Urea

Urea is the final product of the metabolism of amino acids (the building blocks of proteins) in mammals, amphibians, and turtles. In the liver, ammonia reacts with carbon dioxide and through a series of seven steps that are controlled by enzymes (protein catalysts that speed up specific reactions), urea is produced. Each molecule of urea is "built" from two ammonia molecules (NH₃) and one carbon dioxide (CO₂) molecule. Ammonia itself is toxic so many animals have developed metabolic steps that take the ammonia formed and convert it into less toxic and easily dealt with molecules. The extremely high solubility of urea in water (35 oz [1,000 g] of urea dissolve in a gal [l] of water) make it ideal for eliminating nitrogen-based waste products. The concentration of urea rises in many kidney diseases, and blood urea concentration is often used to monitor kidney function. The urea produced by the body is excreted in urine. A healthy adult will excrete about 0.9 oz (25 g) of urea per day. Upon standing the urea in urine will decompose to carbon dioxide and ammonia, accounting for the "ammonia" smell of old urine. In many animals and in some vegetarians, cloudy urine is common. This is the result of a precipitate (insoluble compound) formed from urea and calcium or magnesium ions. Medically urea is used as a diuretic, or substance that promotes water loss through urination. Urea-containing creams are used on wounds.

Figure 1. Structural formula of urea. Illustration by Hans & Cassidy. Courtesy of Gale Group.