What's in blood?
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WHAT'S IN BLOOD?
Blood is composed of a solid, cellular portion, called "formed elements," and a fluid portion, called "plasma."
The solid elements are suspended and carried in the plasma, a yellow liquid (water and dissolved substances), which contains many types of proteins (albumins, globulins, antibodies, fibrinogen), hormones, glucose and other nutrients for cells, carbon dioxide taken away from cells, etc.
The most important cellular elements found in blood are red blood cells (erythrocytes), white blood cells (leukocytes) and platelets. Red blood cells transport oxygen and carbon dioxide. White blood cells defend the body against invasion by bacteria and viruses. Platelets help the blood to clot.
When a blood sample is centrifuged, the heavier cellular elements are packed into the bottom of the tube, leaving plasma at the top. Solid elements constitute approximately 45% of the total blood volume; liquid plasma accounts for the remaining 55%. Red blood cells (erythrocytes) are the most abundant of the solid elements in blood. White blood cells (leukocytes) and platelets form a layer called the "buffy coat" between the packed red blood cells and the plasma.
Erythrocytes
are flattened, biconcave discs, about 7 µm in diameter. Erythrocytes lack a
nucleus and mitochondria. Because of these deficiencies, they do not live long,
but while they do, they must perform a tremendous amount of work.
In each erythrocyte there are approximately 280 million hemoglobin molecules,
which give blood its red color. Each hemoglobin molecule consists of a protein (called
“globin”) and an iron-containing pigment, called “heme.” The iron group
of heme is able to combine with oxygen in the lungs and release oxygen in the
tissues.
Leukocytes, or white blood cells, differ from erythrocytes in several
ways. Leukocytes contain nuclei and mitochondria and can move in an amoeboid
fashion. Because of their amoeboid ability, leukocytes can squeeze through pores
in capillary walls and get to a site of infection.
Leukocytes are classified according to
their stained appearance.
Those leukocytes that have granules in
their cytoplasm are called granular leukocytes or granulocytes.
Granular leukocytes with pink-staining
granules are called eosinophils or acidophils, and those with blue-staining
granules are called basophils. Those with granules that have little affinity for
either stain are neutrophils. Granulocytes have oddly shaped nuclei, with lobes
and strands.
Leukocytes that do not have granules in
their cytoplasm are: lymphocytes and monocytes.
Lymphocytes are usually the second most numerous type of leukocyte; they are small cells with round nuclei and little cytoplasm.
Monocytes,
in contrast, are the largest of the leukocytes and generally have kidney-or
horseshoe-shaped nuclei.
All leukocytes defend the body against invasion by microbes (bacteria and viruses): lymphocytes produce antibodies, granulocytes and monocytes are phagocytic cells.
Platelets, or thrombocytes, are the
smallest of the formed elements and are actually fragments of large cells called
megakaryocytes, found in bone marrow. (This is why the term “formed elements”
is used rather than “blood cells” to describe erythrocytes, leukocytes, and
platelets.)
The fragments that enter the circulation as platelets lack nuclei, but, like
leukocytes, are capable of amoeboid movement.
Platelets
survive about five to nine days and then are destroyed by the spleen and liver.
Platelets play an important role in blood clotting. They constitute the major
portion of the mass of the clot, and activate the clotting factors in plasma
that result in threads of fibrin, which reinforce the platelet plug.
Text
adapted from:
http://www.arrowvale.worcs.sch.uk/sportscollege/blood.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Blood.html
http://www.cardioliving.com/consumer/Circulatory/Blood.shtm
http://www.psbc.org/education/hematology/blood/blood.htm