Urine is a liquid byproduct of the body secreted by the kidneys through a process called urination and excreted through the urethra. The normal chemical composition of urine is mainly water content, but it also includes nitrogenous molecules, such as urea, as well as creatinine and other metabolic waste components. 

Other substances may be excreted in urine due to injury or infection of the glomeruli of the kidneys, which can alter the ability of the nephron to reabsorb or filter the different components of blood plasma.

Normal Chemical Composition of Urine

Urine is an aqueous solution of greater than 95% water, with these remaining constituents, in order of decreasing concentration:

• Urea 9.3 g/L.
• Chloride 1.87 g/L.
• Sodium 1.17 g/L.
• Potassium 0.750 g/L.
• Creatinine 0.670 g/L .
• Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites). 

Urine is sterile until it reaches the urethra, where epithelial cells lining the urethra are colonized by facultatively anaerobic gram-negative rods and cocci. Urea is essentially a processed form of ammonia that is non-toxic to mammals, unlike ammonia, which can be highly toxic. It is processed from ammonia and carbon dioxide in the liver.

Abnormal Types of Urine

There are several conditions that can cause abnormal components to be excreted in urine or present as abnormal characteristics of urine. They are mostly referred to by the suffix -uria. Some of the more common types of abnormal urine include:

• Proteinuria—Protein content in urine, often due to leaky or damaged glomeruli.
• Oliguria—An abnormally small amount of urine, often due to shock or kidney damage.
• Polyuria—An abnormally large amount of urine, often caused by diabetes.
• Dysuria—Painful or uncomfortable urination, often from urinary tract infections.
• Hematuria—Red blood cells in urine, from infection or injury.
• Glycosuria—Glucose in urine, due to excess plasma glucose in diabetes, beyond the amount able to be reabsorbed in the proximal convoluted tubule.