Sentence:  The shadows of the <WSD>hila</WSD> on chest X-ray are mainly formed by the pulmonary arteries (PAs) and some of their main branches and the upper pulmonary veins (PVs). 
Meaning of the word marked with <WSD> tags:  A depression or fissure through which ducts, nerves, or blood vessels enter and leave a gland or organ; a porta.