Beta-blockers are a class of drug that block the action of epinephrine at sites on heart muscle and blood vessel cells walls. This allows the heart muscle to relax reducing the workload on the heart by slowing the heart rate, which allows the heart to pump more efficiently. In blood vessels this decreases blood vessel constriction, enhancing blood flow within the vessels.
Chabner, Davi-Ellen. 2007. The Language of Medicine. 8th ed. Saunders Elsevier, Missouri