A class of cell surface receptors that prefers ADENOSINE to other endogenous purines. Purinergic P1 receptors are widespread in the body including the cardiovascular, respiratory, immune, and nervous systems. There are at least two pharmacologically distinguishable types (A1 and A2, or Ri and Ra). The methylxanthines, e.g., CAFFEINE, bind to these receptors, but also have other unrelated effects.