Sentence:  This viewpoint is further supported by Verloop 's ( ' 48 ) demonstration of thickened bronchial artery and arteriolar muscular coats which are capable of acting as <WSD>valves</WSD> . 
Meaning of the word marked with <WSD> tags:  a structure in a hollow organ (like the heart) with a flap to insure one-way flow of fluid through it