The specific gravity and osmolality of urine (regardless of volume) formed by patients with acute renal failure will reflect
impaired concentrating capacity if a sufficient quantity of nephrons have been damaged (Table 1, p. 8S). The damage may be
reversible or irreversible. Irreversible damage may be nonprogressive or progressive.
A state of pathologic oliguria may develop in patients with primary polyuric renal failure if some prerenal abnormality (vomiting,
decreased water consumption, cardiac decompensation, etc.) develops (Table 1, p. 8S). The oliguria is related to reduced renal
perfusion resulting in reduction in the amount of glomerular filtrate that is formed. If this prerenal cause(s) is reversible
and/or if adequate renal perfusion is restored, polyuria will resume.
Oliguria or a nonpolyuric state may develop as a terminal event in patients with chronic progressive generalized renal disease.
Oliguria in the context of reduction of the volume of urine expelled from the urinary bladder during the voiding phase of
micturition is associated with diseases of the lower urinary system (ureters, urinary bladder, urethra) that impair flow of
urine through the excretory pathway.
Examples of such diseases include: 1) neoplasms, strictures or uroliths that partially occlude the urethral lumen, 2) herniation
of the urinary bladder that partially obstructs urine outflow through the urethra or urine inflow through the ureters and
3) rupture of the urinary bladder.
In healthy dogs, very little urine should be retained in the urinary bladder following micturition (approximately 0.1 to 0.2
ml/kg body weight).
What is the definition of anuria?
The term anuria has been used to indicate the absence of urine formation by the kidneys, and absence of elimination of urine
from the body. It is possible that anuria could occur as a result of complete shutdown of renal function due to lack of renal
perfusion caused by thromboembolic disease or severe bilateral renal medullary papillary necrosis. However, anuria is usually
associated with total obstruction to urine outflow or rents in the lower urinary tract.