cells except the bones and teeth, and does not normally appear in a solid state in the tissues, but in some cases it precipitates as a solid and settles in the tissues as pathologic calcium salt deposits, which are predominantly calcium phosphate and to a lesser extent calcium carbonate. Calcium salts are usually monorefractive, but calcium oxalate is birefractive, and when stained with HE, calcium generally appears as a purplish-blue color; a number of dyes form chelates with calcium, including alizarin red S, red violet, and nuclear solid red. Alizarin Red S, an anthraquinone derivative, is a sodium salt of alizarin sulfonate, which chelates with calcium salts in calcium carbonate or calcium phosphate to form an orange-red complex; these dyes are generally better at recognizing moderate to large amounts of calcium than lightly stained trace calcium deposits, but Alizarin Red S tends to give more reliable results with small amounts of deposits.