Erythropoietin is a hormone produced in the kidney to stimulate the generation of more red blood cell. It is triggered by low oxygen via HIF transcription factors. Makes sense. Oops, oxygen is low, let's make more erythrocytes to carry more oxygen from the lung to consuming cells.
What doesn't make sense is why evolution would locate the production of EPO in the kidneys. These are organs tasked with excretion via the urine - pump and retrieve. They have very little to do with red blood cell gas exchange (lungs), generation (bone marrow), or removal (spleen).
Can somebody think of a good reason why you would locate the synthesis and release of the primary hormone to trigger red blood cell generation in a seemingly unrelated organ like the kidneys?