The cortex contains specialized capillar-
ies called glomeruli, which, together
with a series of tubules, make up the
nephrons, the filtering units of the kid-
ney. The nephrons filter blood under
pressure and then selectively reabsorb
water and certain other substances back
into the blood. Urine is formed from
substances that are not reabsorbed.
The urine is conducted through tubules
to the renal pelvis (the central collecting
area of the kidney) and then through
tubes called ureters to the bladder.
The kidneys also regulate the body’s
fluid balance. To do this, the kidneys
excrete excess water, and when water is
lost from the body (for example as a
result of sweating), they conserve it (see
ADH). In addition, the kidneys control
the body’s acid–base balanceby adjust-
ing urine acidity. The kidneys are also
involved in hormonal regulation of red
blood cellproduction and blood pressure.
kidney biopsyA procedure in which a
small sample of kidney tissue is
removed and examined under a micro-
scope. Kidney biopsyis performed to
investigate and diagnose serious kidney
disorders, such as glomerulonephritis,
proteinuria, nephrotic syndrome, and
acutekidney failure, or to assess the kid-
neys’ response to treatment. There are 2
basic techniques: percutaneous needle
biopsy, in which a hollow needle is
passed through the skin into the kidney
under local anaesthesia; and open sur-
gery under general anaesthesia.
kidney cancer A cancerous tumour of
the kidney.Most kidney cancers origi-
nate in the kidney itself, but in rare
cases cancer spreads to the kidney from
another organ. There are 3 main types
of kidney cancer. The most common,
renal cell carcinoma, usually occurs in
people over 40. Nephroblastoma (also
called Wilms’ tumour) is a fast-growing
tumour that mainly affects children under- Transitional cell carcinoma arises from
cells lining the renal pelvis; it is more
common in smokers or those who have
taken analgesic drugs for a long time.
Symptoms of kidney cancer vary. It is
often symptomless in the early stages,
although later there may be blood in the
urine. All types require surgical removal
of the kidney and sometimes also of the
ureter. For nephroblastoma, surgery is
followed by treatment with anticancer
drugs. Kidney cancer is likely to be fatal
if it has spread to other organs before
treatment is started.
kidney cystA fluid-filled sac in the
kidney.Most kidney cysts are noncancer-
ous. Cysts commonly develop in people
over 50 and may occur singly or multi-
ply in 1 or both kidneys. Most cysts
occur for no known reason and do not
usually produce symptoms unless they
become large enough to cause pain in
the lower back due to pressure. However,
large numbers of cysts in the kidneys
may be associated with polycystic kidney
disease (see kidney, polycystic), which
often leads to kidney failure.Treatment
of simple cysts is not usually necessary,
but aspiration(withdrawal of fluid) or
surgical removal may be carried out if a
cyst is painful or recurs.
kidney disordersThe kidneysare sus-
ceptible to a wide range of disorders.
However, since only 1 normal kidney is
needed for good health, disease is rarely
life-threatening unless it affects both
kidneys and is at an advanced stage.
Congenital abnormalities, such as
horseshoe kidney, are fairly common
and usually harmless. Serious inherited
disorders include polycystic kidney dis-
ease (see kidney, polycystic), Fanconi’s
syndrome, and renal tubular acidosis.
Blood vessels in the kidneys can be
damaged by shock, haemolytic–uraemic
syndrome, polyarteritis nodosa, diabetes
mellitus, and systemiclupus erythemato-
sus. The filtering units may be inflamed
(see glomerulonephritis). Allergic reac-
tions to drugs, prolonged treatment
with analgesic drugs, and some antibi-
otics can damage kidney tubules.
Noncancerous kidney tumoursare rare,
as is kidney cancer. Metabolic disorders,
such as hyperuricaemia, may cause kid-
ney stones (see calculus, urinary tract).
Infection of the kidney is called pyelo-
nephritis. Hydronephrosisis caused by
urinary tract obstruction. In crush syn-
drome, kidney function is disrupted by
proteins released into the blood from
damaged muscle. Hypertensioncan be
a cause and an effect of kidney damage.KIDNEY BIOPSY KIDNEY DISORDERSK