section_path: Pathophysiology/Ischemic/Collateral flow
section_name: Collateral flow
paragraph_text: Collateral flow
  
  The brain can compensate inadequate blood flow in a single artery by the collateral system. This system relies on the efficient connection between the carotid and vertebral arteries through the circle of Willis and, to a lesser extent, the major arteries supplying the cerebral hemispheres. However, variations in the circle of Willis, caliber of collateral vessels, and acquired arterial lesions such as atherosclerosis can disrupt this compensatory mechanism, increasing the risk of brain ischemia resulting from artery blockage.
  
  The extent of damage depends on the duration and severity of the ischemia. If ischemia persists for more than 5 minutes with perfusion below 5% of normal, some neurons will die. However, if ischemia is mild, the damage will occur slowly and may take up to 6 hours to completely destroy the brain tissue. In case of severe ischemia lasting more than 15 to 30 minutes, all of the affected tissue will die, leading to infarction. The rate of damage is affected by temperature, with hyperthermia accelerating damage and hypothermia slowing it down and other factors. Prompt restoration of blood flow to ischemic tissues can reduce or reverse injury, especially if the tissues are not yet irreversibly damaged. This is particularly important for the moderately ischemic areas (penumbras) surrounding areas of severe ischemia, which may still be salvageable due to collateral flow.