Download FSB04 All about blood

FSB04
blo od spatte r
All about blood
Teacher Background Information
Composition of blood
Blood is a fluid that makes up approximately eight
percent of the total weight of humans. This equates to
between four and five litres in females and between
five and six litres in males. (good value to know is
70ml ± 10ml per Kg body weight)
The primary function of blood is to supply nutrients
and elements to tissues and to remove waste
products (such as carbon dioxide and lactic acid).
Blood also enables cells and different substances
(amino acids, fats and hormones) to be transported
between tissues and organs. The normal pH of human
blood is approximately 7.40 (normal range is 7.35-7.45).
Blood that has a pH below 7.35 is acidic, while blood
pH above 7.45 is alkaline (basic).
Blood is broken into two components: the formed
elements (hematocrit) and the plasma. Formed
elements of the blood constitute about 40% of whole
blood. The main types of formed elements in blood
are red blood cells, white blood cells and platelets.
(Varies considerably 25% anemia sufferers, 65%
Nepalese Shirpa’s, 44-50% women, 48 -54% males
– hematocrit also varies depending on the size of
the vessel within the body i.e. can be as low as 15%
in the capillaries).
Red blood cells
Red blood cells (erythrocytes) make up most (98.5%)
of the cells in the blood. In every milliliter of blood,
there are between five and six million red blood
cells. The function of red blood cells is to transport
the respiratory gases. They are a disk shape, which
provides a large surface area for gas exchange and
they are also flexible, which allows them to squeeze
through narrow blood vessels.
Red blood cells are generated by stem cells, which are
found in bone marrow. Red blood cells start to break
down after about 120 days of circulating around the
body. Cells are cleaned up by macrophages (a type of
white blood cell) when they eventually break down or
rupture.
White blood cells
There are two broad categories of white blood cells:
phagocytes and lymphocytes. Phagocytes are cells
that digest waste materials. A macrophage is an
important type of phagocyte.
The two main types of lymphocytes are B cells and
T cells. B cells are responsible for making special
proteins called antibodies. T cells are involved in the
immune defence against foreign or infected cells. For
example: attacking a cell infected with a virus.
Platelets
The function of platelets is to seal leaks in blood
vessels and initiate blood clotting. Platelets are only
small fragments of cells but they are packed with
essential enzymes and chemicals.
. An injury to the lining of a blood
vessel exposes collagen fibers.
2. When a platelet comes into contact with
collagen fibers, the platelet is activated
and becomes larger and sticky.
3. The platelet releases chemicals that activate other
platelets, which starts the clotting process.
Plasma
The other 60% is blood plasma, a fluid that is the
blood’s liquid medium, appearing yellow in colour.
Plasma contains gases, ions, nutrient molecules
and proteins. The proteins in the plasma have many
functions. For example: albumin is the most abundant
protein in human plasma. It is responsible for
maintaining the osmotic pressure needed for proper
distribution of body fluids between intravascular
compartments and body tissues.
IS IT BLOOD – how to tell.
Red stains are often found at the scene of a crime but
are they blood?
A test known as the Kastle-Meyer Color Test is a
presumptive test that is used to determine if a suspect
stain is blood. A presumptive test cannot confirm that the
stain is blood but the test can confirm that it is not blood.
FSB04
blo od spatte r
All about blood
Are you confused?
When the colourless K-M solution is added to the red stain, it will turn a
deep pink colour if blood is present. The problem is that the K-M solution
also turns deep pink if it comes into contact with potatoes!
What happens is, if the result is positive (deep pink colour) then further
tests are performed to make sure it is really blood.
How does it work?
Kastle-Meyer solution contains the chemicals phenolphthalein and
hydrogen peroxide.
Red blood cells contain the protein haemoglobin. Haemoglobin acts as a
catalyst in the reaction.
Haemoglobin causes hydrogen peroxide to break down into water and
oxygen. The oxygen then oxidizes or causes phenolphthalein to loose 2
hydrogen atoms turning it into an ion.
The phenolphthalein ion is a pink colour.
This reaction will not proceed without haemoglobin (or some substance in
potatoes!) acting as the catalyst.
There is another tests that can be used at the crime scene rather than
sending a sample to the lab (which could take quite a lot of time).
FSB04
blo od spatte r
All about blood
Whose blood is it? Human or animal?
Once the investigator knows that the red stain is blood, the next step is to
find out the source of the blood – is it from an animal or from a human?
The classical test that is used to find out if blood is human or animal relies
on a response from the immune system. Your immune system protects
your body from bacteria and viral infections but also reacts when a foreign
substance, such as a protein molecule from the blood of another animal, is
introduced.
This is the basis of the test known as the Precipitin Test.
Blood contains protein molecules called antibodies. Antibodies are
special molecules that are able to identify and then target specific foreign
molecules. The foreign molecules that are targeted are known as antigens.
When the immune system encounters a foreign molecule that is new to
the body, a specific antibody is produced that will target the antigen. The
antibody attaches or binds to the antigen and destroys or neutralises it. The
antibody that is produced is specific to the antigen.
How does it work?
If blood serum from a human is introduced into an animal such as a rabbit,
the rabbit will produce antibodies to the human blood proteins. These
antibodies are called anti-human antibodies.
Blood from that rabbit would therefore contain anti-human antibodies.
If blood from the rabbit was then placed on blood from a crime scene,
the anti-human antibodies in the rabbit’s blood would target and bind to
human blood proteins if the crime scene sample is human blood.
When the anti-human antibodies bind to the human blood proteins a
precipitate or clot forms. The clot is evidence that the blood is of human
origin.
With advances in medical technology it is now possible to test what the
substance is without having to resort to the laboratory. This is using a
device known as the ABAcard® HemaTrace®.
The HemaTrace® is a immuno-chromatographic assay for the forensic
identification of human haemoglobin (Hhb). The testing procedure allows
for a questioned sample to be added to the test kit following haemolysis
with buffer solution. If human Hb is present within the questioned
bloodstain, it will react with a mobile antihuman-antibody impregnated
in the absorbent test strip (stationary phase) forming a mobile antibodyantigen complex. This mobile antibody-antigen complex then migrates
(mobile phase) through the test strip to a test window where an antihuman
FSB04
blo od spatte r
All about blood
Hb antibody is immobilised. The test works on the immune response of
recognition of self or non self.
To interpret results, the presence of two coloured bands, one in the test
area (T) and one in the control area (C) indicates a positive result, whilst
the visualisation of only one band in the control area, would indicated a
negative result.
The ABAcard® HemaTrace® test is a sensitive, reliable and timely “species of
origin” testing process for the confirmatory identification of human blood at
the crime scene.
Test Strip prior to use
Solution migration
Positive result to Human Hb
Images courtesy UWA PhD research student Mark Reynolds.
FSB04
blo od spatte r
All about blood
Is there hidden blood?
Forensic scientists (CSI) are able to use chemical tests to find out if a
suspect surface contains traces of blood that may be invisible to the naked
eye. The test involves the use of a chemical called luminol that produces a
bright blue light when it reacts with blood.
The chemical test cannot be used to relate blood back to a particular
person nor can the test say with certainty that the substance is blood. This
is because luminol also reacts with other substances beside blood.
However, the luminol test is useful because it can locate areas where there
may be tiny amounts of blood. Further tests can then be carried out on
samples that have been identified by luminol, to see if it is really human
blood.
The luminol test
When using luminol to test for blood the area of suspicion must be
darkened. The luminol mixture is sprayed onto the suspect area and when
the bloodstain comes into contact with the luminol there is an emission of
bright blue light.
The chemical reaction is an example of chemiluminescence.
Chemiluminescence is a process where light is produced as a result of a
chemical reaction. Got to FSB30 for the luminol movie.
Images courtesy UWA PhD research student Mark Reynolds.
FSB04
blo od spatte r
All about blood
How does it work?
Luminol powder is mixed with sodium carbonate (Na2CO3) and hydrogen
peroxide (H2O2) plus distilled water. This produces an alkaline or basic
luminol mixture.
The emission of light occurs when luminol is oxidized by an oxidant in an
alkaline (basic) solution BUT this reaction will NOT occur unless a catalyst is
available.
A catalyst is a substance that increases the rate of a chemical reaction but
is not affected or changed by the reaction. The catalyst in the luminol
reaction is usually a metal.
In the luminol mixture mentioned above, the sodium carbonate (Na2CO3)
is added to make the luminol solution alkaline and the hydrogen peroxide
(H2O2) is the oxidant (the substance that donates oxygen molecules).
But remember, the reaction will not occur if the catalyst is not present.
When luminol is being used to detect blood, the catalyst for the reaction is
haemoglobin. Haemoglobin is a molecule that contains iron and is found
in red blood cells.
luminol + hydrogen peroxide oxidised luminol + LIGHT
haemoglobin catalyst
Chemical Structure
Luminol’s (C8H7N3O2) scientific name is 5-amino-2,3-dihydro-1,4phthalazine-dione. Its molecular weight is 177.16, and it has a melting
point of 319ºC-320ºC. It looks like a yellow grainy substance.
Haemoglobin is the catalyst in the oxidation of luminol by hydrogen
peroxide. The end product of the reaction is 3-APA is 3-aminophthalate
plus light.
In the reaction, an intermediary product 3-APA* is formed. 3-APA* is the
excited state which means that it has higher energy than 3-APA. As the
energy decreases or decays, 3-APA * becomes 3-APA and brilliant blue
light is emitted.