An intact blood-brain barrier and normal production, circulation, and absorption of cerebrospinal fluid are critical for normal brain function. Minor disruptions of barrier function are without clinical consequences. Major disruptions accompany most significant acute brain injuries. The anatomic location of the blood-brain barrier is the endothelial cells of arterioles, capillaries, veins, and the epithelial cell surface of the choroid plexus. However, endothelial cells require the presence of glial cells to maintain barrier function. During cardiopulmonary bypass, several factors may result in a temporary disruption of the barrier; the most important are systemic inflammatory response and focal ischemia due to emboli. Lacking a lymphatic system, the brain depends on the circulation of cerebrospinal fluid to remove the products of metabolism, and the circulation of cerebrospinal fluid depends on a vascular systolic pulse wave to drive this fluid antegrade along the brain paravascular spaces. Although it is not possible to identify this paravascular space histologically, its presence is confirmed by tracer methods.