How does the circulatory system work?

Heart diseas: You ask a question far too involved for the space here. Look up circulatory physiology on a search engine. On the simplest level, the heart pumps blood around the body. Many systems control how fast, what blood pressure and a myriad of other factors.

Blood: All the cells that compose your body require a continuous supply of oxygen and nutrients and means of removing waste. These functions are performed by blood. Your heart pumps oxygen rich blood to all areas and receive oxygen poor blood in return - simulataneously, your heart separately pumps this oxygen poor blood to your lungs and receives it to pump everywhere else. Blood goes round and round.