Regulation of pH is one of the most complex mechanisms in human physiology. Indeed, the H+ ion concentration not only contributes to the establishment and maintenance of the body's homeostasis (by defining isohydria) but it also acts as an ionic, electric or osmotic driving force; provides optimum conditions for the proper functions of a plethora of molecules; behaves as an extra- and intracellular signaling system; exerts protective functions, and so on. The versatile role of pH requires delicate, well-orchestrated regulatory machineries that are controlled by a multitude of endogenous mechanisms - this is especially true for the skin whose pH is quite unique within the body. In this chapter, we summarize key endogenous factors and mechanisms that can influence the pH of the skin. Moreover, we highlight the significance of certain molecular systems (i.e., pH-sensing ionotropic and metabotropic receptors) that have recently emerged as potential "pH-regulated sensors and transducers," which are suggested to mediate the cellular effects of pH in various skin compartments and cells.
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