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Case 1 – The first-aid troops1. Describe the pictures of the cells → types, structure
and function
The formed elements of blood are the erythrocytes,
leukocytes and the platelets. Only the leukocytes are
real cells. Erythrocytes have no nuclei or organelles
and platelets are cell fragments. Most of the formed
elements only survive for a few days and they don’t
divide. Only stem cells divide continuously in red
bone marrow to replace them.
Erythrocytes
Erythrocytes are the red blood cells and they are
small, about 7.5 µm in diameter. They are shaped like
biconcave discs, with depressed centers. Mature
erythrocytes are bound by a plasma membrane ,but
lack a nucleus. They are more or less “bags” of
hemoglobin that functions in gas transport. Also
antioxidant enzymes are present, which get rid of harmful oxygen radicals and they
provide structure to the RBC. Spectrin is attached to the cytoplasmic face of RBC plasma
membranes and maintain the biconcave shape, but they are deformable.
Erythrocytes pick up oxygen in the capillaries of the lungs and release it to tissue cells
across other capillaries throughout the body. It also transports 20% of the carbon
dioxide released by cells back to the lungs. Thee structural characteristics contribute to
erythrocyte gas transport functions:
1) Small size + biconcave structure provide a huge surface relative to volume. No
point from the cytoplasm is far from the surface.
2) An erythrocyte is over 97% hemoglobin (discounting water content).
3) They lack mitochondria, so they don’t consume the oxygen that they carry. Very
suitable for oxygen transport.
Hemoglobin binds easily and reversibly to hemoglobin.
Normal values of hemoglobin are 13-18 grams per 100
millimetres of blood in males and 12-16 g/100 ml in
females. It is made up of the rede heme pigment bound
to the protein globin. Globin consists of four
polypeptide chains, two alpha and two beta, each
binding a ring-like heme group. Each heme group bears
an atom of iron set like a jewel in its center. A
hemoglobin molecule can transport four molecules of
oxygen because each iron atom can combine reversibly
with one molecule of oxygen.
When oxygen binds to iron, the hemoglobin is called
oxyhemoglobin and assumes a new three-dimensional
shape and becomes ruby red. When oxygen detaches from iron, it resumes in former
shape and is called deoxyhemoglobin. It becomes dark red.
,Leukocytes
Leukocytes are the white blood cells and are the only formed elements that are
complete cells. They account for less than 1% of total blood volume. They are crucial to
our defence against disease. White blood cells are able to slip out of the capillary blood
vessels, a process called diapedesis, and the circulatory system is simply their means of
transport to areas of the body where they mount inflammatory or immune responses.
The signals that prompt white blood cells to specific locations are cell adhesion
molecules displayed by endothelial cells forming the capillary walls at sites of
inflammation. Then they move through the tissue spaces by amoeboid motion (they
form flowing cytoplasmic extensions that move them along). By following the chemical
trail of molecules released by damaged cells a phenomenon called positive chemotaxis,
they pinpoint areas of tissue damage and infection and gather there in large numbers to
destroy foreign substances and dead cells.
The leukocytes are grouped into two major categories on the basis of structural and
chemical characteristics. Granulocytes contain obvious membrane-bound cytoplasmic
granules and agranulocytes lack obvious granules.
Granulocytes, which include neutrophils, eosinophils and basophils are all roughly
spherical in shape. They are larger and much shorter lived than erythrocytes. They have
lobed nuclei. All granulocytes are phagocytes to some degree.
Neutrophils
Neutrophils are the most numerous white blood cells (50-70% of WBC population).
They are about twice as large as erythrocytes. The cytoplasm contains very fine granules
that are difficult to see. They get their name because their granules take up both basic
(blue) and acidic (red) dyes. Together, the two types of granules give the cytoplasm a
lilac colour. Some of the granules contain hydrolytic enzymes and are regarded
lysosomes. Other (smaller) granules contain a potent “brew” of antimicrobial proteins,
called defensins. They are often called polymorphonuclear leukocytes because their
nucleus consists of three to six lobes.
The neutrophils are our body’s bacteria slayers. Their number increases explosively
during acute bacterial infections. They are chemically attracted to sites of inflammation
and are active phagocytes. They are especially partial to bacteria and some fungi, and
bacterial killing is promoted by a process called a respiratory burst. In this respiratory
burst the cells metabolize oxygen to produce a potent germ-killer oxidizing substances
such as bleach and hydrogen peroxide. In addition, defensin-mediated lysis occurs when
the granules containing the defensins merge with a microbe-containing phagosome.
Eosinophils
Eosinophils account for 2-4% of all leukocytes and are approximately the size of
neutrophils. Their nuclei resembles an old-fashioned telephone receiver, two lobes
connected by a broad band of nuclear material. Large, coarse granules pack to the
cytoplasm. These granules are lysosome-like and filled with a unique variety of digestive
enzymes. They lack enzymes to specifically digest bacteria.
,The most important role of eosinophils is to lead the counter attack against parasitic
worms, that are too large to be phagocytized. They are ingested in food or invade the
body via the skin. Eosinophils reside in the loose connective tissues at the same body
sites, and when they encounter a parasitic worm they gather around and release
enzymes from their cytoplasmic granules onto the parasite’s surface, digesting it away.
They are also important in allergies and asthma.
Basophils
Basophils are the rarest white blood cells, accounting for only 0.5-1%. Their cytoplasm
contains large, coarse, histamine-containing granules. Histamine is an inflammatory
chemical that acts as a vasodilator and attracts other white blood cells to the inflamed
site. Granulated cells similar to basophils, called mast cells, are found in connective
tissues. although mast cell nuclei tend to be an oval, the cells are similar microscopically
and both cell types bind to a particular antibody (immunoglobulin E) that causes the
cells to release histamine.
Agranulocytes include lymphocytes and monocytes, that lack visible cytoplasmic
granules. Although similar to each other structurally, they are functionally distinct and
unrelated cell types. The nuclei are spherical or kidney shaped.
Lymphocytes
Lymphocytes account for 25% or more of the WBC population. It has a large nucleus that
occupies most of the cell volume. The nucleus is usually spherical but may be indented.
Large numbers of lymphocytes exist in the body, but relatively few are found in the
bloodstream. Most are closely associated with the lymphoid tissues, where they play a
crucial role in immunity. T lymphocytes (T cells) function in the immune response by
acting directly against virus-infected cells and tumor cells. B lymphocytes (B cells) give
rise to plasma cells, which produce antibodies (immunoglobulins) that are released to
the blood.
Monocytes
Monocytes account for 3-8% of the WBC population and they are the largest leukocytes.
The nucleus is U or kidney shaped. When circulating monocytes leave the bloodstream
and enter tissues, they differentiate into highly mobile macrophages with prodigious
, appetites. Macrophages are actively phagocytic and they are crucial in the body’s
defence against viruses, certain intracellular bacterial parasites and chronic infections.
Platelets
Platelets are about one-fourth the diameter of a lymphocytes and they are cytoplasmic
fragments of extraordinarily large cells called megakaryocytes. Each platelet exhibits
an outer region and an inner area containing granules. The granules contain an
impressive array of chemicals that cat in the clotting process, including serotonin, Ca2+, a
variety of enzymes, ADP and platelet derived growth factor (PDGF).
Platelets are essential for the clotting process that occurs in plasma when blood vessels
are ruptured or their lining is injured. By sticking to the damaged site, platelets form a
temporary plug that helps seal the break. Because they are anucleate, platelets age
quickly and degenerate in about 10 days if they are not involved in clotting. In the
meantime, they circulate freely, kept mobile but inactive by molecules secreted by
endothelial cells lining the blood vessels.
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