Carbon monoxide is not always a poison gas for human organism: Physiological and pharmacological features of CO

•CO functions as an endogenous signaling molecule.•CO modulates functions of the cardiovascular system.•CO inhibits blood platelet aggregation and adhesion.•CO may play a role as potential therapeutic agent. Carbon monoxide (CO) is an odorless, colorless and non-irritating gas. Even a small amount of CO exposure is possibly associated with specific toxic effects. CO is also produced endogenously in the body as a byproduct of heme degradation catalyzed by heme oxygenase. More recently CO has been identified as a gasotransmitter in various biological systems. However, the biological role and the therapeutic potential of carbon monoxide is not clear. This review summarizes the negative and the positive functions of carbon monoxide in various biological systems, including cardiovascular system.