Like other organs in the body, the kidneys undergo age-associated
anatomical, structural and physiological changes. Starting at 50 years
of age, there is a 3-10% decline in kidney weight for each subsequent
decade of life, with the loss of cortical mass greater than
medullary mass. In fact, the number of glomeruli start to decrease
progressively after age 40, and the number of sclerotic glomeruli
increases, making the distinction between involutional and disease-related sclerosis unclear in some cases. The outer cortical
glomeruli are, in general, more extensively involved than deeper
glomeruli, but glomerular size does not change. In general, the loss of
the glomerular mass is proportional to the loss of tubular mass,
maintaining glomerulotubular balance. In addition to glomerular
sclerosis, there is a gradual increase in interstitial fibrosis, and
there is focal thickening of both glomerular and tubular basement
membranes, probably due to the accumulation of type IV
collagen.