Blood Urea Nitrogen (BUN)

Blood Urea Nitrogen (BUN)



Basic Information image



Definition




• Urea: A highly water-soluble neutral (not acidic or basic) nitrogenous molecule necessary for disposal of nitrogenous waste in mammals


• Blood urea nitrogen (BUN): Serum urea nitrogen (SUN) and BUN are the same concentration because urea nitrogen passively diffuses throughout total body water.


• Azotemia: An increase in blood nitrogen-containing compounds, especially urea nitrogen and creatinine



Typical Normal Range (US units; SI units)


7 to 17 mmol/L, 10 to 24 mg/dL



Physiology


Most urea is synthesized by the liver from dietary ammonia created through gastrointestinal protein catabolism. Urea is highly water-soluble and passively diffuses throughout the total body water within 90 minutes. Most urea is excreted through the kidney via simple glomerular filtration. It is passively reabsorbed in the collecting ducts inversely to the rate of urine flow. A decreased glomerular filtration rate and decreased urine flow will allow for less filtration and more reabsorption, respectively, and increase BUN. Antidiuretic hormone (ADH) allows diffusion of urea from collecting tubules into the renal interstitium where it contributes to the medullary concentration gradient. Urea is also excreted in saliva, sweat, and the gastrointestinal tract; the latter is reabsorbed directly or as ammonia.



Causes of Abnormally High Levels




• Prerenal causes due to increased protein catabolism (such as with small bowel hemorrhage, starvation, high-protein diet, prolonged exercise, infection, and fever) or decreased renal perfusion (shock, endotoxemia, hypovolemia, dehydration, and cardiovascular disease).


• Foals may have spurious increases near birth.


• Renal azotemia can be acute or chronic (vasomotor nephropathy, toxins, interstitial nephritis, duodenitis/proximal jejunitis, hemolytic anemia/hemoglobinuria, or rhabdomyolysis/myoglobinuria).


• Postrenal azotemia is due to urinary obstruction (urethral calculi, nephroliths) or uroperitoneum.

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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on Blood Urea Nitrogen (BUN)

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