Stones occur in the upper or lower urinary tract, and the main cause of renal colic is ureteral spasm. Blood enters the kidney through the renal artery and is filtered by the glomerulus and tubules to form urine, where nutrients are reabsorbed by the body and where waste products are excreted in the form of urine, such as creatinine and urea nitrogen. Some of the precipitated substances, such as urates, will gradually expand when crystals are precipitated, leading to stone formation. The stones move downward with the formation and excretion of urine, triggering renal colic as they move.