Blood leaves the heart through large vesels called arteries , which then split up into smaller ones called arterioles . Those carry blood to near the tissue that requires oxygen, where they split up into many small vessels called capillaries . It is in the capillaries that gas exchange takes place. After passing through the capillaries, blood is no longer oxygen-rich and is called venous blood. It is collected into larger vessels called venules , which join together into large ones called veins .
Veins lead back to the heart, where blood gets pumped to the lungs.
In the lungs the blood exchanges carbon dioxide for oxygen and becomes oxygen-rich, or arterial blood again. From there it comes back to the heart, where its journey around the body starts once again.