Complex sentence: The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two great veins, the superior and inferior venae cavae, which return blood to the right atrium, to be pumped into the right ventricle.
Split into simpler sentences: The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle.