Nitrogenous wastes are produced in the body as a result of protein metabolism. These wastes are toxic and harmful. The main nitrogenous excretory wastes of humans are urea, uric acid, ammonia, creatinine etc. Animals that excrete urea as excreta are called ureotelic animals (land and marine animals including humans). Animals that excrete uric acid as excreta are called uricotelic animals (insects, lizards, snakes, birds). Kidneys dispose of these excretory wastes in the following ways.
1. Urea excretion: excess and unused amino acids of the body are stored in the liver. In the de-amination process, keto acids and NH2 are formed from excess amino acids by the action of deaminase enzymes. NH2 produces toxic ammonia. Ammonia and CO2 react through the ornithine cycle to produce urea. The urea produced stays in the plasma and then reaches the kidneys. Excreted out of the body through the creation of urine.
2. Uric acid excretion: Uric acid is produced as a result of purine alkali metabolism in the liver. Uric acid is relatively less toxic. It reaches the kidneys through the blood. It is then excreted out of the body through urine.
3. Creatinine excretion: There is an amino acid called creatine in the muscles of the body. 2% creatine is used to produce energy in the muscles. Creatinine metabolism results in the formation of creatinine. Creatinine reaches the kidneys through the blood and is excreted in the urine.
4. Other components: Additional drugs, various toxins and hormones are excreted in the urine.