( ( ( ( ( Oxygen molecules ) ( ( pass ( from ( ( the ( air space ) ) ( of ( an alveoli ) ) ) ) ) ( into ( ( its ( capillary bed ) ) ( via ( simple diffusion ) ) ) ) ) ) -- ) ( ( ( oxygen concentration ) ( in ( the air ) ) ) ( ( is greater ) ( than ( it ( is ( in ( the bloodstream ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NN Oxygen) (NNS molecules)) (VP (VBP pass) (PP (IN from) (NP (NP (DT the) (NN air) (NN space)) (PP (IN of) (NP (DT an) (NNS alveoli))))) (PP (IN into) (NP (NP (PRP$ its) (JJ capillary) (NN bed)) (PP (IN via) (NP (JJ simple) (NN diffusion))))))) (: --) (S (NP (NP (NN oxygen) (NN concentration)) (PP (IN in) (NP (DT the) (NN air)))) (VP (VBZ is) (ADJP (JJR greater)) (SBAR (IN than) (S (NP (PRP it)) (VP (VBZ is) (PP (IN in) (NP (DT the) (NN bloodstream)))))))) (. .)))
Oxygen molecules pass from the air space of an alveoli into its capillary bed via simple diffusion -- oxygen concentration in the air is greater than it is in the bloodstream.
(ROOT (S (NP (NNP Oxygen)) (VP (VBZ is) (VP (VBN transferred) (PP (IN into) (NP (DT the) (NN bloodstream))) (PP (IN by) (NP (JJ simple) (NN diffusion))))) (. .)))