Final answer:
Arteries are the blood vessels that eject oxygenated blood into systemic circulation and have a very thick wall to maintain the pressure.
Step-by-step explanation:
The structure that ejects oxygenated blood into systemic circulation and has a very thick wall is the Artery.
Arteries have thicker walls compared to other blood vessels because they are closer to the heart and receive blood that is surging at a higher pressure.
This thick wall helps arteries to maintain the pressure of the blood moving through the system.