Adenosine triphosphate (ATP) is a nucleotide consisting of a purine base (adenine) attached to the first carbon atom of ribose (a pentose sugar). Three phosphate groups are esterified at the fifth carbon atom of the ribose. ATP is incorporated into nucleic acids by polymerases in the processes of DNA replication and transcription. ATP contributes to cellular energy charge and participates in overall energy balance, maintaining cellular homeostasis. ATP can act as an extracellular signaling molecule via interactions with specific purinergic receptors to mediate a wide variety of processes as diverse as neurotransmission, inflammation, apoptosis, and bone remodelling. Extracellular ATP and its metabolite adenosine have also been shown to exert a variety of effects on nearly every cell type in human skin, and ATP seems to play a direct role in triggering skin inflammatory, regenerative, and fibrotic responses to mechanical injury, an indirect role in melanocyte proliferation and apoptosis, and a complex role in Langerhans cell-directed adaptive immunity. During exercise, intracellular homeostasis depends on the matching of adenosine triphosphate (ATP) supply and ATP demand. Metabolites play a useful role in communicating the extent of ATP demand to the metabolic supply pathways. Effects as different as proliferation or differentiation, chemotaxis, release of cytokines or lysosomal constituents, and generation of reactive oxygen or nitrogen species are elicited upon stimulation of blood cells with extracellular ATP. The increased concentration of adenosine triphosphate (ATP) in erythrocytes from patients with chronic renal failure (CRF) has been observed in many studies but the mechanism leading to these abnormalities still is controversial.