BLOOD
BLOOD CLOTTING
B
be replaced by unrelated new ones.
(See also
psychotherapy.)
blood
The red fluid that circulates in the
body's veins, arteries, and capillaries.
Blood is pumped by the heart via the
arteries to the lungs and all other tis-
sues and is then returned to the heart
in veins (see
circulatory system
). Blood
is the body's transport system and plays
an important role in the defence against
infection. An average adult has about
5 litres of blood.
Almost half of the volume of blood
consists of
blood cells
; these include red
blood cells (erythrocytes), which carry
oxygen to tissues; white blood cells
(leukocytes), which fight infection; and
platelets (thrombocytes), which are in-
volved in
blood dotting.
The remainder
of the blood volume is a watery, straw-
coloured fluid called plasma, which
contains
dissolved proteins,
sugars,
fats, salts, and minerals. Nutrients are
transported in the blood to the tissues
after absorption from the intestinal
tract or after release from
storage
depots such as the liver. Waste prod-
ucts, including
urea
and
bilirubin
are
carried in the plasma to the kidneys and
liver respectively.
Plasma proteins include fibrinogen;
which is involved in blood clotting;
im-
munoglobulins
(also called
antibodies
)
and
complement
, which are part of the
immune system;
and
albumin.
Hormones
are also transported in the blood to
their target organs.
blood cells
Cells, also called blood cor-
puscles, present in blood for most or
part of their lifespan. They include red
blood cells, which make up about 45
per cent by volume of normal blood,
white blood cells, and
platelets
. Blood
cells are made in the bone marrow by a
series of divisions from
stem cells.
Red blood cells (also known as RBCs,
red blood corpuscles, or erythrocytes)
transport oxygen from the lungs to
the tissues (see
respiration
). Each RBC
is packed with
haem oglobin
,
enzymes
,
minerals, and sugars. Abnormalities can
occur in the rate at which RBCs are
either produced or destroyed, in their
numbers, and in their shape, size, and
haemoglobin content, causing forms of
protection against
cancer. T-lympho-
cytes manufacture
chemicals, known
as
lymphokines,
which affect the
function of other
cells. In addition,
the T-cells mod-
erate the activity
of B-lymphocytes,
which form
the
antibodies
that can
prevent a second
attack of certain infectious diseases.
Platelets (also known as thrombocytes),
are the smallest blood cells and are
important in
blood clotting
.
The numbers, shapes, and appearance
of the various types of blood cell are of
great value in the diagnosis of disease
(see
blood count
;
blood film
).
blood clotting
The process of blood
solidification. Clotting is important in
stemming bleeding from damaged blood
vessels. However, unwanted blood clot-
ting can occur inside major blood vessels
and cause a
myocardial infarction
(heart
attack) or stroke (see
thrombosis
).
When a blood vessel is damaged, it
constricts immediately to reduce blood
flow to the area. The damage sets off a
anaemia
and
pofycythaemia
(see
blood,
disorders of).
White blood cells (also called WBCs,
white blood corpuscles, or leukocytes)
protect the body against infection and
fight infection when it occurs. The 3
main types of WBC are granulocytes
(also called polymorphonuclear leuko-
cytes), monocytes,
and
lymphocytes
.
Granulocytes are further classified as
neutrophils, eosinophils, or basophils,
and each type of granulocyte has a role
in either fighting infection or in inflam-
matory or allergic reactions. Monocytes
and lymphocytes also play an important
part in the
immune system.
Lympho-
cytes are usually formed in the lymph
nodes. One type, a T-lymphocyte, is re-
sponsible for the
delayed hypersen-
sitivity reactions
(see
allergy
) and
is also involved in
BLOOD CELLS
White
blood cell
80
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