Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…“bicarbonate concentration threshold” (associated with PCO2 ~ 15 mmHg) and without at least partial recovery of blood pHe (Claiborne and Heisler 1986; Pörtner et al. 1998; McKenzie et al. 2003).
remarkably, some of the most CO2 tolerant fishes studied to date tightly regulate the pHi of vital…
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