Kidneys

There are several important changes that occur in kidney function as we grow older. The kidneys filter the blood and dispose of wastes and excess fluid as urine. The kidneys also play a vital role in the "acid-base" balance of the body. Beginning in our mid-40s, most of us experience a decline in kidney function, although they continue to function more than adequately under ordinary circumstances.

Most of the clinically important changes in renal function with age are probably due to changes in the intrarenal vasculature. Blood flow to the kidneys decreases by as much as 10 percent per decade can be decreased by nearly half that of younger people (600 ml/min) in those who are age 80 or older (300ml/min). As we age, the kidneys lose one quarter to one third their mass as both the number and size of nephrons (filtering units) decreases. By age 80, the total number of glomeruli fall by 30 to 40 percent and another 30% may become sclerotic and nonfunctional. These changes reduce the rate at which the blood is filtered by the kidneys.

In addition, the regulation of hormones that respond to dehydration (i.e., vasopressin) and the ability to conserve salt may decline. These renal changes make older adults particularly vulnerable to dehydration. As a result of physiologic changes, the kidneys are less efficient in concentrating urine and eliminate solutes from the blood stream.

For the most part, kidney function is well preserved although it may be slower. Most changes do not cause clinically significant disease or disability, but they do leave the kidney vulnerable to illness or medications that can depress renal function and lead to acute or chronic renal failure. Medication dosages often need to be reduced in the elderly because the reduction in kidney function can affect clearance of some drugs and lead to toxicity or adverse effects. Drugs that are renal toxic should be used cautiously in older adults.