Introduction The kidneys are also involved in promoting erythropoiesis and producing activated vitamin D erythropoiesis (red blood cell production) the kidneys produce a glycoprotein growth factor called erythropoietin (EPO) in hypoxia, there is less O2 being delivered to the kidneys this in turn causes the production of hypoxia-inducible factor-1α (HIF-1α) which acts on the fibroblasts in the renal cortex to transcribe EPO (increased EPO mRNA) EPO promotes the differentiation of proerythroblasts to eventually form into erythrocytes (red blood cells) note that one can commonly see anemia in patients with chronic renal failure since the functioning renal mass is decreased this is why recombinant human EPO is a treatment for anemia of chronic renal failure vitamin D production the kidneys contain an enzyme called 1α-hydroxylase 1α-hydroxylase catalyzes the hydroxylation (at the C1 position) of 25-hydroxycholecalciferol into 1,25-dihydroxycholecalciferol 1,25-dihydroxycholecalciferol is a physiologically active form of vitamin D in the kidney, 25-hydroxycholecalciferol can also be hydroxylated (at the C24 position) into 24,25-hydroxycholecalciferol, an inactive form of vitamin D 1α-hydroxylase is regulated by a number of factors such as plasma Ca2+ concentration a decrease in plasma Ca2+ increases 1α-hydroxylase activity parathyroid hormone increases 1α-hydroxylase activity plasma phosphate concentration a decrease in plasma phosphate increases 1α-hydroxylase activity Prostaglandins the kidneys locally produce prostaglandins (e.g., PGE2) that result in vasodilation of the afferent arteriole this in turn increases renal blood flow nonsteroidal antiinflammatory drugs (NSAIDs) impair prostaglandin synthesis the renoprotective effects of prostaglandins are loss in low blood volume states (e.g., hemorrhage) Dopamine low levels of dopamine dilates the renal arterioles and thus increases renal blood flow secreted by proximal tubular cells to promote natriuresis