Answer: The correct answer for the blank is-
Pulmonary artery.
Right ventricles receive deoxygented blood ( from right atrium) when tricuspid valves ( present between right atrium and right ventricle) open.
When ventricles contract, this blood flows from right ventricles to pulmonary artery, which takes it to the lungs.
In other words, the deoxygenated blood ( carbon dioxide containing blood) is sent to lungs via pulmonary artery so that it is exchanged with the oxygenated blood at lungs, which is then sent to the heart for its circulation in the entire body.