Cerebral Blood Flow: Quantifying Consciousness Although the heart may be the source of energy needed to generate blood flow,and other organs absolutely essential for normal living,the brain is the reason we are alive. The collected book chapters are aimed at addressing this most fundamental organ and its blood flow. These papers reflect detailed descriptions of similar topics presented over a two-day period as part of the 5th International Symposium on Applied Physiology of the Peripheral Circulation held in Pittsburgh, Pennsylvania, in June 2000. The symposium and this volume were organized into four distinct groupings that follow in a logical fashion. The initial papers focus on the basic unique physiology and pathophysiology of the cerebral circulation, including a historical reviewof the means to measure cerebral blood flow and its implications of the past 30 years. Dr. Walter D. Obrist was one of the original investigators to use radiolabeled markers to assess cerebral blood flow. His equations and initial studies form an impressive introduction to where we are now. As with much of the body's special organs, the brain too has many circulatory features unique to itself.These include the unique blood-brain barrier function ofthe endothelium, local neural regulation control, and intracranial pressure effects. These special issues,plus genetic factors that may predispose individuals to developing cere­ bral aneurysm, make up the initial section of the monograph.