The blood vessels in your brain will make the proper adjustment to try and keep the flow of blood and the oxygen that it carries to your brain cells unchanged . That is, until it reaches a critical point and this mechanism can no longer keep up with the brain's demands for food and oxygen. This autoregulation can be lost by direct injury to the brain or in severe catastrophic injuries. Our nerves cells are constantly working and sending messages to distant areas to command movement, produce speech, or drift off to quiet thoughts of our next vacation. An inadequate amount of oxygen going to the brain is called hypoxia , while a total lack of oxygen is anoxia.
Ischemia refers to an inadequate amount of blood flowing to the brain.
These events can happen independently or together. For example, a man with a ruptured spleen and punctured lung after a car wreck will have both the problems of too little blood and too little oxygen. It's a common question. Like most things in life, it varies from person to person, but we usually become dizzy within a few seconds and lose consciousness shortly after that. If the cardiac arrest lasts longer than 3- 4 minutes there is usually generalized damage to the brain.