["Acta Physiologica, Volume 231, Issue 3, March 2021. ", "\nAbstract\n\nAim\nBlood transfusion may improve renal oxygenation during cardiopulmonary bypass (CPB). In an ovine model of experimental CPB, we tested whether increasing blood haemoglobin concentration [Hb] from ~7 g dL−1 to ~9 g dL−1 improves renal tissue oxygenation.\n\n\nMethods\nTen sheep were studied while conscious, under stable isoflurane anaesthesia, and during 3 hours of CPB. In a randomized cross‐over design, 5 sheep commenced bypass at a high target [Hb], achieved by adding 600 mL donor blood to the priming solution. After 90 minutes of CPB, PlasmaLyte® was added to the blood reservoir to achieve low target [Hb]. For the other 5 sheep, no blood was added to the prime, but after 90 minutes of CPB, 800‐900 mL of donor blood was given to achieve a high target [Hb].\n\n\nResults\nOverall, CPB was associated with marked reductions in renal oxygen delivery (−50 ± 12%, mean ± 95% confidence interval) and medullary tissue oxygen tension (PO2, −54 ± 29%). Renal fractional oxygen extraction was 17 ± 10% less during CPB at high [Hb] than low [Hb] (P = .04). Nevertheless, no increase in tissue PO2 in either the renal medulla (0 ± 6 mmHg change, P > .99) or cortex (−19 ± 13 mmHg change, P = .08) was detected with high [Hb].\n\n\nConclusions\nIn experimental CPB blood transfusion to increase Hb concentration from ~7 g dL−1 to ~9 g dL−1 did not improve renal cortical or medullary tissue PO2 even though it decreased whole kidney oxygen extraction.\n\n"]