The Flow of Blood

The circulatory system is a closed, double-looped system, which means that blood is contained within blood vessels and passes twice through the heart on each journey around the body.
Deoxygenated blood (blood low in oxygen and high in carbon dioxide) goes into the right side of the heart through a large vein called the vena cava.
Using the pumping action of the heart, blood is sent to the lungs via the pulmonary artery. This is one of the only places in the body where deoxygenated blood is carried by an artery, not a vein.
Gaseous exchange occurs in the lungs, wherein blood takes in oxygen and gets rid of carbon dioxide.
This newly oxygenated blood then returns to the left side of the heart via the pulmonary veins. This is one of the few places in the body where veins carry oxygenated blood.
The left side of the heart pumps the oxygen-rich blood out to all parts of the body through a major artery called the aorta.
As the blood flows through the body, it delivers oxygen and nutrients to cells and collects waste carbon dioxide and other waste products.
This deoxygenated blood then returns to the right side of the heart, from where the whole process begins again.

Remember:

Arteries: Vessels that carry blood AWAY from the heart.
Veins: Vessels that carry blood TO the heart.
Capillaries: Small, thin-walled vessels that allow exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and body’s cells.
Pulmonary circulation: The part of the circulatory system that carries blood between the heart and lungs.
Systemic circulation: The part of the circulatory system that carries blood between the heart and the rest of the body (except for the lungs).