The kinds of things that go wrong in the brain. Problems with blood flow: blockage in blood vessels, when there is a blockage in the blood flow, there is usually two things that blood carries in it is oxygen and sucrose to keep the cells alive. Oxygen is the most important because a person can only stay without oxygen for about 3 minutes. The brain is very active metabolically so if it runs out of oxygen it gets in deep trouble. Often we can get other organs in the body that can survive much better without a blood supply. If you get your finger cut off, you can put in back together. However, 3 minutes without oxygen, you can just call it quits. It has a very high metabolic rate, it has a very high metabolic requirement, so a very short period without oxygen can be devastating for the brain.