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Red blood cells
The human body contains an estimated 25 trillion red blood cells;
approximately 4.8 million to 5.4 million are found in every microliter
of blood. The structure of a red blood cell is eminently suited to its
primary function, the transport of oxygen from the lungs to body
tissues. Red blood cells are very small (about 6 nanometers wide),
shaped like a disk, and contain a small depression on either side.
Their small size allows them to squeeze through the tiniest of blood
vessels (capillaries). In addition, their size allows a greater
diffusion of oxygen across the blood cells" plasma membranes than if
the cells were larger—because blood contains so many of these small
cells, their combined surface areas translate into an extremely large
surface area for the diffusion of oxygen. The disk shape and the
depressions on either side also contribute to a greater surface area.
TRANSPORT OF OXYGEN. Red blood cells are unusual in that they do
not contain nuclei or mitochondria, the cellular organelles in which
aerobic metabolism (the breakdown of nutrients that requires oxygen) is
carried out. Instead, red blood cells acquire energy through metabolic
processes that do not require oxygen. The lack of nuclei and
mitochondria therefore allow the red blood cell to function without
depleting its cargo of oxygen, leaving more oxygen for the body tissues.
The molecule that binds oxygen in red blood cells is called hemoglobin.
Hemoglobin is a large, globular protein consisting of four protein
chains surrounding an iron core. Hemoglobin is densely packed inside
the red blood cell; in fact, hemoglobin accounts for a third of the
weight of the entire red blood cell. Each red blood cell contains about
250 molecules of hemoglobin.
In the lungs, oxygen diffuses across the red blood cell membrane and
binds to hemoglobin. As blood circulates to the tissues, oxygen
diffuses out of the red blood cells and enters tissues. The waste
product of aerobic metabolism, carbon dioxide, then diffuses across red
blood cells and binds to hemoglobin. Once circulated back to the lungs,
the red blood cells discharge their load of carbon dioxide, which is
then breathed out of the lungs. However, only 7% of carbon dioxide
generated from metabolism is transported back to the lungs for
exhalation by red blood cells; the majority is transported in the form
of bicarbonate, a component of plasma.
HEMOPOIESIS. Red blood cells are formed in red bone marrow from
precursor cells called pluripotent stem cells. The process of red blood
cell formation is called hemopoiesis (alternatively, hematopoiesis). In
adults hemopoiesis takes place in the marrow of ribs, vertebrae, the
breastbone, and the pelvis. On average, a red blood cell lives only
three to four months. Constant wear and tear on the red blood cell
membrane, caused by squeezing through tiny capillaries, contributes to
the red blood cell"s short life span. Worn out red blood cells are
destroyed by phagocytic cells (cells that engulf and digest other
cells) in the liver. Parts of red blood cells are recycled for use in
other red blood cells, such as the iron component of hemoglobin.
White blood cells
White blood cells are less numerous than red blood cells in the human
body; each microliter of blood contains 5,000 to 10,000 white blood
cells. The number of white blood cells increases, however, when the
body is fighting off infection. Their numbers are maintained until the
immune system detects the presence of a foreign invader. When the
immune system is activated, chemicals called lymphokines stimulate the
production of more white blood cells.
White blood cells function in the body"s defense against invasion and
are key components of the immune system. They usually do not circulate
in the blood vessels, and are instead found in the interstitial fluid
and in lymph nodes. Lymph nodes are composed of lymphatic tissue and
are located at strategic places in the body. Blood filters through the
lymph nodes, and the white cells present in the nodes attack and
destroy any foreign invaders.
TYPES OF WHITE BLOOD CELLS. The human body contains five types
of white blood cells: monocytes, neutrophils, basophils, eosinophils,
and lymphocytes. Each type of white blood cell plays a specific role in
the body"s immune defense system.
Under a microscope, three kinds of white blood cells appear to contain
granules within their cytoplasm. These three types are the neutrophils,
basophils, and eosinophils. Together, these three types of white blood
cells are called granulocytes. The granules are specific chemicals that
are released during the immune response. The other two types of white
blood cells, the monocytes and lymphocytes, do not contain granules.
These types are known as the agranular leukocytes.
Monocytes, which comprise 3% to 8% of the white blood cells, and
neutrophils, which comprise 60% to 70% of white blood cells, are called
phagocytes. They ingest and digest cells, including foreign
microorganisms such as bacteria. Monocytes differentiate into cells
called macrophages. Macrophages can be fixed in one place, such as in
the brain and lymph nodes, or can "wander" to areas where they are
needed, such as the site of an infection. Neutrophils have an
additional defensive property: they release granules of lysozyme, an
enzyme that destroys cells.
Basophils comprise 0.5% to 1% of the total composition of white blood
cells and function in the body"s inflammatory response. Allergies are
caused by an inflammatory response to relatively harmless substances,
such as pollen or dust, in sensitive individuals. When activated,
basophils release various chemicals that cause the characteristic
symptoms of allergies. Histamines, for instance, cause the runny nose
and watery eyes associated with allergic reactions; heparin is an
anticoagulant that slows blood clotting and encourages the flow of
blood to the site of inflammation, inducing swelling.
Eosinophils, which comprise 2% to 4% of the total composition of white
blood cells, are believed to counteract the effects of histamine and
other inflammatory chemicals. They also phagocytize bacteria tagged by
antibodies.
Lymphocytes, which comprise 20% to 25% of the total composition of
white blood cells, are divided into two types: B lymphocytes (also
called B cells) and T lymphocytes (also called T cells). The names of
these lymphocytes are derived from their origin. T lymphocytes are
named for the thymus, an organ located in the upper chest region where
these cells mature; and B lymphocytes are named for the bursa of
Fabricus, an organ in birds where these cells were discovered.
T lymphocytes play key roles in the immune response. One type of T
lymphocyte, the helper T lymphocyte, activates the immune response when
it encounters a macrophage that has ingested a foreign microorganism.
Another kind of T lymphocyte, called a cytotoxic T lymphocyte, kills
cells infected by foreign microorganisms. B lymphocytes, when activated
by helper T lymphocytes, become plasma cells, which in turn secrete
large amounts of antibodies.
All white blood cells arise in the red bone marrow. However, the cells
destined to become lymphocytes are first differentiated into lymphoid
stem cells in the red bone marrow. These stem cells undergo further
development and maturation in the spleen, tonsils, thymus, adenoids,
and lymph nodes.
Platelets
Platelets are not cells; they are fragments of cells that function in
blood clotting. Platelets number about 250,000 to 400,000 per liter of
blood. Blood clotting is a complex process that involves a cascade of
reactions that leads to the formation of a blood clot. Platelets
contain chemicals called clotting factors. These clotting factors first
combine with a protein called prothrombin. This reaction converts
prothrombin to thrombin. Thrombin, in turn, converts fibrinogen
(present in plasma) to fibrin. Fibrin is a thread-like protein that
traps red blood cells as they leak out of a cut in the skin. As the
clot hardens, it forms a seal over the cut.
This process works for relatively small cuts in the skin. When a cut is
large, or if an artery is severed, blood loss is so severe that the
physical pressure of the blood leaving the body prevents clots from
forming. In addition, in the inherited disorder called hemophilia, one
or more clotting factors are lacking in the platelets. This disorder
causes severe bleeding from even the most minor cuts and bruises.
Platelets have a short life span; they survive for only five to nine
days before being replaced. Platelets are produced in red bone marrow
and are broken off from other red blood cells.
Role in human health
