<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> The kidneys are two bean - shaped organs found on the left and right sides of the body in vertebrates . They are located at the back of the abdominal cavity in the retroperitoneal space . In adults they are about 11 centimetres (4.3 in) in length . They receive blood from the paired renal arteries; blood exits into the paired renal veins . Each kidney is attached to a ureter, a tube that carries excreted urine to the bladder...</P> <P> The nephron is the structural and functional unit of the kidney . Each adult kidney contains around one million nephrons . The nephron utilizes four processes to alter the blood plasma which flows to it: filtration, reabsorption, secretion, and excretion . Via one or more of these mechanisms, the kidney participates in the control of the volume of various body fluid compartments, fluid osmolality, acid - base balance, various electrolyte concentrations, and removal of toxins . Filtration occurs in the glomerulus: one - fifth of the blood volume that enters the kidneys is filtered . Examples of substances reabsorbed are solute - free water, sodium, bicarbonate, glucose, and amino acids . Examples of substances secreted are hydrogen, ammonium, potassium and uric acid . Examples of substances that are excreted are urea, ammonium, and uric acid . The kidneys also carry out functions independent of the nephron . For example, they convert a precursor of vitamin D to its active form - calcitriol - and synthesize the hormones erythropoietin and renin . </P> <P> Renal physiology is the study of kidney function . 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 . Urology addresses diseases of kidney (and urinary tract) anatomy: these include cancer, renal cysts, kidney stones and ureteral stones, and urinary tract obstruction . </P>

Where does all filtration take place in the kidneys