Sentence:  For example , the marked susceptibility of the monkey to respiratory infection might be related to its delicate , long alveolar ducts and short , large bronchioles situated within a parenchyma entirely lacking in protective supportive tissue <WSD>barriers</WSD> such as those found in types 1 , and 3 , . 
Meaning of the word marked with <WSD> tags:  anything serving to maintain separation by obstructing vision or access