( ( Although ( air ( everywhere ( contains ( 21 ( percent oxygen ) ) ) ) ) ) ( , ( ( the ( atmospheric pressure ) ) ( ( decreases ( as ( ( ( altitude increases ) ( ( ( so less ) and ) ( less oxygen ) ) ) ( becomes ( available ( to ( the ( human body ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN Although) (S (NP (NN air)) (ADVP (RB everywhere)) (VP (VBZ contains) (NP (CD 21) (NN percent) (NN oxygen))))) (, ,) (NP (DT the) (JJ atmospheric) (NN pressure)) (VP (VBZ decreases) (SBAR (IN as) (S (NP (NP (NN altitude) (NNS increases)) (ADJP (ADJP (RB so) (JJR less)) (CC and) (ADJP (RBR less) (JJ oxygen)))) (VP (VBZ becomes) (ADJP (JJ available) (PP (TO to) (NP (DT the) (JJ human) (NN body)))))))) (. .)))
Although air everywhere contains 21 percent oxygen, the atmospheric pressure decreases as altitude increases so less and less oxygen becomes available to the human body.
(ROOT (S (NP (NNP Air)) (VP (VBP pressure) (SBAR (S (NP (PRP it)) (VP (VBZ decreases) (PP (IN as) (NP (NN altitude) (NNS increases))))))) (. .)))