In those cases, the spread between the bid & ask goes to the market maker as compensation for making a market in a stock. For a liquid stock that is easy for the market maker to turn around and buy/sell to somebody else, the spread is small (narrow).
In cerebral hypoxia, sometimes only the oxygen supply is interrupted. Brain cells are extremely sensitive to a lack of oxygen. Some brain cells start dying less than 5 minutes after their oxygen supply disappears. As a result, brain hypoxia can rapidly cause severe brain damage or death.
Blood isn't what keeps the brain alive per se, but rather the dissolved oxygen in the blood. Typically, the human brain can go for about 3 to 5 minutes without oxygen before irreversible brain damage begins to occur, and 9 minutes is usually the point where brain death will occur and all electrical function stops. But in certain situations, the brain can go as long as an hour without oxygen.