[k] Poststreptococcal Glomerulonephritis

THE KIDNEY

Poststreptococcal Glomerulonephritis

A 17-year-old girl develops poststreptococcal glomerulonephritis. In this condition the glomeruli are damaged by an acute inflammatory reaction. This inflammatory response damages the filtration barrier and the integrity of the capillary endothelium and the net negative charge on the glomerular basement membrane are lost. Which of the following substances would appear in greater quantity in the urine of this girl? A. Red blood cells B. Glucose C. Sodium D. Proteins

A Because of the damage to the capillary endothelium and basement membrane, red blood cells would be found in the urine; this is termed hematuria. Importantly, red blood cells can also appear in the urine even when the filtration barrier is not damaged. For example, red blood cells can appear in the urine as a result of bleeding in any part of the lower urinary tract. Such bleeding occurs with kidney stones, and occasionally as a result of a bacterial infection that occurs in the lower urinary tract and causes bleeding. In women, urine can also contain menstrual blood.

B Because glucose is filtered and completely reabsorbed by the proximal tubule, it is not normally found in the urine. Its presence in the urine indicates either (a) an elevated plasma glucose level, such that the filtered load (i.e., GFR x P glucose) is greater than the Tm for glucose reabsorption by the proximal tubule, or (b) an abnormality in glucose transport. Because glucose is freely filtered by the normal glomerulus, damage to the ultrafiltration barrier would not increase its filtration.

C Na+ normally appears in the urine of healthy individuals. Like glucose, Na+ is freely filtered by the normal glomerulus. Therefore damage to the filtration barrier does not increase the rate of Na+ excretion.

D. Normally the urine contains essentially no protein because the glomerulus prevents the filtration of plasma proteins. However, when the glomerulus is damaged, large amounts of plasma proteins can be filtered. If the amount filtered overwhelms the reabsorptive capacity of the proximal tubule, protein appears in the urine (proteinuria).