Urea concentration is measured as urea nitrogen. Measurement of urea nitrogen in serum can be used to screen for decreased GFR, and the test is often called blood urea nitrogen (BUN).
Urea is an organic chemical compound, and is essentially the waste produced
by the body metabolizing protein. Naturally, the compound is produced when the
liver breaks down protein or amino acids, and ammonia; the kidneys then transfer
the urea from the blood to the urine. Excess nitrogen is excreted from the body
Urea nitrogen concentrations depends on:
1. Hepatic urea production
The urea production rate depends on hepatic function and digestion and
catabolism of protein. In liver disease, for example, urea formation is
decreased. While, urea formation increases with protein catabolism or increased
protein digestion in the intestine.
2. Renal tubular flow rate
Urea can be filtered through the glomerulus and passively diffuses out of the
tubules at a rate depending on renal tubular flow rate; the remainder of the
filtered urea is expelled in urine. About 40% of filtered urea can be reabsorbed
at high flow rate; at low flow rate, around 60% of filtered urea is reabsorbed
and added back to the urea concentration of blood. Therefore, high UN levels can
be seen with reduced GFR.
Increased urea nitrogen may appear due to factors including:
▪ Increased catabolism of protein: Fever, burns, corticosteroid
administration, starvation, physical activity.
▪ Increase of protein digestion: high-protein diet, and hemorrhage into the
▪ Reduced GFR: pre-renal, renal or post-renal causes.
Decreased urea nitrogen may occur due to reasons like:
▪ Decrease of protein intake or protein anabolism: limit of protein
▪ Increased excretion: any cause of polyuria, such as Diabetes,
▪ Decreased production: liver disease.
The above-mentioned is about introduction of urea nitrogen. If there is still
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