Kidneys filter around 1,700 liters of blood daily to generate 1 liter of concentrated fluid called urine, and in doing, so they excrete harmful waste products, maintain acid base balance, reabsorb essential substances, and regulate vascular tone. Hormones like erythropoietin, which stimulate red blood cell production, are also produced by kidneys. They also activate vitamin D, playing a major role in calcium homeostasis. Each kidney weights about 150 grams. They lie in the lumbar region and consist of an outer cortex and inner medulla. Medullary pyramids are related to a minor calyx: two or three minor calyces open into a major calyx; these combine to form the renal pelvis leading into the ureter, which leaves the kidney at the hilum.