( However ( , ( ( ( the hookups ) ( to ( the ( heart-lung machine ) ) ) ) ( ( ( are often ) ( done ( from ( ( ( ( blood vessels ) ( in ( the leg ) ) ) ( rather than ) ) ( ( the ( major ( blood vessels ) ) ) ( that ( ( ( enter and ) leave ) ( the heart ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (NP (NP (DT the) (NNS hookups)) (PP (TO to) (NP (DT the) (JJ heart-lung) (NN machine)))) (VP (VBP are) (ADVP (RB often)) (VP (VBN done) (PP (IN from) (NP (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN leg)))) (CONJP (RB rather) (IN than)) (NP (NP (DT the) (JJ major) (NN blood) (NNS vessels)) (SBAR (WHNP (WDT that)) (S (VP (VBP enter) (CC and) (VBP leave) (NP (DT the) (NN heart)))))))))) (. .)))
However, the hookups to the heart-lung machine are often  done from blood vessels in the leg rather than the major blood vessels that enter and leave the heart.
(ROOT (S (NP (NP (NNP Blood)) (PP (IN from) (NP (DT the) (NNS lungs)))) (VP (VBZ enters) (PP (IN into) (NP (DT the) (JJ left) (NN atrium)))) (. .)))