Sentence:  Therefore , an explanation of this alveolar bronchial artery supply might be the nutritive requirement of an increased amount of supportive tissue , not primarily diffusing in nature , in the region of the <WSD>alveolus</WSD> . 
Meaning of the word marked with <WSD> tags:  a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways