Sentence:  The pulmonary artery , in addition to supplying the distal portion of the respiratory bronchiole , the alveolar duct , and the <WSD>alveoli</WSD> , continues on and directly supplies the thin pleura ( fig. 8 ) . 
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