The brain is the most important organ in the body because it controls all of the body's functions. While a person can be technically "alive" without her brain, she cannot experience life without it. Oxygen is very important to the brain in order to use glucose (sugar) as fuel. A lack in a sufficient supply of oxygen will prevent the brain from working properly. This leads to all sorts of neurological signs and symptoms and, if the oxygen is absent for a few minutes or more, death.
Changes in Behavior
A person whose brain is being deprived of oxygen will change his or her behavior. This is because the brain controls behavior, thought and other personality functions. Changes in behavior are seen in people who are intoxicated with carbon monoxide or a chemical that impairs the brain's oxygen use. These changes include acting lethargic, losing interest and not properly responding to a stimulus.
Loss of Consciousness
Even a little bit of time without oxygen will start to shut down the brain, leading to loss of consciousness. The exact amount of time varies from person to person, but anyone who is being deprived of oxygen will eventually pass out. Even if there is no brain death, oxygen insufficiency for a few minutes may place a person in a permanent coma or persistent vegetative state.
Loss of Memory
One of the functions of the brain is to make and store memories. As hypoxia, or lack of oxygen, progresses, the brain is no longer able to create memories. This is why people who drown, are intoxicated with carbon monoxide or receive a strong blow to the head, may not remember the event if they survive. Another reason for loss of memory is death of brain tissue that does not receive oxygen for an extended period of time (a few minutes in most people). As the tissue dies, the memories stored in it are permanently lost since brain tissue cannot be regenerated.