Simplify the following text for a Grade 9 native English-speaking person.
Original: Atherosclerosis is the most common cause of stenosis ( narrowing of the blood vessels ) of the heart 's arteries and , hence , angina pectoris .
Simplified: Angina pectoris ( angina of the heart ) is pain which is due to ischemia of the heart muscle . A common cause of angina pectoris is when the heart is working harder than normal e.g. during exercise .