ing the urine to capture the stones or stone frag-
ments when they pass, for laboratory analysis.
Despite the pain they cause, kidney stones do not
usually cause permanent damage to the kidneys.
People who have had kidney stones should
make dietary modifications, such as calcium
restriction, only if the doctor specifically recom-
mends them. Though doctors once believed
dietary calcium was a key culprit in the develop-
ment of kidney stones, recent research shows that
when blood levels of calcium are too high (HYPER-
CALCEMIA) the cause is more likely to be overab-
sorption from the gastrointestinal tract than
excessive consumption. Cutting back on dietary
calcium in such a situation can have the opposite
and undesired consequence of increasing gastroin-
testinal absorption of calcium. Dietary calcium is
essential for BONE STRENGTHand health, tissue HEAL-
ING, and proper NERVEand MUSCLEfunction.
Risk Factors and Preventive Measures
Men are more likely than women to develop kid-
ney stones. As well, kidney stones appear to run
in families, suggesting a genetic or hereditary
component. People who have RENAL TUBULAR ACI-
DOSISor INFLAMMATORY BOWEL DISEASE(IBD) have
increased risk for developing nephrolithiasis.
Lifestyle measures to reduce the risk for kidney
stones include drinking six to eight 8-ounce
glasses of water and getting physical exercise daily.
These measures increase the volume of urine,
helping dissolve minerals that might crystallize,
and keeps the urine moving through the urinary
system. The doctor may prescribe medication to
reduce the risk of kidney stones in people who
have history of RECURRENCE.
See also CYSTINURIA; HYPEROXALURIA; MINIMALLY
INVASIVE SURGERY; URINARY TRACT INFECTION (UTI);
UROLITHIASIS.
nephron The microscopic functional unit of the
kidney. Each kidney contains more than a million
nephrons, each of which extends from the renal
cortex into the renal medulla in fairly linear fash-
ion. Two elements make up the nephron: the renal
tubules and the renal corpuscle. The renal corpus-
cle contains the GLOMERULUS, the coiled network of
capillaries that bring BLOODinto the nephron, and
Bowman’s capsule, the podlike structure that
encases the glomerulus. The pressure of the blood
as it enters the glomerulus forces molecules of
water, electrolytes, and other substances through
the thin glomerular wall into Bowman’s capsule.
This mixture, called filtrate, collects in the capsule
and drains into the renal tubule. Each segment of
the tubule reabsorbs different substances from the
filtrate as it passes through them. A second net-
work of capillaries separate from the glomerulus,
the peritubular capillaries, entwines the renal
tubule to allow the reabsorbed materials to reenter
the blood circulation.
The first portion of the tubule to exit Bowman’s
capsule, the proximal tubule (also called the prox-
imal convoluted tubule), runs along the renal cor-
puscle, heading inward toward the renal medulla
though it remains within the renal cortex. The
proximal tubule reabsorbs about two thirds of the
sodium and two thirds of the water the filtrate
contains, and reabsorbs calcium when vitamin D
is present. The next segment, the loop of Henle,
drops deep into the renal medulla, makes a sharp
loop, and rises back up into the renal cortex in
somewhat of a hairpin appearance. Different por-
tions of the loop of Henle reabsorb sodium, potas-
sium, chloride, magnesium, calcium, and water.
The loop of Henle plays a significant role in the
concentration and dilution of the URINE, and is the
target of some types of diuretic medications. The
distal tubule (also called the distal convoluted
tubule) continues up through the renal cortex and
wraps around the renal corpuscle, ultimately join-
ing with the collecting tubule (also called the col-
lecting duct). The distal tubule reabsorbs sodium
and bicarbonate and secretes potassium. The final
segment of the renal tubule is the collecting
tubule, which funnels the remaining filtrate
toward the renal pelvis for excretion via the
URETERas urine. Only water reabsorption takes
place from the collecting tubule.
For further discussion of the nephron within
the context of the urinary system’s structure and
function please see the overview section “The Uri-
nary System.”
See also BLADDER; FANCONI’S SYNDROME; KIDNEYS;
URETHRA.nephropathy Progressive, irreversible damage to
the KIDNEYS that occurs as a result of systemicnephropathy 209