The Blood and Lymph 119marrow also warehouses minerals it requires for
cell synthesis and the bones need for STRENGTHand
growth, such as calcium. As well, the bone mar-
row stores B-cell lymphocytes and plasma cells,
leukocytes integral to the body’s IMMUNE RESPONSE.
Oxygen transport: erythrocytes The erythro-
cytes, also called red blood cells (RBCs), pick up
oxygen molecules in the LUNGSand carry them to
the cells. After delivering the oxygen, the erythro-
cytes then retrieve carbon dioxide molecules, the
waste byproducts of cellular METABOLISM, and cart
them back to the lungs for elimination from the
body through respiration. This OXYGEN–CARBON
DIOXIDE EXCHANGEis the foundation of the body’s
survival. No cells in the body can survive longer
than 10 to 15 minutes (three to five minutes for
BRAINand heart cells) without oxygen.
Erythrocytes acquire their capacity to carry
oxygen from the pigmented protein HEMOGLOBIN,
which is high in iron. The pigment also gives ery-
throcytes their red color. The iron hemoglobin
contains allows the hemoglobin to bind with the
oxygen molecules. A healthy, normal erythrocyte
contains about 300 million molecules of hemoglo-
bin; each molecule of hemoglobin can bind with
four molecules of oxygen. Iron enters the body
from dietary sources. Iron deficiency is the most
common cause of ANEMIA, a condition in which
the blood cannot meet the body’s oxygenation
needs.
Erythrocytes are concave on both sides, giving
them the FLEXIBILITY to nearly fold in half to
squeeze through the narrowest of the body’s blood
vessels, the arterioles, venules, and capillaries. As
well, erythrocytes lack nuclei, the “command”
structures common to cells that contain deoxyri-
bonucleic acid (DNA). DNA gives the cell its repli-
cation instructions; without it a cell cannot
reproduce. The absence of a nucleus further aids
the erythrocyte’s flexibility, however, which is
most important for delivering oxygen deep within
the body’s tissues.
Because erythrocytes cannot proliferate, the
red bone marrow churns out a steady supply of
new ones, releasing them into the circulation at a
rate of 2 to 3 million per second. Erythrocytes
enter the bloodstream in a slightly immature
stage, called reticulocytes. They reach full matu-
rity after about 24 hours in circulation and live in
the bloodstream for 110 to 120 days, after which
the SPLEENfilters them from the blood and breaks
them down (hemolyzes) into their component
structures. The LIVERfurther metabolizes the com-
ponents of hemolyzed erythrocytes, recycling their
ingredients for use in synthesizing new erythro-
cytes as well as to manufacture BILEand other bio-
chemical substances. Macrophages within the
liver, migratory monocytes called Kupffer cells,
then consume whatever remains of the erythro-
cytic waste.
Stop the bleeding: platelets The smallest cell
elements in the blood, platelets, are encased in
protein coatings that become adhesive (sticky)
when chemical messengers released at the site of
bleeding enter the bloodstream. The chemicals
activate PLATELET AGGREGATION, in which platelets
swarm to the site of bleeding and stick to each
other as well as to the collagen fibers at the site to
form a hemostatic plug. This activation also
enables platelets to change shape, elongating or
contracting as necessary to bridge the gaps among
the collagen fibers to form a weblike structure that
ensnares other cells and substances. As the coagu-
lation cascade unfolds the plug expands and hard-
ens, eventually forming the clot that halts the
bleeding. On the surface of the SKIN, this clot is a
scab. Within a blood vessel, it is a thrombus.
Platelets arise from the largest cells in the red
bone marrow, megakaryocytes, and actually are
fragments of megakaryocytic cytoplasm rather
than independent cells. They are irregularly
shaped and loosely defined, a structure ideally
suited to their purpose. Platelets also lack nuclei
and live in the circulation for about 10 days.
Roughly a third of the body’s total platelet volume
resides in the spleen, which releases them into the
circulating blood in response to bleeding.
Defend and protect: leukocytes The leukocytes,
also called white blood cells (WBCs), are the foun-
dation of the body’s IMMUNE RESPONSE. They take
one of three forms: LYMPHOCYTE, MONOCYTE, or
GRANULOCYTE. Each has specialized functions
within the immune response. Lymphocytes attack
invading pathogens, and monocytes and granulo-
cytes consume the remains of the pathogenic
invaders. Lymphocytes circulate primarily in the
lymph. Monocytes circulate in the blood for about
24 hours after the bone marrow releases them and