Polarized light micrograph of of urea crystals, also known as carbamide, a nitrogenous compound. As a physiological regulator of nitrogen excretion in mammals it constitutes about half of the total urinary solids, and is also found in sweat, blood, bile, and milk. In vivo, urea is formed in the liver via the urea cycle from ammonia, and is the end product of protein catabolism, dissolved in blood and excreted by the kidney in the urine. Urea is mainly used as nitrogen fertilizer. Nitrogen promotes leaf growth and forms proteins and chlorophyll. It is an important raw material for the chemical industry (resins, plastics, dyings, paints, adhesives, aviation fuels, lubricating oils), for explosives (urea nitrate), for medical uses (dermatology), for pharmaceutical uses (hypnotics, sedatives, diuretics), and for the automobile industry (Bluetec system) among others. Field of view size: 2.97 x 4.45 mm (0.117 x 0.175 in)