How is urea produced in humans


Synonym: carbamide, carbonyl diamide, carbonic acid diamide
English: urea

1 definition

urea is the end product of the urea cycle, which is used in the human body to detoxify the metabolic toxin ammonia, which arises from the breakdown of amino acids. By multiplying by the factor 0.46, the urea nitrogen can be calculated from the urea.

2 physiology

Most organisms have a tendency to reuse the ammonia that comes from the breakdown of amino acids. This is made possible by the glutamate dehydrogenase reaction.

But ammonia is a highly toxic substance. Even a slight increase in the ammonia concentration in the serum leads to impairment of the efficiency of the central nervous system, in extreme cases a comatose state occurs (ammonia intoxication). It is therefore necessary to quickly remove the ammonia formed in the metabolic nitrogen-containing compounds. Most vertebrates and invertebrates excrete the unneeded portion of the ammonia formed either as urea, ammonia or uric acid.

  • Ureotelian organisms (terrestrial vertebrates, humans) form urea from the amino nitrogen.
  • Ammonotelian organisms (many aquatic animals and bony fish) release the nitrogen as ammonia.
  • Uricotelian organisms (birds and land-dwelling reptiles) excrete the amino nitrogen as uric acid.

In humans, as Ureotel, the predominant part (80 - 90%) of ammonia is broken down into a non-toxic substance, urea, which is readily soluble in water, and thus detoxified and excreted. The urea nitrogen concentration in the serum is about 50 to 100 times the toxic ammonia concentration.

As a small, uncharged molecule, urea can freely cross cell membranes and distributes itself in the body like water. Some of the urea is broken down in the intestine (split into carbon dioxide and ammonia).

3 laboratory medicine

The urea concentration in serum and urine is an indicator of protein turnover. The greater the protein turnover, the higher the urea excretion, controlled by so-called adaptive enzymes of the urea cycle. The synthesis of urea mainly takes place in the liver, only in this organ are all the enzymes of the urea cycle present in higher activity.

Urea is filtered glomerularly in the kidneys and partially reabsorbed. The determination of the urea concentration is therefore suitable for clinical diagnostics

Only when the glomerular filtration rate falls below 75% of the norm is the upper serum limit value for urea exceeded. Since the urea concentration in serum also depends on the protein metabolism, urea is not as suitable as creatinine for assessing kidney function. The urea determination is therefore usually used in addition to the determination of creatinine.

In acute tubular necrosis, the serum urea concentration can decrease while creatinine increases. The reason for this is that the cell necrosis hinders the back diffusion.

Urea levels can also be used to assess catabolism and protein intake in critical care medicine.

3.1 material

1 ml of serum is required for the examination.

3.2 Measurement method

The most common method is the enzymatic / photometric determination of the serum urea concentration.

3.3 Reference range

clienteleNormal value [mg / dl]
Women under 50 years 15 to 40
over 50 years 21 to 43
Men under 50 years 19 to 44
over 50 years 18 to 55
children 1 to 3 years 11 to 36
4 to 13 years 15 to 36
14 to 19 years 18 to 45

The reference range of urea depends on the method and should be taken from the respective printout.

3.4 Interpretation

3.4.1 Increased urea concentration

The urea concentration in the serum is increased in:

3.4.2 Decreased urea concentration

The urea concentration in the serum is decreased in:

3.5 Notes

  • 15 to 35 g of urea are excreted in the urine every day.
  • The routine double determination of creatinine and urea in case of renal insufficiency is not justified, since the urea concentration is strongly dependent on protein intake, catabolism and diuresis.
  • Another measuring method for determining the urea concentration is the measurement of the nitrogen content of urea in the serum.

see also:Urea nitrogen

4 literature

  •; accessed on March 11, 2021