Urinary Disorders Q 22

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You expect a patient in the oliguric phase of renal failure to have a 24 hour urine output less than:

  

    A. 200ml

    B. 400ml

    C. 800ml

    D. 1000ml

    

    

Correct Answer: B. 400ml

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Oliguria is defined as urine output of less than 400ml/24hours. Renal causes of oliguria arise as a result of tubular damage. As a result of the tubular damage, the kidney loses its normal function i.e., production of urine while excreting the waste metabolites. In addition to this, direct damage to the renal tubules leads to a back leak of filtered uremic metabolites from the tubular lumen into the bloodstream. Hence, in these cases, decreased production of urine leads to oliguria.

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Option A: The most common prerenal cause is reduced blood flow to the kidney secondary to intravascular volume depletion, heart failure, sepsis, or as a side effect of medication. Oliguria secondary to prerenal causes usually resolves with the restoration of normal renal perfusion.

Option C: As a result of the decreased renal blood flow, various neurohormonal pathways are activated that result in the increased production of renin, angiotensin, aldosterone as well as catecholamines, and prostaglandins. Activation of these pathways leads to increased water and salt reabsorption resulting in the production of low quantities of concentrated urine while maintaining adequate glomerular filtration rate (GFR) and renal blood flow (RBF) to meet the metabolic requirements of the kidneys.

Option D: Oliguria can arise as a result of the normal physiological response of the body or due to an underlying pathology affecting the kidney or urinary tract. The human body has a normal physiological mechanism of conserving fluids and electrolytes in episodes of hypovolemia. These mechanisms are under close neurohormonal control and are completely reversible without any subsequent injury to the kidneys.

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