What is the lifespan of red blood cells primarily influenced by?

The lifespan of red blood cells (RBCs), which is typically around 120 days, is primarily influenced by the spleen. The spleen plays a crucial role in filtering and recycling aged or damaged red blood cells from circulation. As RBCs age, they become more susceptible to being captured and destroyed by macrophages in the spleen. This organ not only removes defective RBCs but also helps in the recycling of hemoglobin and iron for the production of new blood cells in the bone marrow.

While other organs like the liver and kidneys are involved in various aspects of blood and metabolic function, their roles are not directly focused on the lifespan of red blood cells. The liver is more involved in metabolism and detoxification, and the kidneys help regulate blood composition and filter waste products from the blood. The lungs facilitate gas exchange but are not directly involved in the lifecycle of red blood cells. Therefore, the spleen's function as a filter and a recycling center for RBCs underlines its primary influence on their lifespan.