Despite its name, vitamin D is actually an inactive precursor that undergoes a series of transformations to become the fully active hormone that regulates plasma calcium levels. In the human body vitamin D is derived from two sources:  Skin, in which vitamin D3 (cholecalciferol) is rapidly produced by the action of ultraviolet light on the precur-sor 7-dehydrocholesterol. The skin is the major source of vitamin D3, especially in regions where food is not supple-mented with vitamin D. Typically, 30 minutes to 2 hours of sunlight exposure per day can provide enough vitamin D to fulfill daily body requirements for this vitamin.  Diet, from which vitamin D3 is absorbed by the small intestine in association with chylomicrons. In the blood vitamin D3 is bound to vitamin D–binding protein and transported to the liver. The first transforma-tion occurs in the liver and involves hydroxylation of vitamin D3 to form 25-OH vitamin D3. This compound is released into the bloodstream and undergoes a second hydroxyla-tion in the proximal tubules of the kidney to produce the highly active 1,25-(OH)2 vitamin D3 (calcitriol). The pro-cess is regulated indirectly by an increase in plasma Ca2concentration, which triggers secretion of PTH, or directly by a decrease in circulating phosphates, which in turn stim-ulates activity of 1 -hydroxylase responsible for conver-sion of 25-OH vitamin D3 into active 1,25-(OH)2 vitamin D3. Active 1,25-(OH)2 vitamin D3 stimulates intestinal absorption of Ca2and phosphate and mobilization of Ca2from bones. It is therefore necessary for normal development and growth of bones and teeth. The related compound vitamin D2 (ergocalciferol) undergoes the same conversion steps as vitamin D3 and produces the same biologic effects. Patients with end-stage chronic kidney diseases have inadequate conversion of vitamin D into active metabolites resulting in vitamin D3 deficiency. In adults,  vitamin D3 defi-ciency is manifested by impaired bone mineralization and reduced bone density. Therefore, patients with chronic kidney diseases, especially those on prolonged renal hemodialysis are often supplemented with vitamin D3 and calcium to avoid severe disturbance of calcium homeostasis because of sec-ondary hyperparathyroidism, a condition prevalent in these patients. Vitamin D3 deficiency in childhood results in rickets, a disease that causes abnormal bone ossification.
and specific secretion by the cells of the kidney. The final urine is conveyed by the ureters to the urinary bladder, where it is stored until discharged via the urethra. The final urine contains water and electrolytes as well as waste products, such as urea, uric acid, and creatinine, and breakdown products of various substances.
The kidney also functions as an endocrine organ.
Endocrine activities of the kidneys include  Synthesis and secretion of the glycoprotein hormone erythropoietin (EPO), which acts on the bone marrow and regulates red blood cell formation in response to decreased blood oxygen concentration. EPO is synthesized by endothelial cells of the peritubular capillaries in the renal cortex and acts on specific receptors expressed on the surface of erythrocyte progenitor (Er-P) cells in the bone marrow.
The recombinant form of erythropoietin (RhEPO) is used for the treatment of anemia in patients with end-stage renal disease. It is also used to treat anemia resulting from bone marrow suppression that develops in AIDS patients undergoing treatment with antiretroviral drugs, such as azidothymidine (AZT).