Sentence:  As early as 1913 Ghoreyeb and Karsner demonstrated with perfusion studies in dogs that bronchial artery flow would remain constant at a certain low level when pressure was maintained in the pulmonary artery and vein , but that increases in bronchial artery flow would occur in response to a relative drop in pulmonary artery <WSD>pressure</WSD> . 
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)