Three different sets of kidneys develop in the embryo from the urogenital ridges.
Effects
of Aging on the Urinary System
Three different sets of kidneys develop in the embryo from
the urogenital ridges. These are
paired elevations of the intermediate mesoderm that will form the urinary and
reproductive organs. Only the final set will persist and become the adult
kidneys. In the fourth week of development, the pronephros or prekidney
forms as a tubule system, then quickly degenerates when a second and lower
system forms. The pronephros is completely gone by the sixth week, but its pronephric duct, con-necting it to the
cloaca, persists. It will be used later by the developed kidneys. The second
system, called the mesonephros or middle kidney, claims the prone-phric
duct and is eventually called the mesonephric
duct. This system also degenerates,
when the third system or metanephros develops. It begins at about
five weeks as hollow ureteric buds.
These push supe-riorly from the mesonephric duct into the urogenital ridge,
causing the local mesoderm to form nephrons. The ureteric buds’ distal ends
form the renal pelvises, calyces, and collecting ducts. The unexpanded
proxi-mal parts of the ureteric buds, which are later called ureteric ducts, form the ureters.
The kidneys develop in the pelvis and then ascend to their
final locations. Because of this, they receive blood from continually higher
sources. Lower blood vessels usually degenerate, but some-times persist. When
they do, multiple renal arteries are common. By the third month, the
metanephric kidneys excrete urine. The majority of the amni-otic fluid
surrounding the fetus is fetal urine. The fetal kidneys do not operate nearly
as efficiently as they will following birth, since placental exchange allows
the urinary system of the mother to clear most unwanted substances from fetal
blood. During metanephros development, the cloaca subdivides, forming the
rectum, anal canal, and urogenital sinus. The urinary and genital ducts
will empty into the urogenital sinus,
and the urinary bladder and urethra will develop from this structure.
Kidney problems increase with age, includ-ing the formation
of calculi or kidney stones, also
called nephrolithiasis. Other changes
because of aging include a decline in the number of functional nephrons, up to
40% in some individuals. Patients may experience a reduction in the glomeruli,
dam-age to the filtration apparatus of the glomeruli that remain, and lessened
renal blood flow. The nephrons and collecting system may become less responsive
to the ADH, and both water and sodium reabsorp-tion is lower. More sodium ions
are lost in the urine. The sphincter muscles lose tone, which can lead to
incontinence. Ability to control urination may be partially or completely lost
after Alzheimer’s dis-ease, stroke, or other central nervous system
events.Urinary retention in males may develop because of an enlarged prostate
gland that compresses the urethra and restricts urine flow.
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