00:01
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.
00:19
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.
01:16
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
blood clotting
from occasional episodes of thrombosis.
01:31
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.
02:53
The platelet response to vessel injury has three major
components, adhesion, aggregation, and release.
03:01
And it is important that we go through each
of these in more detail.
03:09
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.
03:54
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.
04:31
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.
04:47
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.
05:23
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.
06:24
Now, It looks a little complicated. So let
us try and make some sense of this.
06:31
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 blue 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.
08:26
But our view of the coagulation
system has moved on a little bit
and we are trying to update it now to
a little slightly different concept.
08:35
Let me take you through this.
08:38
We now think the factor VIIa and tissue factor
are critical for the initiation of coagulation.
08:48
There is a small amount of
activated factor VIIa in our body.
08:52
It's as if there is a low level of turnover
perhaps ready to be fired off when necessary.
08:59
But we 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.
09:13
Factor VIIa is a very potent molecule.
09:17
But it's thought there are very low levels
of activated factor VII around in our blood
and whenever there is vascular injury, tissue
factor is quickly activated, and binds to VIIa.
09:32
That generates a combination
which can do two things.
09:38
In itself, it can generate factor
Xa as you see on the right,
which remember is one of the critical
common pathways to coagulation.
09:49
But also it generates small
amounts of factor IXa as well.
09:54
That factor Xa can generate some thrombin, but
that is not enough to cause stable coagulation.
10:02
However, thrombin itself can activate three
things, which you will see on the diagram:
factor VIII, factor V and factor XI.
10:13
These together can provide a huge
amplification to this clotting cascade.
10:20
You can see how that happens.
10:22
Factor VIIIa is a cofactor, it is not an enzyme, but
it acts with activated IXa to form activated factor X.
10:33
X can then use the cofactor factor Va to
activate prothrombin and generate thrombin.
10:43
Then of course we can generate fibrinogen.
10:46
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.
10:59
Try and put this together in
an overview of hemostasis.
11:04
And just on the left at the top you
will see the damage to the vessel wall.
11:08
And what we have to get to is the thing
at the bottom, the stable hemostatic plug.
11:15
So after that damage to the vessel wall, we are
seeing platelet adhesion through von Willebrand factor
and we're also seeing some vasoconstriction.
11:25
The platelet release reaction as well can
stimulate the vessel to constrict more strongly.
11:31
That platelet adhesion leads to
aggregation and platelet release.
11:37
But we need to trigger the coagulation system as
well and that is shown on the right-hand side.
11:43
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.
11:58
The coagulation system produces fibrin,
which enmeshes that platelet plug
and makes the stable hemostatic plug.
12:08
Of course, no biological system can
go unchecked and we see on this slide
some of the negative regulators of
coagulation, protein C and S and antithrombin,
and also plasmin, which
can break down the fibrin.
12:25
Within the laboratory, we have a
number of tests of hemostatic function.
12:30
Of course, the full blood count
is the key place to start.
12:33
We need to know the number of platelets within the
blood, but clotting tests are also very important.
12:40
There are two that I particularly
want to focus on today.
12:43
One is the activated partial
thromboplastin time, 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 seconds
for blood to clot in this way.
13:02
The second major test that we often do
is the prothrombin time or the PT.
13:07
This measures different factors -
VII, X, V prothrombin and fibrinogen.
13:12
And this involves a more rapid
coagulation 10 to 14 seconds.
13:18
Now you may have heard of this represented as
the INR, the International Normalized Ratio
and that is where you measure
a patient's prothrombin time
and you express it as a ratio of
a control value in the laboratory.
13:33
And the reason why this is so key is that
this is the test we use to monitor warfarin,
which is a very common drug, which is
given to people with thrombotic problems.
13:45
On the right, we have a table where
we see some of the common causes
of a prolonged prothrombin
time or prolonged APTT.
13:56
So if you look at the prothrombin time,
you will see top there is warfarin,
which is a drug that we give to
the patients with thrombosis.
14:04
We will also see that liver disease and vitamin K
deficiency can lead to this test being prolonged.
14:12
Indeed, warfarin is an inhibitor of vitamin K.
14:16
And below that, a disorder called
disseminated intravascular coagulation,
really a very devastating condition
where coagulation is profoundly impaired
and then you will see that the APTT
and the prothrombin time increased.
14:33
We take the APTT itself, we will see that
heparin is the first thing that we put there
And indeed, the test was largely developed to test
for how much heparin we were giving to patients
and to monitor that.
14:49
But critically, it is also increased in hemophilia,
a rare but very important disorder as we shall see.
14:57
And again, a range of other conditions as well.
15:00
Finally on the slide on the bottom left, I want
to mention the test of platelet aggregometry.
15:08
So we can actually measure in the laboratory
how well platelets are able to aggregate
in response to factors such as collagen or ADP.
15:18
And in patients with platelet functional
abnormalities, that will be suppressed.