In humans, the kidneys are located high in the abdominal cavity, one on each side of the spine, and lie in a retroperitoneal position at a slightly oblique angle. Images showing the human trunk, with positions of the organs show, and kidneys seen at the vertebral level of T12 to 元. Whereas according to some opinions, "renal" should be replaced with "kidney" in scientific writings such as "kidney artery", other experts have advocated preserving the use of renal as appropriate including in "renal artery". The word “ renal” is an adjective meaning “relating to the kidneys”, and its roots are French or late Latin. Urology addresses diseases of kidney (and urinary tract) anatomy: these include cancer, renal cysts, kidney stones and ureteral stones, and urinary tract obstruction. Nephrology is the medical specialty which addresses diseases of kidney function: these include chronic kidney disease, nephritic and nephrotic syndromes, acute kidney injury, and pyelonephritis. Renal physiology is the study of kidney function. Nephrectomy is frequently used to cure renal cell carcinoma. Dialysis and kidney transplantation are used to treat kidney failure one (or both sequentially) of these are almost always used when renal function drops below 15%. Procedures used in the management of kidney disease include chemical and microscopic examination of the urine ( urinalysis), measurement of kidney function by calculating the estimated glomerular filtration rate (eGFR) using the serum creatinine and kidney biopsy and CT scan to evaluate for abnormal anatomy. Each adult human kidney contains around 1 million nephrons, while a mouse kidney contains only about 12,500 nephrons. The nephron is the structural and functional unit of the kidney. For example, they convert a precursor of vitamin D to its active form, calcitriol and synthesize the hormones erythropoietin and renin. The kidneys also carry out functions independent of the nephron. Examples of substances secreted are hydrogen, ammonium, potassium and uric acid. Examples of substances reabsorbed are solute-free water, sodium, bicarbonate, glucose, and amino acids. Filtration occurs in the glomerulus: one-fifth of the blood volume that enters the kidneys is filtered. The kidney participates in the control of the volume of various body fluids, fluid osmolality, acid–base balance, various electrolyte concentrations, and removal of toxins. Each kidney is attached to a ureter, a tube that carries excreted urine to the bladder. They receive blood from the paired renal arteries blood exits into the paired renal veins. They are located on the left and right in the retroperitoneal space, and in adult humans are about 12 centimetres ( 4 + 1⁄ 2 inches) in length. The kidneys are two reddish-brown bean-shaped organs found in vertebrates.