Sentence:  In criticism of the latter 's views , his conclusions were based upon dog lung injection studies in which all of the vascular channels were first filled with a <WSD>solution</WSD> 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:  a homogeneous mixture of two or more substances; frequently (but not necessarily) a liquid solution