The coronary arteries supply blood to the heart muscle, and capillary beds branch out in the head, arms, abdomen, and legs to supply blood throughout the rest of the body. Within the capillaries, oxygen diffuses into the body tissues and carbon dioxide diffuses into the blood. The capillaries join together to form venules, which convey blood to the veins. The now oxygen-poor blood returns to the heart through the vena cava, which empties it back into the right atrium. The oxygen-poor blood then flows into the right ventricle and the cycle repeats itself.