Circulation is the movement of blood throughout the body through one continuous circuit of blood vessels. Different organs have unique functions and, therefore, have different requirements, circulatory patterns, and regulatory mechanisms. Several of the most vital organs (including the brain, heart, and kidneys) have autoregulatory properties, meaning that they are able to maintain a relatively constant blood flow despite fluctuations in mean arterial pressure (MAP). In other cases, locally produced factors (such as adenosine, CO2, or NO) can produce local vasoconstriction or vasodilation, regulating blood flow under specific physiologic conditions.