Encyclopedia of Health and Medicine

R–S
reticulocyte An ERYTHROCYTE (red BLOOD cell)
that enters the blood’s circulation from the BONE
MARROW just before it has reached maturity. Reticulocytes
are somewhat larger than erythrocytes
and normally make up about 1 percent of the erythrocytes
in circulation. A reticulocyte matures
into an erythrocyte after being in circulation for
about a day. Reticulocytes are still continuing to
synthesize (make) HEMOGLOBIN, so contain somewhat
less hemoglobin than mature erythrocytes.
An increased number of reticulocytes in circulation
indicates the BONE marrow is producing erythrocytes
more rapidly than normal. Accelerated
erythropoiesis may suggest various underlying
causes, such as undetected internal bleeding,
hemolytic ANEMIA, and extended exposure to high
altitude (which increases the body’s need for oxygen).
The reticulocyte count also rises in PREG-
NANCY and with some medications such as
NONSTEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS),
levodopa taken to treat PARKINSON’S DISEASE, and
sulfonamide ANTIBIOTIC MEDICATIONS.
A decreased number of reticulocytes in circulation
suggests chronic INFECTION, exposure to radiation,
aplastic anemia, or iron-deficiency anemia.
The reticulocyte count also may drop with
CHEMOTHERAPY, the antibiotic chloramphenicol, and
the immunosuppressant medication azathioprine
typically taken after ORGAN TRANSPLANTATION to prevent
organ rejection or severe RHEUMATOID ARTHRITIS.
For further discussion of reticulocytes within
the context of blood and lymph structure and
function please see the overview section “The
Blood and Lymph.”
See also CELL STRUCTURE AND FUNCTION; HEMA-
TOPOIESIS; IMMUNOSUPPRESSIVE THERAPY.
Rhesus (Rh) blood type
See BLOOD TYPE.
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right lymphatic duct A major LYMPH VESSEL that
collects LYMPH draining from the right upper body
and head. The right lymphatic duct is about a
quarter-inch in diameter and two inches long,
adjacent to the subclavian VEIN beneath the
clavicle (collarbone). It empties into the right
subclavian vein, delivering lymph to the bloodstream.
For further discussion of the right lymphatic
duct within the context of blood and lymph structure
and function, please see the overview section
“The Blood and Lymph.”
See also CISTERNA CHYLI; THORACIC DUCT.
sickle cell disease An inherited genetic disorder
of defective HEMOGLOBIN, a protein compound erythrocytes
(red blood cells) contain that binds with
oxygen. Though the primary effect of sickle cell
disease, also called sickle cell ANEMIA, is anemia
(insufficient oxygen in the blood), the condition
also causes significant PAIN and damage to organs
throughout the body. In the United States sickle
cell disease is significantly more common in
African Americans. Around the world, sickle cell
disease is most common among people of African,
northern Mediterranean, Indian, and Middle Eastern
descent. About 70,000 Americans have sickle
cell disease and another 2 million have sickle cell
trait. Sickle cell disease is the most common inherited
blood disorder.
Sickle cell disease gets its name from the characteristic
sickle shape of the erythrocytes. The
deformity results from the defective hemoglobin,
called hemoglobin S, which the erythrocytes carry.
When hemoglobin S releases oxygen during the
OXYGEN-CARBON DIOXIDE EXCHANGE, it polymerizes—
its structure undergoes molecular changes that
cause its molecular weight to increase. This stiff-