Where does oxygen poor blood go after leaving the heart, why?

Well, blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. As theatrium contracts, blood flows from your right atrium into your right ventricle through the open tricuspid valve

They'll be transported to the lungs, for gas exchange. 

These oxygen-poor blood would leave the right ventricle of the heart, then through the pulmonary artery, and then they enter the lungs. The main reason for them to go through the lungs is that to carry out the gas exchange process. During this process, waste in the blood (such as carbon dioxide) are removed and more oxygen is diffused into these blood. This process is very important because all the other body organs need oxygen and nutrients to function, so gas exchange in lungs is the only way for blood to get more oxygen.

After the gas exchange process, the blood would be transported back to the left atrium of the heart, through the pulmonary vein, and lastly transported to many other body organs to supply them with oxygen and nutrients through the aorta.