BLOOD CELLS
B
CONSTITUENTS OF BLOOD
Blood is pumped around the body in
veins and arteries, transporting oxygen
from the lungs to the tissues and
carbon dioxide from the tissues to
the lungs. Blood also carries nutrients
such as sugars, fats, and proteins that
have been absorbed from the intestine
and hormones produced by a variety
of glands. Waste products that are
released from cells are carried in the
blood to be broken down in the liver
or excreted from the kidneys.
R e d b l o o d c e ll
P l a t e l e t
W h ite b l o o d c e ll
Normal blood smear
This is the appearance of
normal blood under a
microscope. The dominant
feature is the abundance of
red blood cells, which make
up almost half of the volume
of blood. One white blood cell
(a lymphocyte) can be clearly
seen; the platelets are the
tiny dark particles.
White blood cells
These cells protect the body against
infection and fight itwhen it occurs.
They are bigger than red blood cells
but fewer in number. Each of the
three main types (granulocytes,
monocytes, and lymphocytes)
plays a different role in
dealing with infection.
P la s m a
Platelets
The smallest type of blood cell
produced in the bone marrow.
They play an important role in
blood clotting.
Plasma
The fluid part of the blood that
consists mostly of water. It carries
substances such as proteins, fats,
glucose, and salts.
Red blood cells
These disc-shaped
cells are formed in the
bone marrow and carry
oxygen from the lungs
to the rest of the body.
They have a large
surface area and a
flexible shape.
Platelets
Red blood cells
blood cells, each of which is disc-
shaped, about 0.0075 mm in diameter,
and thicker around the edge than at the
centre. This shape gives each cell a rela-
tively large surface area, which helps it
absorb and release oxygen molecules,
and allows it to distort as it squeezes
through narrow blood vessels. The sur-
face structure of red blood cells varies
slightly among individuals, and this
provides the basis for classifying blood
into groups (see
blood groups).
RBCs are packed with large quantities
of
haemoglobin,
a pigmented protein
that contains iron. Haemoglobin binds
(combines chemically) with oxygen to
form oxyhaemoglobin, which carries
oxygen to body tissues. Oxyhaemoglo-
bin is responsible for the bright red
coloration of oxygenated blood, which
flows mainly through the arteries. Most
venous blood is darker because it con-
tains the unbound (deoxygenated) form
of haemoglobin.
Every
RBC
also
contains
enzymes
(substances that promote biochemical
reactions), minerals, and sugars that
provide energy for the cell’s
metabolism
(chemical processes) and maintain its
shape, structure, and elasticity.
Aging and destruction
The normal lifes-
pan of RBCs in the circulation is about
120 days. As they age, their internal
chemical machinery wears out, they
lose elasticity, and they become trapped
in small blood vessels in the spleen and
other organs; they are then destroyed by
a type of white blood cell called a
macrophage. Most of the components
of haemoglobin molecules are reused,
but some are broken down to form the
waste product
bilirubin
.
Disorders
Abnormalities can occur in
the rate at which RBCs are produced or
destroyed; in their numbers; and in their
shape, size, and haemoglobin content,
all
of which
may
cause
forms
of
anaemia
and
polycythaemia
(see
blood
disorders box).
WHITE BLOOD CELLS
These are also called WBCs, white blood
corpuscles, or leukocytes. They protect
the body against infection and fight
infection when it occurs. White blood
cells are bigger than red blood cells (up
to 0.015 mm in diameter) but are far
less numerous (about 7,500 per ml of
blood). They generally spend a shorter
part of their lifespan than red blood
cells in the blood itself. The three main
types of WBC are granulocytes (also
called polymorphonuclear leukocytes),
monocytes, and
lymphocytes
.
Granulocytes
Granulocytes are further
classified as neutrophils, eosinophils, or
basophils, each type having a specific
role. The most important are neutro-
phils, which isolate and destroy invading
bacteria.
Neutrophils
remain
in
the
blood for only about six to nine hours
before moving
through
blood-vessel
walls into tissues. Eosinophils play a
part in allergic reactions and increase in
numbers in response to certain parasitic
infections.
Basophils are involved in
inflammatory and allergic reactions.
Monocytes
These
cells
also
play
an
important role in the
immune system.
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