R–S

reticulocyte An ERYTHROCYTE (red BLOOD cell)
that enters the blood’s circulation from theBONE
MARROWjust before it has reached maturity. Retic-
ulocytes are somewhat larger than erythrocytes
and normally make up about 1 percent of the ery-
throcytes 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 some-
what less hemoglobin than mature erythrocytes.
An increased number of reticulocytes in circu-
lation indicates the BONEmarrow is producing ery-
throcytes 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 oxy-
gen). 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 circula-
tion suggests chronic INFECTION, exposure to radia-
tion, 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 TRANSPLANTATIONto pre-
vent 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.

right lymphatic duct A major LYMPH VESSELthat

collects LY M P Hdraining 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 blood-

stream.

For further discussion of the right lymphatic

duct within the context of blood and lymph struc-

ture 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 ery-

throcytes (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 PAINand 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 East-

ern descent. About 70,000 Americans have sickle

cell disease and another 2 million have sickle cell

trait. Sickle cell disease is the most common inher-

ited blood disorder.

Sickle cell disease gets its name from the char-

acteristic 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-

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