Humans and other mammals have dual circulatory systems: one system is for pulmonary circulation (circulation to the lungs; pulmo = lungs), and the other system is for systemic circulation (the rest of the body).As each atrium and ventricle contract, blood is pumped into certain major blood vessels, and from there, continues through the circulatory system. The pulmonary pathway begins with deoxygenated blood. It is depleted because it has just exchanged oxygen for carbon dioxide across the membranes. It goes into the right atrium and into the right ventricle. Then it goes through the pulmonary semilunar valve and into the pulmonary artery. This artery carries the blood back to the lungs where it becomes oxygenated again. This is where the systemic system takes over.
Freshly oxygenated blood returns to the heart via the pulmonary veins. They go into the LEFT atrium, then the left ventricle. Next it travels (thanks to heart contractions) into the aorta which then in turn branches out into the smaller arterioles and arteris, then to tiny capillaries where oxygen is exchanged for CO2.
