It is important to stretch before exercising because for example if you were to be doing a strenuous workout it causes the blood vessels in your legs to expand. Which then causes more blood into your legs and feet. If you were to stop after a while of exercising your heart rate slows down and blood can pool into your lower body which could cause you to possibly faint.
Its the tricuspid valve that is located between the left atrium and left ventricle?
Search the internet for the answers :3