Re: Why urea is toxic?

Willey-As far as keeping urine in the body being harmful, you are correct! 
 The urinary bladder does not allow any absorption into the circulatory 
system so the urine's toxicity is not a factor.  The bladder is merely a 
storage place until the urine is voided.  Urea and uric acid are both 
toxic, with urea being more so due to its solubility.  Urea is a waste 
product of protein metabolism and uric acid is from purine metabolism.  In 
most animals, uric acid is 100% excreted but in humans, only about 10% is. 
 We do not have uricase, an enzyme that helps it become allantoin in 
animals.  Some uric acid is filtered from the blood by the kidneys and some 
is secreted by the kidneys.  The filtered part is almost all reabsorbed 
into the plasma and the secreted portion is excreted.  In some people, the 
uric acid in their plasma precipitates and forms urate crystal and this can 
lead to gouty arthritis and/or kidney stones.  Urea is eliminated by the 
kidneys largely due to transamination activity in the liver.  The amine 
group from glutamic acid is removed by oxidative deamination as ammonia and 
combined with CO2 to form urea.  About 28 gms/day are eliminated.  It is 
the ammonia that is VERY toxic.  Alot of the ammonia is detoxified by the 
brain as it produces it.  The CNS is very sensative to ammonia levels and 
too much can alter the levels of pyruvate, lactate, glycogen and glucose. 
These upsets obviously will effect the Kreb's cycle and our ability to 
produce ATP. It also can upset certain receptors in the CNS which can lead 
to neurotransmission defects.  These can lead to irritability, vomiting, 
sleepiness, edema of the brain, coma and death.  Hyperammonemia, as it is 
called, is rather rare but can occur and it once again points out the 
importance of our kidneys!  In addition, osmolarity and pH balance would be 
upset so that our circulatory functions such as acidity/alkalinity and 
interstitial fluid balance would be upset. I hope this answer helps.