Which organ is responsible for monitoring and destroying worn-out blood cells?

The spleen is the organ responsible for monitoring and destroying worn-out blood cells. It plays a crucial role in the body's immune response and is involved in filtering blood and recycling iron from hemoglobin. As blood passes through the spleen, it detects old or damaged red blood cells, which are then removed and broken down. This process helps maintain healthy blood and supports the overall functionality of the circulatory system.

The liver also plays a role in processing blood and recycling components, but its primary function includes bile production and metabolizing nutrients rather than the direct removal of red blood cells. The kidneys are essential for filtering waste from the blood and regulating electrolytes and fluid balance, but they do not directly destroy worn-out blood cells. The heart's main function is to pump blood throughout the body and does not participate in blood cell maintenance. Thus, the spleen is key in monitoring and destroying old blood cells, making it the correct answer.