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:expression of nutrient transporters and the thickness of the cell layers separating the maternal and fetal capillaries (interhaemal thickness), representing the barrier to diffusion of nutrients and gases (Constância et al. 2002; Sibley et al. 2004).
Category: