Measuring oxygen handling by RBCs. (A) Microfluidic chamber showing 16 channels supplied from 2 different solutions, and feeding into a central chamber where cells are superfused. (B). Rate of solution exchange was gauged by including 15 μM fluorescein in 1 of the solutions. The exchange time constant is better than 33 ms. (C) Imaging of RBCs loaded dually with CellTracker DeepRed and Calcein. (D) Fluorescence ratio (DeepRed/Calcein) during a rapid solution exchange, from normoxia to anoxia and back. The datapoints (black) are fitted to a monoexponential curve (red). (E) Histogram of time constant (τ) and (F) oxygen carrying capacity (κ) obtained for RBCs drawn from a nonanemic donor. A total of 1263 cells imaged from 18 fields of view and 2 loadings.