If the blood supply is cut off drastically or for a long time, muscle cells suffer irreversible injury and die. Sometimes a coronary artery temporarily contracts or goes into spasm. When this happens the artery narrows and blood flow to part of the heart muscle decreases or even stops. What causes a spasm is unclear, but it can occur in normal blood vessels as well as vessels partially blocked by atherosclerosis. If a spasm is severe, a heart attack may result.
High levels are a large factor in clogging arteries.
Your blood pressure is also checked each time you donate. Cholesterol levels are also checked periodically.