Fascinating and Confusing Cardiovascular System

In this chapter of the cardiovascular system, I learned a lot of information's about the Cardiovascular System and how it works. But, from time to time I would get confused about how the blood flows throughout the body. I understand how the cardiovascular system is about how the blood get pump out of the heart and throughout the rest of the body. What I don't understand is how the heart does it. In this section it confused me about how the blood gets into the heart and out of the heart and what about the oxygen how does that work. I learned that the oxygen is carried within the blood and transfer throughout the body through blood. The blood in the body carries nutrients, waste, and oxygen with it throughout the body. The Cardiovascular System chapter is an interesting chapter but on some sections I am somewhat confused on it.