Answer:
Veins
Step-by-step explanation:
Within the body, there are five primary classifications of blood vessels: arteries, veins, arterioles, venules, and capillaries -- each of these inevitably flows into another in a perfect "circle" that keeps blood circulating in order to perfuse oxygen to the many tissues present throughout the body.
Arteries usually with oxygen-rich blood pump this blood away from the heart and into the smaller arterioles. Arterioles become the one-cell thick capillaries where gas exchange occurs (oxygen hops off of the hemoglobin protein, carbon dioxide hops on). These capillaries then become venules now circulating oxygen-deficient blood that needs to return to the heart. The venules join together to form veins which then pump feed larger volumes of this oxygen-poor blood back to the heart and lungs to restart the process.