As people age, there is a slow, steady decline in the weight of the kidneys. After about age 30 to 40, about two thirds of people (even those who do not have kidney disease) undergo a gradual decline in the rate at which their kidneys filter blood. However, the rate does not change in the remaining one third of older people, which suggests that factors other than age may affect kidney function.
As people age, the arteries supplying the kidneys narrow. Because the narrowed arteries may no longer supply enough blood for normal-sized kidneys, kidney size may decrease. Also, the walls of the small arteries that flow into the glomeruli thicken, which decreases the function of the remaining glomeruli. Accompanying these losses is a decline in the ability of the nephrons to excrete waste products and many drugs and an inability to concentrate or dilute urine and to excrete acid. Despite age-related changes, however, sufficient kidney function is preserved to meet the needs of the body. Changes that occur with age do not in and of themselves cause disease, but the changes do reduce the amount of reserve kidney function that is available. In other words, both kidneys may need to work at nearly their full capacity to carry out all the normal kidney functions. Thus, even minor damage to one or both of the kidneys may result in a loss of kidney function.
The ureters do not change much with age, but the bladder and the urethra do undergo some changes. The maximum volume of urine that the bladder can hold decreases. A person's ability to delay urination after first sensing a need to urinate also declines. The rate of urine flow out of the bladder and into the urethra slows. Throughout life, sporadic contractions of bladder wall muscles occur separately from any need or appropriate opportunity to urinate. In younger people, most of these contractions are blocked by spinal cord and brain controls, but the number of sporadic contractions that are not blocked rises with age, resulting sometimes in episodes or urinary incontinence. The amount of urine that remains in the bladder after urination is completed (residual urine) increases. As a result, people may have to urinate more frequently and have a higher risk of urinary tract infections.
In women, the urethra shortens and its lining becomes thinner. These changes in the urethra decrease the ability of the urinary sphincter to close tightly, increasing the risk of urinary incontinence. The trigger for these changes in a woman's urethra seems to be a declining level of estrogen during menopause.
In men, the prostate gland tends to enlarge with aging, gradually blocking the flow of urine (see Prostate Disorders: Benign Prostatic Hyperplasia (BPH)). If untreated, blockage may become nearly complete or complete, causing urinary retention and possibly kidney damage.
Last full review/revision November 2012 by Navin Jaipaul, MD, MHS