Shock is a state of generalized cellular hypoperfusion where oxygen delivery is inadequate to meet metabolic needs. Shock is divided into three stages: a nonprogressive stage where compensatory mechanisms allow recovery without therapy, a progressive stage where shock worsens without therapy until death, and an irreversible stage where all known therapies are inadequate. Shock is classified as hypovolemic, cardiogenic, neurogenic, or vasogenic depending on the underlying cause of inadequate blood flow.