Surplus red blood cells are stored in which organ?

The spleen serves as a reservoir for surplus red blood cells. This organ plays a crucial role in filtering blood and managing the body's blood supply. When the body experiences a need for more red blood cells, such as during times of increased demand for oxygen in tissues, the spleen can release these stored cells back into the bloodstream. Additionally, the spleen is involved in recycling iron from old or damaged red blood cells, further contributing to maintaining healthy blood levels.

While the heart is essential for pumping blood throughout the body, it does not store red blood cells. The liver is primarily involved in processing nutrients, producing bile, and metabolizing substances but does not serve as a storage site for red blood cells. The kidneys are focused on filtering blood and producing urine, and while they do play an important role in maintaining overall blood composition, they do not store red blood cells like the spleen does. Thus, the spleen is uniquely positioned as the organ responsible for storing surplus red blood cells, making it the correct answer.