What is the primary organ responsible for pumping blood in a closed circulatory system?

In a closed circulatory system, the primary organ responsible for pumping blood is the heart. The heart functions as a muscular organ that contracts rhythmically to propel blood throughout the body. This system is characterized by blood being contained within vessels, providing efficient transportation of nutrients, gases, and waste products. The heart's structure, with its chambers and valves, allows for the effective separation and direction of blood flow, ensuring that oxygenated and deoxygenated blood do not mix.

The other organs listed do not play a central role in pumping blood. The liver is primarily involved in metabolic processes and detoxification, the aorta is a major artery that carries blood away from the heart but does not actively pump blood, and the lungs are essential for gas exchange but do not pump blood themselves. Thus, the heart is the critical organ for maintaining circulation in a closed system.