Urine formation: Urine is formed inside the kidney in the nephron. A nephron is a functional unit of the kidney. The blood containing waste like urea enters the glomerulus which filters the blood. Water, urea, and other salts like glucose are filtered out in the renal tubule. The filtered fluid is called glomerular fluid. When the filtrate containing useful substances as well as the waste substances passes through the tubule, the useful substances like glucose, amino acids, most salts, and water are reabsorbed into the blood through the blood capillaries surrounding the tubule. Certain substances which are harmful and not needed by the body like urea, remain behind in the tubule. This yellowish liquid is called urine.