Like bicarbonate and lactate, its hard to say exactly how much urea one has at any given moment.
Urea is neither acidic nor alkaline. It is merely two amine groups strapped to a carbonyl group.
It does not dissociate in solution.
When the body loses water, urea reabsorption is increased, and the urea concentration increases.
The reason for this seems to be an increase in the renal medulalry urea concentration, which is then reflected in the total serum urea. This effect is mediated by vasopressin-sensitive transport proteins on the endothelium of the collecting duct- by reabsorbing more urea, you set up a larger concentration gradient in the medulla, and therefore you can reabsorb more water (and make more concentrated urine).