What is NOT a direct function of the kidney?

The other three answer choices are direct functions of the kidney, while regulating O2 levels is not. Yes, the kidney indirectly regulates blood O2 by the release of erythropoietin. Its direct function would be to stimulate red blood cell synthesis from the bone marrow thereby (very slowly over a period to time) working to increase blood O2 levels. So, the kidney has a direct function on the bone marrow, not the O2 levels directly.