BLOOD PRESSURE
blood culture
A laboratory test performed on a sample
of
blood
to determine the presence of
microorganisms such as bacteria. (See
also
culture.)
blood donation
The process of giving blood for use in
blood transfusion.
Blood donors give up
to 5 00 ml of blood (about one-tenth of
total blood volume), usually about twice
a year. Donated blood is routinely tested
for a range of infectious agents, such as
hepatitis B
and
hepatitis C,
and antibodies
to
HIV.
After being classified into
blood
groups,
the blood is stored in a blood
bank, either whole or separated into its
components (see
blood products).
Apheresis is a specific type of blood
donation in which only a particular
component of blood (such as plasma,
platelets, or white blood cells) is with-
drawn from the donor.
blood film
A test that involves smearing a drop of
blood on to a glass slide for examin-
ation under a microscope. The blood
film is stained with dyes to make the
blood cells show up clearly.
The test allows the shape and appear-
ance of blood cells to be checked for
any abnormality, for example the sickle-
shaped red blood cells characteristic of
sickle cell anaemia.
The relative propor-
tions of the different types of white
blood cells can also be counted. This
examination, known as a differential
white cell count, may be helpful in
diagnosing infection or
leukaemia.
Blood
films are also used in diagnosing infec-
tions in which the parasites can be seen
inside the red blood cells; an example of
such an infection is
malaria
.
Blood film tests are usually carried
out together with a full
blood count
.
blood gases
Measurement of the concentrations of
oxygen
and
carbon
dioxide
in
the
blood. The acidity-alkalinity (pH) and
bicarbonate levels are also measured.
The test is carried out on a sample of
blood that has been taken from an
artery, usually at the wrist or the groin.
It is useful in diagnosing and moni-
toring
respiratoryfailure.
Bicarbonate and
acidity reflect the
acid-base balance
of
the body, which may be disturbed in
conditions such as diabetic ketoacido-
sis, aspirin poisoning,
hyperventilation
(overbreathing), or repeated vomiting.
Blood oxygen can also be monitored
continuously without the need to take
blood samples by using an
oximeter.
blood glucose
The level of
glucose
in the blood. Abnor-
mally high blood glucose (sometimes
called blood sugar) levels may be an
indication of
diabetes mellitus.
(See also
hyperglycaemia; hypoglycaemia.)
blood glucose monitoring
A method of analysing a person’s blood
glucose
(sugar) levels that requires only
a drop of blood taken from a pinprick
on the fingertip. The blood is applied to
a test strip, which has an area impreg-
nated with a chemical that reacts with
the glucose in the blood sample. The
glucose level is shown either by a visi-
ble colour change on the strip or by
placing the strip in a digital meter. Peo-
ple with
diabetes mellitus
must perform
regular blood glucose monitoring tests
to monitor their blood glucose control.
(See also
hyperglycaemia; hypoglycaemia.)
blood groups
Systems of classifying blood according
to the different
antigens
(marker pro-
teins) on the surface of
red blood cells
(RBCs) and the
antibodies
in the blood
plasma. The antigens affect the ability of
the RBCs to provoke an
immune response
.
The two principal blood grouping sys-
tems used are the ABO system and the
rhesus system.
ABO GROUPS
In this system, the presence or absence
of two types of antigen (A and B) on the
BLOOD GROUP
COMPATIBILITY
Group A is compatible with A and O,
and group B with B and O. Group AB
is compatible with all groups, but O
is compatible with no other group.
Donor blood group
A
B
AB
O
Recipient
blood
group
A
O
o
B
o
o
AB
o
o
o
o
O
o
Key
O
Compatible
• Incompatible
surface of the red blood cells determines
a person’s blood group. People with the
A antigen (blood group A) have anti-B
antibodies; people with the B antigen
(blood group B) have anti-A antibodies;
those with both antigens (blood group
AB) have neither type of antibody; and
those with neither antigen (blood group
O) have both types of antibody.
RHESUS FACTORS
The rhesus system involves several anti-
gens, the most important of which is
called factor D. People with this factor
are Rh-positive; those without it are Rh-
negative. The importance of the Rh
group relates mainly to pregnancy in
Rh-negative women because, if the baby
is Rh-positive, the mother may form
antibodies against the baby’s blood (see
rhesus incompatibility).
USES
Blood group typing is essential for safe
blood transfusion.
The ABO and rhesus
groups are used to categorize blood
stored in blood banks, so that donor
blood that is compatible with that of
the patient can be selected before trans-
fusion takes place.
Because a person’s blood group is
inherited, identification of blood group
may be used in paternity testing. Gen-
etic analysis allows identification of the
blood of a criminal suspect with virtual
certainty (see
genetic fingerprinting).
blood level
The concentration of a given substance
in the blood plasma or serum that may
be measured by
blood tests
.
blood loss
See
bleeding.
blood poisoning
A common name for
septicaemia
with
toxaemia,
a life-threatening illness that is
caused by multiplication of bacteria and
formation of toxins in the bloodstream.
Septicaemia may occur as a compli-
cation of an infection in an organ or
tissue. In some infective conditions,
sep-
tic shock
may be caused by toxins that
are released by bacteria. Treatment for
blood poisoning is with
antibiotic drugs
and intensive therapy for shock. (See
also
bacteraemia
.)
blood pressure
The pressure exerted by the flow of
blood through the main arteries. The
pressure at two different phases is mea-
sured. Systolic, the higher pressure, is
B
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