Astrocytic foot processes surround brain capillaries and, during development, induce endothelial cells to form tight junctions. The endothelial tight junctions are the basis of the blood-brain barrier, a system of controlled transcapillary transport which maintains homeostasis in the CNS. Endothelial tight junctions are found only in brain capillaries. Loss of the integrity of the endothelial barrier causes fluid to leak into the interstitial space, leading to vasogenic cerebral edema. This raises intracranial pressure and can collapse brain capillaries, resulting in arrest of cerebral perfusion. Cerebral edema is caused by a variety of pathological processes, including ischemic insults, inflammation, and malignant brain tumors whose capillaries do not have tight junctions. Astrocytes are less vulnerable than neurons to ischemic injury but they are damaged if there is lactic acidosis. Such damage causes intracellular fluid accumulation (cytotoxic edema). Cytotoxic edema involves the cerebral cortex, whereas vasogenic edema is more pronounced in the white matter. Vasogenic edema is more important clinically than cytotoxic edema.