( ( Spleen ( The spleen ) ) ( ( is ( ( an organ ) ( in ( ( the ( upper ( abdominal cavity ) ) ) ( that ( ( plays ( an ( important role ) ) ) ( in ( ( ( ( storing ( red ( blood cells ) ) ) , ) ( as ( well as ) ) ) ( ( ( filtering out ) ( old ( red ( blood cells ) ) ) ) ( from ( the ( circulating ( blood supply ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Spleen) (DT The) (NN spleen)) (VP (VBZ is) (NP (NP (DT an) (NN organ)) (PP (IN in) (NP (NP (DT the) (JJ upper) (JJ abdominal) (NN cavity)) (SBAR (WHNP (WDT that)) (S (VP (VBZ plays) (NP (DT an) (JJ important) (NN role)) (PP (IN in) (S (VP (VP (VBG storing) (NP (JJ red) (NN blood) (NNS cells))) (, ,) (CONJP (RB as) (RB well) (IN as)) (VP (VBG filtering) (PRT (RP out)) (NP (JJ old) (JJ red) (NN blood) (NNS cells)) (PP (IN from) (NP (DT the) (JJ circulating) (NN blood) (NN supply)))))))))))))) (. .)))
Spleen The spleen is an organ in the upper abdominal cavity that plays an important role in storing red blood cells, as well as filtering out old  red blood cells from the circulating blood supply.
(ROOT (S (NP (NP (DT The) (JJ main) (NN function)) (PP (IN of) (NP (DT the) (NN spleen)))) (VP (VBZ is) (S (VP (TO to) (VP (VP (VB filter) (NP (DT the) (NN blood))) (CC and) (VP (VB remove) (NP (JJ unwanted) (JJ red) (NN blood) (NNS cells))))))) (. .)))
The main function of the spleen is to filter the blood and remove unwanted red blood cells.