Urinary System

Primary functions of the urinary system include: 1) excretion of waste products of metabolism; 2) maintenance of a constant extracellular environment through conservation and excretion of water and electrolytes; 3) production of the hormone erythropoietin, which regulates hematopoiesis, 4) production of the enzyme renin, which regulates blood pressure and sodium reabsorption; and 5) metabolism of vitamin D to its active form (1,25-dihydroxycholecalciferol).

Congenital and inherited anomalies of the urinary system comprise a group of anatomic defects that, although uncommon, may have subclinical to serious functional consequences.

Bovine cystitis is an inflammation of the urinary bladder of cattle that may ascend the ureters to cause infection of the kidneys (pyelonephritis). A similar condition is seen in sheep. The condition is sporadic and worldwide in distribution. Cystitis and pyelonephritis are most often seen after parturition (in one study, the average days to onset after parturition was 83), with multiparous cows being at highest risk. In locations where the disease has been studied, the prevalence is low (<1%–2%). Cystitis and pyelonephritis are rare in male cattle.

For a more general introduction to urolithiasis, see Urolithiasis in Small Animals.

Most infectious diseases of the urinary system in small animals are aerobic bacterial infections. Common organisms include Escherichia coli, Staphylococcus, Enterococcus, and Streptococcus. Less common organisms causing infection include Klebsiella, Proteus, and Pseudomonas. Pasteurella is more common in cats than dogs. Mycoplasma is an uncommon cause of urinary tract infection and is usually found as a coinfection with bacteria. Leptospirosis is a worldwide zoonotic disease caused by filamentous bacteria that infect the kidney and many other organs. Fungi, yeast, and parasites uncommonly infect the urinary system.

Failure of the filtration function of the kidneys leads to the development of azotemia (an excess of nitrogenous compounds in the blood), which may be classified as prerenal, renal, postrenal, or of mixed origin. Prerenal azotemia develops whenever mean systemic arterial blood pressure declines to values <60 mmHg and/or when dehydration causes plasma protein concentration to increase. Conditions that may lead to development of prerenal azotemia include dehydration, congestive heart failure, and shock. Prerenal azotemia generally resolves with appropriate treatment, because kidney structure has not been altered, which allows normal function to resume once renal perfusion has been restored. Renal azotemia refers to a reduction in glomerular filtration rate (GFR) of ~75% during acute or chronic primary renal (or intrarenal) diseases. Postrenal azotemia develops when the integrity of the urinary tract is disrupted (eg, bladder rupture) or urine outflow is obstructed (eg, urethral or bilateral ureteral obstruction). Once adequate urine flow is restored, postrenal azotemia will resolve.