As a neuroscientist with a focus on cerebral circulation and brain function, I can tell you that the brain is extremely sensitive to a lack of blood flow, which is known as ischemia. The brain requires a constant supply of oxygen and nutrients, which are delivered by the blood. When blood flow is cut off, the brain can start to suffer damage within seconds. Brain cells, or neurons, begin to die off after about 5 to 10 seconds of complete ischemia. However, the extent of damage and the rate at which it occurs can vary depending on several factors, including the individual's health, the specific region of the brain affected, and the degree of blood flow reduction. Unconsciousness can occur within 10 seconds of significant blood flow cessation to the brain. After about 4 minutes of no blood flow, brain damage becomes severe, and after about 10 minutes, the damage is likely irreversible. This is why quick response times are critical in situations like stroke or cardiac arrest, where blood flow to the brain may be compromised. read more >>

The brain can survive for up to about six minutes after the heart stops. The reason to learn cardiopulmonary resuscitation (CPR) is that if CPR is started within six minutes of cardiac arrest, the brain may survive the lack of oxygen. After about six minutes without CPR, however, the brain begins to die.read more >>