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 <WSD>relative</WSD> drop in pulmonary artery pressure . 
Meaning of the word marked with <WSD> tags:  estimated by comparison; not absolute or complete