rēnēs

Latin renes was a plurale tantum -- a word that existed only in the plural, like English scissors or trousers. You had renes (kidneys) but almost never ren (a kidney). The Romans understood that these organs came in matched pairs, deep in the body, difficult to examine, and essential to life in ways they could not fully explain. Celsus, in his De Medicina around 30 CE, described kidney stones and their agonizing passage but had limited understanding of the organ's filtering function.

The adjective renalis appeared in medieval medical Latin and entered European medical vocabulary through the school of Salerno in southern Italy, the first great medical university of medieval Europe. Founded in the ninth century, Salerno synthesized Greek, Latin, and Arabic medical knowledge. Arabic physicians, particularly Avicenna in his Canon of Medicine around 1025, had described kidney function more accurately than any Roman source.

English borrowed renal in the 1650s. Richard Bright, a physician at Guy's Hospital in London, described the kidney disease that bears his name in 1827 -- Bright's disease, now called nephritis. His careful correlation of clinical symptoms with autopsy findings helped establish the kidneys as organs worthy of specialized study. Nephrology (from Greek nephros, the Greek cousin of Latin renes) became a medical specialty in the twentieth century.

Modern renal medicine is defined by the dialysis machine, first used successfully by Willem Kolff in the Netherlands in 1943, during the German occupation. Kolff built his artificial kidney from sausage casings, orange juice cans, and a washing machine motor. The Latin word for an organ the Romans barely understood now names a medical field that can replace that organ's function entirely with a machine.