The liver converts ammonia into urea by combining carbon dioxide and ammonia. The urea reaches the kidneys through blood and converted into urine. The process of urine formation takes place in the nephrons of the kidney and consists of the following steps: Glomerular filtration: The initial part of the nephron is called the glomerulus which consists of a tuft of capillaries inside the cup-shaped structure. When the blood passes through these capillaries, the urea, water, amino acids, salts are filtered from the blood. Tubular reabsorption: The middle part of the nephron consists of tubules in which the absorption of water and salts from the filtrate takes place. Tubular secretion: The liquid left behind the tubules of the nephron is urine. The nephron carries the urine to the collecting duct and then to the ureter and remain stored in the urinary bladder and released out through urethra.