Answer the following question given this paragraph: 

A heart attack happens when the blood cannot reach the heart because a blood vessel is blocked. If cardiac muscle is starved of oxygen for more than roughly five minutes, it will die. Cardiac muscle cells cannot be replaced, so once they die, they are dead forever. Coronary heart disease is the leading cause of death of adults in the United States. The image below shows the way in which a blocked coronary artery can cause a heart attack and cause part of the heart muscle to die ( Figure below ). Maybe one day stem cells will be used to replace dead cardiac muscle cells.


Q: What happens when the blood cannot reach the heart because a blood vessel is blocked?


A:
heart attack