BUN Flashcards
Comment on origin of urea in serum.
Ammonia is a primary metabolite derived from dietary protein (intestinal absorption) and tissue protein turnover (protein catabolism), urea is synthesised from ammonium ions by hepatocytes and primary excreted in urine.
Why is BUN in an abdominocentesis sample a poor choice for diagnosis of a uroabdomen?
Ineffective osmole – equibrilates rapidly (~90 minutes) over concentration gradients over most cell membranes.
How may decreased urea concentrations effect urine concentration?
Urea (and sodium) are the major contributors to the renal medullary concentration gradient, driving the formation of concentrated urine in the collecting ducts. Decreased urea in urine may lead to decrease in the concentration gradient – urine may be more dilute.
List 3 mechanisms of increased urea synthesis
High protein diet (mild, transient), GI haemorrhage, disorders that increase endogenous protein catabolism (fever, inflam, infection, neoplasia, DKA etc)
List 3 mechanisms of decreased urea synthesis
Hepatic insufficiency, urea cycle enzyme deficiency (rare), low protein diet (mild effect)
List 3 alternate routes of urea excretion and comment on their significance wrt serum BUN.
Saliva, sweat, GI tract – minimal to no effect in dogs & cats but GI ammonia excretion is significant in horses & ruminants to BUN not sensitive for declining GFR.
Why does dehydration cause a greater increase in serum BUN than would be expected by decreased GFR alone?
Because in a normal animal ~50% of urea is reabsorbed with water in the proximal tubules. Lower tubular flow rates increase resorption by increasing time for diffusion.
How much urea is normally reabsorbed in the collecting ducts? What hormone modulates this?
~10% in health – reabsorbed with water, modulated by ADH.
How long is urea stable for in a urine sample?
1 day at room temp, several days at 4-6C, 2-3 months if frozen.
