Final answer:
The inferior vena cava is the vessel that carries oxygen-poor blood from the lower body to the right atrium, which is part of the pulmonary circulation where gas exchange occurs in the lungs.
Step-by-step explanation:
The vessel responsible for transporting oxygen-poor blood from the bottom half of the body to the right atrium of the heart is the inferior vena cava. This large vein collects deoxygenated blood from the lower body and returns it to the heart, entering the right atrium for the pulmonary circulation. Pulmonary circulation is critical, as it involves the movement of blood to the lungs, where carbon dioxide is exchanged for oxygen before the blood is sent back to the left side of the heart through the pulmonary veins and then to the rest of the body.