Renal Pathophysiology

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RENAL PATHOPHYSIOLOGY: THE ESSENTIALS

Urine Volume The urine volume is variable in patients with renal disease and is generally of little diagnostic importance. Although the GFR may be reduced, the urine vol ume is determined not by the GFR alone but by the difference between the GFR and the amount of water reabsorbed. Thus, the urine output often remains normal (equal to water intake) in patients with advanced chronic renal disease because tubular reabsorption can be decreased to balance the reduction in fil tered load. In numerical terms, a normal subject with a GFR of 180 L/day must reabsorb 179 L (more than 99% of that filtered) to excrete 1 L. A patient with severe renal disease and a GFR as low as 10 L/day (7 mL/min) can also excrete 1 L if only 9 L is reabsorbed (90% of that filtered). The ability to make this com pensation is frequently impaired in acute renal failure, where the urine output is often less than the intake leading to progressive fluid retention. One setting in which the urine volume is important diagnostically is when there is virtually no output ( < 50 mL/day), a finding that is called anu ria . Anuria is primarily seen only in certain forms of AKI, particularly com plete bilateral obstruction and marked renal hypoperfusion in shock. Less often, severe glomerulonephritis or bilateral vascular occlusion (as in the hemolytic-uremic syndrome or a dissecting aneurysm) may be responsible. In comparison, patients with acute tubular necrosis often have a reduced urine output (oliguria < 400 mL/day) but are rarely anuric. Increased urine volume is seen in the setting of diabetes insipidus (water diuresis) or osmotic diuresis from uncontrolled diabetes or solute excretion of previously administered salt or protein. This can result in abnormal elec trolyte values (especially sodium and potassium; see Chapter 3). Patients with kidney disease often present with nonspecific symptoms. The physical exam may be notable for elevated blood pressure, edema, pulmonary congestion, rash, or other organ-specific abnormalities. A reduced GFR could result from acute or chronic kidney disease, and these are discussed in the following chapters. The differential diagnosis of renal disease includes disorders of the collecting system and bladder (postrenal); conditions that result in renal hypoperfusion (prerenal); and intrinsic diseases that affect glomeruli, tubules, or blood vessels. The urinalysis and urinary protein assessment are key elements to help establish the etiology. Proteinuria can be measured on a 24-hour urine collection or esti mated from random urine samples. The amount and type of protein found in the urine can provide diagnostic clues as to whether the condition primarily involves the glomeruli or tubules. Albuminuria characterizes diabetes in addition to other glomerular diseases. Tubular proteinuria tends to be lower grade and a mixture of tubular proteins, whereas paraproteins (immunoglobulin molecules) can be filtered and excreted with variable effects on kidney function. Analysis of the urine sedi ment provides additional information on the potential etiologies. The presence of dysmorphic red blood cells and/or red blood cell casts is suggestive of glomerulone phritis, whereas large amounts of albumin and normal renal function are suggestive SUMMARY

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