( Therefore ( , ( we ( ( must ( think ( about ( ( the ventilation ) ( ( ( ( to ( ( perfusion ratio ) ( for ( each alveolus ) ) ) ) , ) ( not just ) ) ( for ( ( the lungs ) ( as ( a whole ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Therefore)) (, ,) (NP (PRP we)) (VP (MD must) (VP (VB think) (PP (IN about) (NP (NP (DT the) (NN ventilation)) (PP (PP (TO to) (NP (NP (NN perfusion) (NN ratio)) (PP (IN for) (NP (DT each) (NNS alveolus))))) (, ,) (CONJP (RB not) (RB just)) (PP (IN for) (NP (NP (DT the) (NNS lungs)) (PP (IN as) (NP (DT a) (NN whole)))))))))) (. .)))
Therefore, we must think about the ventilation to perfusion ratio for each alveolus, not just for the lungs as a whole.
(ROOT (S (NP (NP (DT The) (NNS lungs)) (PP (IN of) (NP (NNS mammals)))) (VP (VBP are) (ADJP (JJ unique) (PP (IN in) (S (VP (VBG having) (NP (NNS alveoli))))))) (. .)))