Encyclopedia of Health and Medicine

kidneys 201
proceed only when the appropriate assessments
certify the person has suffered BRAIN DEATH.
UNIVERSAL DONOR CARD
Most states honor the universal donor card, a
wallet-size document affirming a person’s intent
to donate his or her organs upon death, as a
legal document. Many states incorporate the universal
donor card into the driver’s license.
A transplant team removes donor kidneys
using sterile surgical technique and a procedure
similar to nephrectomy (surgical removal of a kidney
such as to treat RENAL CANCER), carefully preserving
the blood vessels and URETER. After
removing the kidneys, called organ harvesting, the
transplant team places them in a cold solution that
can sustain them for 36 to 48 hours and sutures
closed the incisions made to gain access to the kidneys.
The organ-processing procedure includes
screening of the donor kidneys for any diseases
they could convey to the recipient. In the United
States an independent organization called the
United Network for Organ Sharing (UNOS) oversees
the collection and distribution of all cadaver
donor organs in compliance with strict guidelines
intended to ensure equity in the process of matching
donor organs with recipients. The transplant
surgery must take place within 36 to 48 hours of
the kidney’s harvesting, sooner if possible.
See also ORGAN TRANSPLANTATION; QUALITY OF LIFE;
SURGERY BENEFIT AND RISK ASSESSMENT.
kidney failure
See RENAL FAILURE.
kidneys A pair of organs responsible for filtering
wastes and excess water from the BLOOD, excreting
both from the body as URINE. The kidneys maintain
the body’s fluid and electrolyte balances, and
also produce hormones that regulate the production
of new erythrocytes (ERYTHROPOIETIN [EPO])
and BLOOD PRESSURE (RENIN). With each heartbeat
about 20 percent of the body’s blood supply surges
through the kidneys. The body’s entire blood supply
passes through the kidneys about two dozen
times a day. Though the kidneys are essential for
life, a single functioning kidney can adequately
sustain life in most people. The kidneys are fully
functional from birth.
Renal Structure
The kidneys are dark reddish brown in color, four
to five inches long, and about two inches across.
An adult kidney weighs five to six ounces, and is
the same shape as the bean that bears its name.
The kidneys rest against along the spinal column
at the back of the abdominal cavity, one on each
side of the SPINAL CORD and within the protective
enclosure of the rib cage. The kidneys are
retroperitoneal—that is, they lie outside the posterior
layer of the peritoneum, the membrane that
protects the abdominal structures. The left kidney
is about an inch higher than the right. A cushion
of fatty tissue surrounds each kidney, helping protect
it as well as hold it in place. An adrenal gland
resides atop each kidney though does not physically
or functionally integrate with the kidney.
A thin but tough membrane called the renal
capsule surrounds the kidney, helping contain and
protect its blood-rich tissues. The outer layer of
the kidney is the renal cortex and the inner layer
the renal medulla. The renal ARTERY, renal VEIN,
and URETER junction with the kidney where it
indents, an area called the hilus. Deeper within
the kidney at this junction is the renal pelvis, a
deltalike region of the kidney that drains urine
into the ureter. The functional unit of the kidney
is the NEPHRON, a microscopic structure, a set of
tubules that carry out the functions filtration, and
a coil of capillaries, the GLOMERULUS, which brings
in the blood for filtration. Each kidney contains
over a million nephrons, each of which functions
independently. The nephrons extend through the
renal cortex and the renal medulla. The renal cortex
contains the blood vessels that bring blood to
the nephrons and the glomerulus for each
nephron, as well as a portion of filtering tubule.
The renal medulla consists of 8 to 12 wedgeshaped
segments, called pyramids. The pyramids
contain the filtering tubules, including the loop of
Henle and the collecting tubule, for each nephron.
Blood circulates primarily through the renal cortex,
while the structures of the renal medulla
direct water and waste products (urine) toward
the renal pelvis and elimination via the ureter.
Renal Function
The pressure of the blood as it flows through the
glomeruli helps force molecules of water and