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aging, urinary system changes that occur with
At birth the structures of the urinary system are
fully developed and function under the automatic
control of the NERVOUS SYSTEM. The newborn’s KID-
NEYSfilter BLOODand make URINE. The BLADDERcol-
lects the urine and, when it fills to a point that
triggers the micturition REFLEX, it empties to drain
urine via the URETHRAto outside the body. Volun-
tary control over URINATION develops between
three and five years of age, the rite of passage
from babyhood to childhood. The urinary system
typically then functions at a steady level for
decades, unless disease alters its structures
(notably the KIDNEYS).

Changes in the Kidneys and Bladder

Beginning around age 40 the number, size, and
efficiency of nephrons, the filtering units of the
kidneys, begins to diminish. At birth each kidney
contains a million or more nephrons. By age 70
the kidneys have lost about 30 percent of the
nephrons they contained at birth. They are
smaller overall in size and take longer to filter the
blood that flows through them. They may allow
more water to enter the urine and keep more
electrolytes in the blood circulation. The imbal-
ance, even when slight, often affects BLOOD PRES-
SUREand other vital functions and increases the
risk for DEHYDRATION.
Other changes in the body often affect the kid-
neys as well as other structures of the urinary sys-
tem. With aging fibrous tissue throughout the
body begins to lose elasticity, becoming more rigid.
This reduced FLEXIBILITY can harden and narrow
the blood vessels that supply the kidneys, slowing
blood flow into the kidneys and through the
nephrons. It also diminishes the bladder’s ability
to distend (expand), decreasing bladder capacity.

Age-related changes in NERVEand BRAINfunction

also slow the micturition REFLEX, allowing the

bladder to become more full before triggering the

urge to urinate. These changes can result in URI-

NARY URGENCYand URINARY FREQUENCY.

The Effects of Other Changes

and Health Conditions

Age-related changes in the reproductive system—

MENOPAUSEin women and BENIGN PROSTATIC HYPER-

PLASIA(BPH) in men—affect the urinary system as

well. The normal and usually harmless enlarge-

ment with aging of the PROSTATE GLANDin men can

constrict the urethra, interfering with the flow of

urine during urination. Relaxation of the pelvic

structures that accompanies the decline of the lev-

els of ESTROGENS in women who are past

menopause affects the woman’s ability to control

the flow of urine, allowing problems such as stress

incontinence (urine leakage with coughing, sneez-

ing, or laughing). As well, stretching and tearing

of the pelvic muscles and ligaments that may have

occurred during PREGNANCYand CHILDBIRTH may

weaken these structures, allowing the bladder to

sag and pressure the VAGINA(CYSTOCELE).

HYPERTENSION (high blood pressure) and DIA-

BETES, two conditions that become increasingly

common with advancing age, are particularly haz-

ardous to the kidneys and between them account

for about 80 percent of RENAL FAILURE(acute and

chronic) and END-STAGE RENAL DISEASE(ESRD). Early

and appropriate treatment for these conditions

can significantly slow their actions on the kidneys,

highlighting the importance of routine health

screening for them. The risks for BLADDER CANCER,

RENAL CANCER, NEPHROPATHY, NEPHROLITHIASIS(kidney

stones), and UROLITHIASIS (bladder stones) also

increase with age.

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