We however are going to look at a simple overview of this system. The heart is the main pump. The heart is divided into four chambers. The top two chambers are the atriums and the bottom two chambers are the ventricles. The atriums both contract at the same time as do the two ventricles.
Blood enters the heart via the superior and the inferior vena cava.
These are the two largest veins in the body. The right atrium receives the blood first. The right atrium contracts and forces the blood into the right ventricle. When the right ventricle contracts the blood is pumped into both lungs via the pulmonary artery. This portion of the circulatory system is sometimes referred to as pulmonary circulation or lesser circulation.