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Red Blood Cells (RBCs) a.k.a.
erythrocytes
Rbc's constitute the
largest component of the formed elements (99.9%).
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The hematocrit
is the percentage volume of blood contributed by the
formed elements. It is also called volume of packed
red cells (VPRC) and packed cell volume (PCV).
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The normal
hematocrit is about 46 (40 to 54) in men,
and 42 (37 to 47) in women.
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Structure of RBCs
Rbc's are highly
specialized cells that lack a nucleus, mitochondria or
ribosomes. Their ability to transport O2 and
CO2 is due to the presence of hemoglobin and
other protein within the cell. |
The
biconcave shape of the rbc's increases the
surface area for exchange and enhances the strength and
flexibility of the cells. |
The biconcave shape
enables rbc's to move more quickly through capillaries
by forming stacks called
rouleaux and the ability of the cell to bend
upon itself enables the cells to squeeze through smaller
capillaries.
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RBC life span
The loss of protein
making organelles (nucleus, ribosomes) and other
organelles (mitochondria) decrease the ability of rbc's
to produce structural proteins and enzymes. This
combined with the wear and tear that rbc's experience
cause these cells to have a short life span.
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The typical rbc's
live about 120 days before bursting or being
phagocytized.
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RBCs and Hemoglobin
Hemoglobin (Hb)
makes up 95% of the protein within the rbc's. Hemoglobin
is a red pigment that is bright red (arterial blood)
when combined with oxygen but becomes a deep red color
(venous blood) when deoxygenated. |
On a molecular
level, hemoglobin consists of four protein subunits,
two alpha hemoglobin units and two beta hemoglobin
units. Each hemoglobin subunit holds a molecule called
heme that has an iron ion that can reversibly
bind with oxygen. The hemoglobin molecule can also
reversibly bind with CO2 and about 23%
of CO2 is carried in this way. |
Hemoglobin enables
rbc's to pick up O2 and give up CO2
in the lungs and release O2 and pick up CO2
in the peripheral tissues.
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White Blood Cells (WBCs) a.k.a.
leukocytes
Leukocytes perform
their primary functions in the peripheral tissues where
they are present in large numbers. The cardiovascular
system is primarily a means to travel to the peripheral
tissues. |
Wbc's leave the
circulation by the process of
diapedesis that enables them to squeeze between
endothelial cells. They are attracted to specific
locations by
chemotaxis by which cells use chemical gradients
to guide their movements. |
Leukocytes defend
against invasion by pathogens and help the body remove
toxins, wastes and damaged cells. |
When there are
inadequate numbers of wbc's this condition is called
leukopenia; when there is an excessive number of
wbc's this condition is called
leukocytosis. |
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There are two major classes
of white blood cells:
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1. Granular Leukocytes
which have specific granules in their cytoplasm.
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2. Agranular Leukocytes
which lack specific granules in their cytoplasm.
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Granular Leukocytes
Neutrophils |
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Neutrophils are the most numerous wbc constituting about
70% of circulating wbc's in a normal individual.
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The
specific granules of neutrophils contain lysozyme and
other
bacteriostatic and
bacteriocidal agents. |
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The
nucleus is dense and contorted with expanded regions
connected by slender threads so that the nucleus may
appear as beads on a string. Because of this appearance
these cells have be called
polymorphonuclear cells or PMN's.
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Neutrophils are highly mobile and are the first to
arrive at an injury site. They actively phagocytize
bacteria and cellular debris. They die quickly but
release chemicals recruit other leukocytes to the
troubled site. |
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Eosinophils |
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Eosinophils have specific granules that stain with
eosin an acidic red dye and the nucleus is
bilobed. |
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These
cells are 2 to 4% of the circulating white blood
cells in a normal individual. |
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Eosinophils phagocytize antigen-antibody complexes and
releases chemicals that moderates the
deleterious effects of inflammatory reactions.
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The
specific granules also have chemicals that are effective
against parasites, particularly helminthic parasites. |
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Eosinophil counts increase during allergic reactions
and parasitic infections. |
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Basophils |
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Basophils are the rarest of the wbc's (less than 1%)
and have specific granules that stain with basic dyes.
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The
specific granules of basophils contain
vasoactive chemicals similar to those of mast cells
(histamine and heparin). |
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When
released, the chemicals of the specific granules
exaggerate the inflammatory response by increasing
vascular permeability. |
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Basophils are involved with
hypersensitivity reactions and
anaphylaxis.
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