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 solution under <WSD>pressure</WSD> and then were injected with various sized colored particles designed to stop at the arteriolar level . 
Meaning of the word marked with <WSD> tags:  the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit)