Your heart and blood vessels make up your overall blood circulatory system. Your blood circulatory system is made up of four subsystems.
Arterial circulation is the part of your overall blood circulatory system that involves arteries, like the aorta and pulmonary arteries.
Arteries are blood vessels that carry blood away from your heart. Healthy arteries are strong and elastic. They become narrow between beats of the heart, and they help keep your blood pressure consistent. This helps blood circulate efficiently through your body.
Arteries branch into smaller blood vessels called arterioles. Arteries and arterioles have strong, flexible walls that allow them to adjust the amount and rate of blood flowing to various parts of your body.
Venous circulation is the part of your overall blood circulatory system that involves veins, like the vena cava and pulmonary veins. Veins are blood vessels that carry blood to your heart.
Veins have thinner walls than arteries. Veins can widen as the amount of blood passing through them increases.
Capillary circulation is the part of your overall blood circulatory system where oxygen, nutrients, and waste pass between your blood and parts of your body.
Capillaries connect the arterial and venous circulatory subsystems. Capillaries are very small blood vessels.
The importance of capillaries lies in their very thin walls. Unlike arteries and veins, capillary walls are thin enough that oxygen and nutrients in your blood can pass through the walls to the parts of your body that need them to work normally.
Capillaries' thin walls also allow waste products like carbon dioxide to pass from your body's organs and tissues into the blood, where it's taken away to your lungs.
Pulmonary circulation is the movement of blood from the heart to the lungs and back to the heart again. Pulmonary circulation includes both arterial and venous circulation.
Blood without oxygen is pumped to the lungs from the heart (arterial circulation). Oxygen-rich blood moves from the lungs to the heart through the pulmonary veins (venous circulation).
This answer from the National Heart, Lung and Blood Institute has been reviewed and/or edited by Dr. William D. Knopf.