The tl;dr; is that the spleen is mostly responsible for "macro" filtering of the blood. It removes relatively large foreign bodies, as well as red blood cells t

The tl;dr; is that the spleen is mostly responsible for "macro" filtering of the blood. It removes relatively large foreign bodies, as well as red blood cells that are starting to wear out (they last 3-4 months, on average before getting filtered out). Because of its role, there are many blood vessels in the spleen, making it very prone to bleeding if it is damaged. It (along with the liver) is one of the more common sites of serious internal bleeding after physical trauma.