BLOOD CELLS
the optic nerve, which may be caused
by diabetes mellitus, hypertension, a
tumour,
injury,
or
temporal arteritis
(inflammation of arteries in the scalp);
optic neuritis
(inflammation of the optic
nerve that may occur in
multiple sclero-
sis); the toxic (poisonous) effects of
certain chemicals; and certain nutritional
deficiencies.
Brain
Nerve impulses from the retina
eventually arrive in a region of the
cere-
brum
(the main mass of the brain)
called the visual cortex. Blindness can
result if there is pressure on the visual
cortex from a
brain tumour
or a
brain
haemorrhage,
or if the blood supply to
the visual cortex has been reduced fol-
lowing a
stroke.
DIAGNOSIS AND TREATMENT
It is frequently possible to detect the
cause of blindness by direct examina-
tion of the eye, using such techniques
as
ophthalmoscopy, slit-lamp examination,
and
tonometry.
The conduction of nerve
impulses can be measured by means of
evoked responses.
Treatment of blindness depends on
the underlying cause. If the loss of
vision cannot be corrected, the patient
may then be registered as legally blind
or partially sighted, and will become
eligible for certain benefits and services.
(See also
eye; vision, loss of.)
blind spot
The small, oval-shaped area on the retina
of the eye where the optic nerve leaves
the eyeball. The area is not sensitive to
light because it has no light receptors
(nerve endings responsive to light). The
blind spot can also be used to describe
the part of the
visual field
in which
objects cannot be detected.
blister
A collection of fluid beneath the outer
layer of the skin that forms a raised
area. The fluid is serum that has leaked
from blood vessels in underlying skin
layers after minor damage; it provides
protection for the damaged tissue.
Common causes of blisters are
burns
and friction. Blisters may also occur in
some skin diseases, including
eczema
,
epidermolysis bullosa, impetigo
,
erythema
multiforme, pemphigus, pemphigoid,
and
dermatitis herpetiformis,
and in
some
types of
porphyria.
Small blisters develop
in
chickenpox, herpes zoster
(shingles),
and
herpes simplex.
Blisters are generally
best left intact; large or unexplained
blisters need medical attention.
bloating
Distension of the abdomen, commonly
due to wind in the stomach or intestine
(see
abdominal swelling).
blocked nose
See
nasal congestion; nasal obstruction.
blocking
The inability to express true feelings or
thoughts,
usually
due
to
emotional
or mental conflict. In Freudian-based
psychotherapy,
blocking is thought to
result from the repression of painful
emotions in early life. A very specific
form of thought blocking occurs in
schizophrenia:
trains of thought are per-
sistently interrupted involuntarily to be
replaced by unrelated new ones.
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 tissues
and is then returned to the heart in
veins (see
circulatory system
). Blood is
the body’s transport system and also
plays an important role in the defence
against infection. An average adult has
about 5 litres of blood.
Almost half the blood’s volume con-
sists of
blood cells,
including red blood
cells (erythrocytes), which carry oxygen
to the tissues; white blood cells (leuko-
cytes), which defend the body against
infection; and platelets (thrombocytes),
which are involved in
blood clotting.The
rest of the blood volume is made up of
plasma, a watery, straw-coloured fluid
containing proteins, sugars, fats, salts,
and minerals.
Nutrients
are
transported
in
the
bloodstream to the tissues after absorp-
tion from the intestinal tract or after
release from storage depots such as the
liver. Waste products, 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;
immunoglobulins
(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-brain barrier
A system of tight, impermeable junc-
tions between the cells that form the
walls of the capillaries (tiny blood ves-
sels) within the
central nervous system.
The blood-brain barrier has a protec-
tive function; it allows only certain
substances and drugs in the blood-
stream to gain access to the central
nervous system, especially to the brain.
blood cells
Cells, also called blood corpuscles, that
are present in blood for most or part of
their lifespan. They include red blood
cells (which make up about 45 per cent
of the volume of normal blood), white
blood cells, and platelets. All blood cells
are made in the bone marrow by a
series of divisions from a single type of
cell called a
stem cell
.
RED BLOOD CELLS
These cells are also known as RBCs, red
blood corpuscles, or erythrocytes. They
transport oxygen from the lungs to the
tissues (see
respiration).
Formation
Red blood cells are formed
from stem cells in the bone marrow by
a process called erythropoiesis, which
takes about five days. Their formation
requires an adequate supply of nutri-
ents, including iron, amino acids, and
the vitamins B12 and folic acid. The rate
at which RBCs are formed is influenced
by a hormone called erythropoietin,
which is produced by the kidneys.
Immature red blood cells that have
just been released into the bloodstream
from the bone marrow are called retic-
ulocytes; within two to four days, these
develop into mature cells.
Structure and function
In l ml of blood
there are approximately 5 million red
B
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