Dr liberman is my surgeon. He is works renowned and is very knowledgeable. He did my first done surgery, and my brain surgery. One thing I respect about him is that he has great bedside manner as well, and doesn't mind explaining things like this to people. I'm sure if you reach out to him, he would explain happily. But there are some good answers here already to sum it up pretty well. Blood is basically suffocating to the brain because of the clotting agents, and blood breaks down over time unlike cerebral spinal fluid. The breaking down of blood cells causes processes to happen that are not safe for the brain or spinal fluid.