The physicochemical structure of bone: cellular and noncellular elements.

Bone responds to systemic acid base perturbations through an ionic exchange with the systemic extracellular fluid which, in its extreme form, culminates in dissolution of the skeletal tissue as observed in chronic acidotic states. As a major reservoir for the body stores of calcium, phosphorus, carbonate, citrate, magnesium and sodium the bone may use these ions in order to buffer massive loads of protons during acute metabolic acidosis. The total skeletal mass consists of two components: (1) an organic unmineralized matrix which includes mainly type 1 collagen and which comprises about one third of total bone weight and (2) the inorganic mineral phase of bone which constitutes about two thirds of the skeleton and is mainly composed of hydroxyapatite crystals (the main form of bone mineral). States of chronic metabolic acidosis lead to a gradual and progressive resorption of both the organic and inorganic components of bone. This process is carried out both by a direct physicochemical effect of the excess protons and through activation of the two main bone cell types, the osteoblasts and osteoclasts. In order to fully comprehend the nature of bone-systemic circulation interface, it is essential to understand the microanatomy and the physicochemical structure of bone.