Also known as Blood Urea Nitrogen — BUN
Urea is a form of nitrogenous waste formed from ammonia via the urea cycle in the liver. It freely crosses lipid membranes and is filtered into the glomerular filtrate in the renal glomerulus. A proportion of this filtered urea is reabsorbed passively before final excretion and it forms a vital of the concentration gradient needed to prevent passive diuresis.
An increase in serum concentrations of urea and creatinine is known as azotaemia and it is found almost exclusively in animals with kidney disease.
Blood urea coentration may be elevated independently of creatinine for the following reasons:
- Gastro-intestinal haemorrhage, as with gastro-duodenal ulceration.
- Diet with high protein or purine content, such as liver.
- Increased catabolism of body tissues, particularly muscle, which may occur in pyrexic animals or those with systemic disease.
Reduced urea concentration may occur due to:
- Over-enthusiastic fluid therapy, diluting the urea present in the blood.
- Disease that cause polyuria and/or polydipsia may cause 'medullary washout' of urea. Common examples include pyschogenic polydipsia, diabetes insipidus and Cushing's disease.
- Reduced synthesis with porto-systemic shunts, microvascular dysplasia or liver failure.
- A diet with a low protein content or malnutrition.
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