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24.4B: Chemical Composition of Urine

Normal urine consists of water, urea, salts, and pigments.

 

LEARNING OBJECTIVES

 

Describe how normal urine consists of water, urea, salts and pigment

 

KEY TAKEAWAYS

Key Points

 

  • Urine is a liquid by-product of the body secreted by the kidneys through a process called urination and excreted through the urethra.
  • Urine is an aqueous solution of greater than 95% water. Other constituents include urea, chloride, sodium, potassium, creatinine and other dissolved ions, and inorganic and organic compounds.
  • Urea is a non-toxic molecule made of toxic ammonia and carbon dioxide. Any abnormal constituents found in urine are an indication of disease.
  • The presence of red blood cells in urine is referred to as haematuria.
  • The presence of proteins, which are normally too large to pass through the tubules, can be an indication of damage to the tubules, and is called proteinuria.

 

Key Terms

 

  • urine: A liquid excrement that consists of water, salts, and urea, and is made in the kidneys then released through the urethra.
  • diabetes: A group of endocrine diseases whereby a person has high blood sugar due to an inability to produce, metabolize, or respond to the hormone insulin.

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 a minimum of 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.