Final answer:
Old RBCs are targeted for destruction in the spleen due to their reduced flexibility and increased damage susceptibility after their 120-day lifespan. The spleen's specialized tissues, like the red pulp with macrophages, efficiently filter and recycle these cells.
Step-by-step explanation:
Only old red blood cells (RBCs) are destroyed in the spleen because they have a limited lifespan and the spleen is well-equipped to filter and remove these senescent or damaged cells. Over their approximately 120-day lifespan, RBCs become less flexible and more prone to damage. The spleen's structure, which includes red pulp containing macrophages, captures these aged cells for degradation. This process is part of the body's routine maintenance to keep the circulatory system functioning optimally. Macrophages in the spleen, liver, and bone marrow phagocytize the old RBCs, breaking them down and recycling their components.