Simplify the following text for a Grade 10 native English-speaking person.
Original: Angina pectoris , commonly known as angina , is severe chest pain due to ischemia ( a lack of blood , thus a lack of oxygen supply ) of the heart muscle , generally due to obstruction or spasm of the coronary arteries ( the heart 's blood vessels ) .
Simplified: Angina pectoris ( angina of the heart ) is pain which is due to ischemia of the heart muscle .