When you get a jolt of adrenaline coursing through your body, you may be in the process of evading a lion -- or sneaking back into your workplace after a long lunch break. Regardless, the adrenaline increases the force with which your heart beats, speeds up your heart rate and increases your blood pressure.
But even if you aren't rushing into a burning building to save lives, you have an ever-present level of adrenaline in your system that increases or decreases according to what your body thinks you need.
Beta blockers lower blood pressure by blocking the effects of adrenaline (also known as epinephrine). As the effects of adrenaline are neutralized, blood vessels open up, heart rate slows down and the heart beats less forcefully (but pumps with better efficiency).