What do arteries,veins, and capillaries do?

Arteries carry blood away from the heart, while veins carry blood towards the heart. Capillaries carry blood from arteries to veins.

Arteries generally carry oxygenated blood from the heart to the tissues and organs during the systolic heartbeat cycle. They are under high pressure.
(the only exception is the pulmonary arteries which carries deoxygenated blood from the heart to the lungs).

Veins generally carry deoxygenated blood from the tissues and organs back to the heart during the diastolic heartbeat cycle. They are under lower pressure.
(the only exception is the pulmonary veins which carry oxygenated blood from the lungs to the heart).

The capillaries join the arteries to the veins and are very small in size. Tissue fluid can leak out through the capillaries into the surrounding cells and is then later retaken up by the lymphatic system and returned to the blood circulation via the lymphatic veins.