Hello and welcome to this lecture on bleeding
disorders. During this lecture, you will achieve
a number of learning outcomes. We will see
that hemostasis is dependent on a combined
response to injury from the blood vessel,
the platelet, and the coagulation system.
We will see that a range of inherited and
acquired disorders can lead to bleeding disorders
and we will see that vessel and platelet problems
lead to purpura and bruising whereas coagulation
deficiency is characterized by bleeds into
muscles and joints. When your vessels get
injured, a number of processes start. After
the initial injury, the vessels can vasoconstrict
to reduce the blood flow. Then platelets attach
to exposed collagen on the surface of the
vessel. These then undergo platelet adherence
and an aggregation reaction. We will talk
about those in more detail. Finally, coagulation
factors ensure that thrombin is generated
in order to produce fibrinogen and that stabilizes
the clot. This process is really quite remarkable.
If you prick your finger, you expect that
blood to clot within seconds and here we spend
most of our life without any problems of inappropriate
from occasional episodes of thrombosis.
Let us look at a key player in the system,
the platelet. The platelets are shed from
the cytoplasm of megakaryocytes. Megakaryocytes
live in the bone marrow, the very large cells
and they can produce thousands of platelets.
On the right, you will see the diagram of
a platelet. Besides the platelet is small
around three times 0.5 microns and they contain
a number of granules. Just look at the main
features of the cell. They have that gray
canaliculus system. The membrane invaginates
into the platelet and provides a very large
surface area on which coagulation can take
place. The major regulator of platelet production
is thrombopoietin and that is produced by
the liver and the kidneys and the main function
of the platelet is to form a plug on the damaged
vessel. You will see another diagram as well,
the types of granules the platelet has. Alpha
granules containing factors such as platelet
derived growth factor, coagulation factors
and also the dense granules.
The platelet response to vessel injury has three major
components, adhesion, aggregation, and release.
And it is important that we go through each
of these in more detail.
Here we have a nice diagram on the right of
the platelet in pink adhering to a damaged
vessel where the brown represents the exposed
collagen on the damaged vessel. On the top
right is represented the first process in
this reaction platelet adhesion. Look at that
blue molecule, that is GPib on the platelet
surface and that cross-links through a factor
called von Willebrand factor, which binds
to the exposed collagen . So that initially
draw the platelets to the site of damage.
Then another molecule becomes very important.
You will see right at the top of the diagram
is an integrent and is there represented as
alpha2b, beta-3. It is also known as GPIIbIIIa;
that is how I will refer to it during the
next few minutes. GPIIbIIIa gets activated
on the amount of this protein has increased
on the surface of the platelet. As we move
down to the second section, you will see that
it is now being involved in cells, binding
to von Willebrand factor and to collagen.
But what happens at the bottom then becomes
important. The GPIIbIIIa can crosslink with
fibrinogen on the left and von Willebrand
factor on the right to draw in more platelets.
That is the platelet aggregation reaction
and then you can see that we formed a primary
plug on the damaged vessel. Finally, the platelets
get activated and they release the granules
into the microenvironment. You will see on
the bottom there. The activation leading through
a list of ATP and thromboxin and these stimulates
the coagulation reaction and they also lead
to the platelet swelling in size and forming
a more stable plug in the damaged vessel.
But a platelet plug on its own is not sufficient.
It needs to be strengthened by a coagulation
cascade leading to fibrin. This is a remarkable
system. It is what we called a biological
amplification system whereby very small amounts
of initiation substances can proteolytically
activate a cascade of circulating precursor
proteins. It has been estimated the one molecule
can lead to the generation of 200 million
molecules downstream which I think it makes
its remarkable amplification. The key outcome
of this is the generation of thrombin, which
converts fibrinogen into fibrin and it is
this fibrin that enmeshes those platelet aggregates
we have just learned about and converts that
unstable complex into a stable plug in the
vessel. We are going to have to look at the
coagulation cascade in a little bit more detail.
Now, It looks a little complicated. So let
us try and make some sense of this.
Let me take you to the bottom right of that diagram.
We have started the bottom and work out, which
is little unconventional, but I think it makes
some sense and conceive that our aim is to
generate fibrin. We need to enmesh the platelet
plug to produce fibrin, fibrinogen needs to
be activated. For that to be activated we
need to generate thrombin from prothrombin
and the key enzyme that activates prothrombin
is the activated factor Xa. The first of those
traditional factor numbers that you may recognize.
So there are three proteins, factor Xa, prothrombin,
and fibrinogen, which are the core common
pathway that we need to remember in this coagulation
cascade. Traditionally, we have considered
that there are two ways of generating activated
factor X. On the top left you will see what
was known as the extrinsic system where protein
called tissue factor that binds to factor
VIIa and activates factor X. On the bottom,
it is known as the intrinsic system. We will
see factors such as XII, XI, IX being serially
activated and leading again to activation
of factor X. This has been quite useful because
we use common clotting tests to assess these
different systems and you will see those represented
on all the diagram. In dark green is the thrombin
time, which we will learn later is widely
used in medicine, then the bottom activated
partial thromboplastin time. Finally for completeness,
thrombin generating fibrinogen into fibrin
can be measured by the thrombin time.
But our view of the coagulation system is moved
on a little bit and we are trying to update
it now to a little slightly different concept
and we will take you through this.
We now think the factor VIIa and tissue factor are
critical for the initiation of coagulation.
There is a small amount of activated factor
VIIa in our body. If there is low level of
turnover perhaps ready to be fired off when
necessary. Now have an updated view of how
the clotting cascade may work and the key
factors we feel now of factor VIIa and tissue
factor. Factor VIIa is the very protein molecule.
There are very low levels of activated factor
VII around in our blood and whenever there
is vascular injury tissue factor is quickly
activated combines to VIIa. That generates
a combination which can do two things.
In itself, it can generate factor Xa as you see
on the right which remember is one of the
critical common pathways to coagulation. But
also it generates small amounts of factor
IXa as well. The factor Xa can generate some
thrombin, but that is not enough to cause
stable coagulation. However, thrombin itself
can activate three things, which you will
see on the diagram factor VIII, factor V and
factor XI. These together can provide a huge
amplification to this clotting cascade. You
can see how that happens. Factor VIIIa is
a cofactor. It is not an enzyme, but it acts
with activated IXa to form activated factor
X. X can then use the cofactor factor Va to
activate prothrombin and generate thrombin.
Then of course we can generate fibrinogen.
So you can see that the slightly updated view
of the clotting cascade provides a mechanism
for the initial activation and then the explosive
amplification. Try and put this together in
an overview of hemostasis. Just a little left
of the top you will see the damaged vessel
wall. The more we have to get to is the thing
at the bottom with stable hemostatic plug.
So after that damage to the vessel wall, we
are seeing platelet adhesion through von Willebrand
factor and also see some vasoconstriction.
The platelet release reaction as well can
stimulate the vessel to constrict more strongly.
That platelet adhesion leads to aggregation
and platelet release. But we need to trigger
the coagulation system as well and that is
shown on the right-hand side.
Tissue factor is the key factor, which activates this
leading to a coagulation cascade, which takes place
on the platelet granules and on the platelet
membrane. The coagulation system produces
fibrin, which enmeshes platelet plug and makes
the stable hemostatic plug. Of course, no
biological system can go unchecked and we
have seen on the slide some of the negative
regulators of coagulation, protein C and S
and antithrombin and also plasmin, which can
break down the fibrin.
Within the laboratory, we have a number of
tests of hemostatic function. Of course, the
full blood count is the key place to start.
We need to know the number of platelets within
the blood, but clotting tests are also very
important. Two that are particularly want
to focus on today. One is the activated partial
thromboplastin from the APTT and as you will
see on the slide, it measures several factors
VIII, IX, XI and VII and normally in the laboratory
it takes 30 or 40 seconds for blood to clot
in this way. The second major test we often
do is the prothrombin time or the PT.
This measures different factors VII, X, V prothrombin
and fibrinogen and this involves a more rapid
coagulation 10 to 14 seconds. Now you may
have heard of this represented as the INR,
the International Normalized Ratio and that
is where you measure the patient's prothrombin
time and you express it as a ratio of a control
value in the laboratory and the reason is
so key is this is the test we used to monitor
warfarin, which is a very common drug, which
is given to people with thrombotic problems.
On the right, we have a table where we see
some of the common causes of a prolonged prothrombin
time or prolonged APTT. So if you look at
the prothrombin time, you will see top there
is warfarin, which is the drug that we give
to the patients with thrombosis. We will also
see the liver disease and vitamin K deficiency
can lead to this test being prolonged. Indeed
warfarin is an inhibitor of vitamin K and
below that a disorder called disseminated
intravascular coagulation, a really very devastating
condition where coagulation is profoundly
impaired and you will see the APTT and the
prothrombin time has increased. We take the
APTT itself. We will see that heparin is the
first thing we put there and indeed the test
was largely developed to test for how much
heparin we are giving to patients and to monitor
that. But critically it is also increased
in hemophilia, a rare but very important disorder
as we shall see and again a range of other
conditions as well.
Finally on the slide on the bottom left, I
want to mention the test of platelet aggregometry.
So we can measure in the laboratory how well
platelets are able to aggregate in response
to factors such as collagen or ADT and in
patients with platelet functional abnormalities
that will be suppressed.