urea (carbamide)

^[1]^[2]^[3]

Function

waste product of nitrogen catabolism, i.e. catabolism of proteins, nucleic acids etc
formed in the liver from NH3 via the urea cycle; NH3 is a degradation product of nitrogen-containing compounds
urea formed in the liver is excreted into the kidney
in the kidney, urea plays a role in the countercurrent exchange system of nephrons, allowing reabsorption of water & ions from urine
urea is reabsorbed in the inner medullary collecting ducts of the nephrons, thus raising osmolarity in the renal medullary interstitium surrounding the thin ascending limb of the loop of Henle, which in turn causes water to be reabsorbed
the urea transporter 2 allows some reabsorbed urea to flow back into the thin ascending limb of the tubule, through the collecting ducts, & into the excreted urine
the countercurrent exchange system is controlled by vasopressin which allows the body to create hyperosmotic urine, with a higher concentration of dissolved substances than in blood plasma
this serves to prevent the loss of water, maintain blood pressure, & to maintain a normal Na+ in plasma

urea is considered a medical food by the FDA^[3]
urea 15 g BID mixed with fruit juice is used for treatment of euvolemic hyponatremia (SIADH) with low plasma osmolality & high urine osmolality not responding to fluid restriction^[3]

↑ Wikipedia: Urea http://en.wikipedia.org/wiki/Urea
↑ Department of Veterans Affairs, VA National Formulary
↑ ^3.0 ^3.1 ^3.2 Rondon-Berrios H, Tandukar S, Mor MK et al Urea for the Treatment of Hyponatremia. Clin J Am Soc Nephrol. 2018 Nov 7;13(11):1627-1632. PMID: https://www.ncbi.nlm.nih.gov/pubmed/30181129 PMCID: PMC6237061 Free PMC article.