Sentence:  In criticism of the latter 's views , his conclusions were based upon dog <WSD>lung</WSD> injection studies in which all of the vascular channels were first filled with a solution under pressure and then were injected with various sized colored particles designed to stop at the arteriolar level . 
Meaning of the word marked with <WSD> tags:  either of two saclike respiratory organs in the chest of vertebrates; serves to remove carbon dioxide and provide oxygen to the blood