Which organ is primarily involved in metabolism?

The liver is the organ primarily involved in metabolism due to its critical role in processing nutrients absorbed from the digestive tract. It performs a variety of functions, including the conversion of sugars into glucose and storing it for energy, the breakdown and synthesis of proteins, the detoxification of harmful substances, and the regulation of fat metabolism. Additionally, the liver plays a key role in the synthesis of cholesterol, bile acids, and other important biochemicals necessary for digestion and metabolism.

Other organs mentioned in the question, while essential for different bodily functions, do not primarily handle metabolism in the same comprehensive manner as the liver. The heart focuses on circulation and pumping blood throughout the body, the lungs are involved in gas exchange and respiration, and the kidneys primarily function in waste excretion and fluid balance. Thus, the liver's multifaceted operations make it the cornerstone of metabolic processes.