Urea is a nitrogen-containing waste product that the body produces to manage metabolic byproducts from dietary protein. To maintain internal balance, the body must regularly remove this compound. The journey of urea, from its molecular origins to its departure from the body, involves several organs and biological pathways.
Nitrogen from Protein Metabolism
The creation of urea begins with dietary proteins. When you eat protein-rich foods like meat or beans, your digestive system breaks them down into amino acids. These are absorbed into the bloodstream and transported to cells for growth, repair, and building new proteins.
When the body has excess amino acids or needs energy when carbohydrates are scarce, they can be repurposed. To use amino acids for energy or convert them to fat, the body must first remove the nitrogen-containing amino group. This process, called deamination, separates the nitrogen from the molecule’s carbon skeleton.
The removal of the amino group results in the formation of ammonia (NH3). Ammonia is highly toxic, and its accumulation can disrupt metabolic processes by raising cellular pH to dangerous levels. The body must quickly convert this harmful substance into a safer compound for elimination.
The Urea Cycle in the Liver
The liver is the primary site for converting ammonia into urea through a series of reactions called the urea cycle. The cycle takes ammonia from amino acid breakdown, combines it with carbon dioxide, and processes it to form urea. This pathway occurs within liver cells, spanning both the mitochondria and the cytoplasm.
The cycle begins inside the mitochondria, where ammonia and carbon dioxide create carbamoyl phosphate. This molecule combines with ornithine to form citrulline. The citrulline is then transported out of the mitochondria into the cell’s cytoplasm for the next steps.
In the cytoplasm, citrulline reacts with other molecules like aspartate, which donates the second nitrogen atom. Specific enzymes guide the cycle until arginine is formed. In the final step, the enzyme arginase splits arginine into urea and ornithine. The urea is now ready for transport, and the regenerated ornithine returns to the mitochondria to restart the cycle.
Excretion of Urea
After its synthesis in the liver, urea is released into the bloodstream. Its primary destination is the kidneys, which serve as the body’s main filtration system. The kidneys filter waste products like urea from the blood while retaining substances like nutrients and water.
Each kidney contains over a million microscopic filtering units called nephrons. As blood enters the kidneys, it flows into the glomerulus, a network of tiny capillaries within each nephron. High pressure forces water, urea, salts, and other small solutes from the blood into a collecting structure called the Bowman’s capsule.
This filtered fluid, now called filtrate, travels through a series of renal tubules. During this passage, a process of selective reabsorption allows the body to reclaim most of the water, glucose, and ions back into the blood. A large portion of the urea is left behind in the remaining fluid.
This fluid becomes concentrated as urine, which contains the filtered urea and other wastes. The urine flows from the nephrons into collecting ducts and then down the ureters. It is finally stored in the bladder before being expelled from the body.