Urea
A nitrogen-containing substance normally cleared from the blood by the kidney into the urine. Diseases that compromise the function of the kidney often lead to increased blood levels of urea, as measured by the blood urea nitrogen (BUN) test.
Urea is of major historical significance. It was the first organic chemical compound ever synthesized. The German chemist Friedrich Wohler in 1828 attempted to make ammonium cyanate from silver cyanide and ammonium chloride and, in the process, accidentally made urea. Wohler wrote his mentor Jons Berzelius, “I must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea.”
This pioneering experiment disproved the theory of vitalism, the concept that organic chemicals could only be modified chemically, but that living plants or animals were needed to produce them.
Wohler had also discovered that urea and ammonium cyanate had the same chemical formula but very different chemical properties. This was due to isomerism, the phenomenon in which two or more chemical compounds have the same number and type of atoms but, because those atoms are arranged differently, each compound has different chemical properties.
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- Ureaplasma
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- Uremia
The presence of excessive amounts of urea in the blood, which may be a sign of kidney disease or failure. It is also referred to as azotemia. See also urea.
- Ureter
One of the two tubes that carry urine from the kidneys to the bladder. Each ureter arises from a kidney, descends, and ends in the bladder.
- Urethra
The tube that leads from the bladder and transports and discharges urine outside the body. In males, the urethra travels through the penis and carries semen as well as urine. In females, the urethra is shorter than in the male, and it emerges above the vaginal opening.