the majority of which will be neutrophils. Proery-
throcytes will become erythrocytes (more than 99
percent) or platelets (less than 1 percent). Numer-
ous substances influence and regulate erythro-
poiesis. Among them are
- ERYTHROPOIETIN (EPO), a HORMONE the KIDNEYS
secrete that stimulates the bone marrow to
increase differentiation of proerythrocytes and
thus increase erythrocyte production - intrinsic factor, or erythrocyte-maturing factor,
which the STOMACH secretes to facilitate ery-
throcyte maturation - vitamin B 12 , also called extrinsic factor, which
interacts with intrinsic factor - iron, which is an essential component of HEMO-
GLOBIN (the protein complex within erythro-
cytes that binds with oxygen)
An erythrocyte goes through several stages of
development before reaching a mature enough
stage, that of reticulocyte, to leave the bone mar-
row. After 24 hours in circulation in the blood, the
reticulocyte evolves to its final stage of maturity
and becomes a fully functional erythrocyte.
Erythrocytes circulate in the blood for about 120
days. The red bone marrow releases 2 million retic-
ulocytes per minute into the blood circulation; the
spleen extracts a comparable number of old ery-
throcytes from the circulation to maintain the cor-
rect proportion of erythrocytes in the blood.
Platelets arise from proerythrocytes that differ-
entiate to become megakaryoblasts and then
megakaryocytes. The megakaryocytes release frag-
ments of their cytoplasm, which become platelets.
While megakaryocytes are the largest cells in the
bone marrow, platelets are the smallest particles in
the blood. The spleen retains about 30 percent of
the platelets the bone marrow produces, releasing
them when a COAGULATIONcascade sends chemical
signals summoning platelets to the site of clot for-
mation.
Leukopoiesis
Leukopoiesis, the production of white blood cells,
takes place in both the bone marrow (granulo-
cytes) and the lymph tissues (monocytes and lym-
phocytes). In general, all three types of leukocytes
make up less than 1 percent of the blood cells in
circulation. Many factors influence leukopoiesis,
including immune status and whether an INFEC-
TION is present in the body. Leukocytes also
undergo a series of developmental evolutions
before reaching maturity. Lymphocytes the lymph
tissues release are immature and migrate to the
THYMUS(T-cell lymphocytes) or the bone marrow
(B-cell lymphocytes) to mature.DISORDERS OF HEMATOPOIESIS
AMYLOIDOSIS ANEMIA
BONE MARROWfailure LEUKEMIA
LEUKOPENIA LYMPHOCYTOPENIA
LYMPHOMA MULTIPLE MYELOMA
MYELODYSPLASIA SYNDROME MYELOFIBROSIS
NEUTROPENIA POLYCYTHEMIA VERA
THROMBOCYTHEMIA THROMBOCYTOPENIA
vitamin B 12 deficiencySee also CELL STRUCTURE AND FUNCTION; HEMOLYSIS.hemapheresis The process of withdrawing BLOOD
from the body, filtering it through a machine
called a cell separator to extract a desired blood
component, and returning the rest of the blood to
the person. There are two forms of hemapheresis,
therapeutic and donor. Therapeutic hemapheresis,
also called apheresis, removes damaged or defec-
tive components from the blood, which allows the
body to naturally replace the components with
healthy structures. Donor hemapheresis collects
blood components for use in BLOOD TRANSFUSIONS.CLINICAL APPLICATIONS FOR
THERAPEUTIC HEMAPHERESIS
GLOMERULONEPHRITIS GOODPASTURE’S SYNDROME
hyperviscosity LEUKEMIA
MALARIA MULTIPLE SCLEROSIS
MYASTHENIA GRAVIS organ transplant rejection
PEMPHIGUSvulgaris protein-bound DRUGtoxicity
RHEUMATOID ARTHRITIS SICKLE CELL DISEASE
thrombocytosis thrombotic thrombocytopenic
transfusion reaction PURPURAFor hemapheresis, the phlebotomist inserts an
intravenous needle into a VEINin each arm. One
needle attaches to tubing that allows blood to flow
out of the body and into the cell separator. The
other needle attaches to tubing that brings thehemapheresis 141