Urea

Urea is an organic compound with the chemical formula (NH₂)₂CO. Urea is also known by the International Nonproprietary Name (rINN) carbamide, as established by the World Health Organization. For example, the medicinal compound hydroxyurea (old British Approved Name) is now hydroxycarbamide. Other names include carbamide resin, isourea, carbonyl diamide, and carbonyldiamine. Urea was first discovered in urine in 1773 by the French chemist Hilaire Rouelle. It was the first organic compound to be artificially synthesized from inorganic starting materials, in 1828 by Friedrich Wöhler, who prepared it from silver isocyanate through a reaction with ammonium chloride.

Urea is highly soluble in water and has a pK_a close to zero. Therefore it is an efficient way for the human body to expel excess nitrogen. Its high solubility is due to extensive hydrogen bonding with water: up to eight hydrogen bonds may form - two from the oxygen atom, one from each hydrogen atom and one from each nitrogen atom. Control of urea by antidiuretic hormone allows the body to create hyperosmotic urine (urine that has more ions in it--is "more concentrated"--than that same person's blood plasma). Preventing the loss of water in this manner is important if the person's body needs to save water in order to maintain a suitable blood pressure or (more likely,) in order to maintain a suitable concentration of sodium ions in the blood plasma.

The urea molecule is planar and retains its full molecular point symmetry, due to conjugation of one of each nitrogen's P orbital to the carbonyl double bond. Each carbonyl oxygen atom accepts four N-H-O hydrogen bonds, a very unusual feature for such a bond type. This dense (and energetically favourable) hydrogen bond network is probably established at the cost of efficient molecular packing: The structure is quite open, the ribbons forming tunnels with square cross-section from ingested food are either used to synthesize proteins and other biomolecules or oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a transaminase, the amino group is then fed into the urea cycle. Urea is neither acidic nor basic, so it is a perfect vehicle for getting rid of nitrogen waste. Urea production occurs in the liver and is regulated by N-acetylglutamate.

Urea is, in essence, a waste product. It is found in and extracted from urine. However, it also plays a very important role in that it helps set up the countercurrent system in the nephrons. The countercurrent system in the nephrons allows for reabsorption of water and critical ions. Urea is reabsorbed in the inner medullary collecting ducts of the nephrons, thus raising the osmolarity in the medullary interstitium surrounding the thin ascending limb of the Loop of Henle. The greater the osmolarity of the medullary interstitium surrounding the thin ascending Loop of Henle, the more water will be reabsorbed out of the renal tubule back into the interstitium (and thus back into the body). Some of the urea from the medullary interstitium that helped set up the Countercurrent System will also flow back into the tubule, through urea transporter 2, into the thin ascending limb of the loop of Henle, through the collecting ducts, and eventually out of the body as a component of urine.