To understand the possible disorders associated with the blood clotting
(coagulation) system, consider the goal of the system: to prevent excess
bleeding. If the clotting system can not adequately form clots (thrombi),
then the result is a bleeding disorder (hemophilia); if the clotting system
forms clots too easily, then the result is formation of excess clots (thrombophilia).
For more information on how blood forms clots, please see the web page
on general clotting information.
Platelet disorders occur when there are too few platelets, too many platelets
or a normal number of platelets that do not function in the normal manner.
Having too few platelet or platelets that do not function well (for example,
aspirin use) can lead to a bleeding tendency (hemophilia). Likewise, too
many platelets can predispose to a tendency to clot excessively (thrombophilia).
Platelet function can be altered in many different situations; many medications
to treat diseases such as strokes and heart attacks were specifically
designed to alter the ability of platelets to form clots. There are also
a number of diseases that can alter how well a platelet can function.
Coagulation Cascade Disorders:
The second portion of the clotting system, the coagulation cascade, also
has the potential to cause both an inability to form clots (hemophilia)
or an excessive ability to form clots (thrombophilia). Hemophilic states
result when there are decreased levels of the clotting factors (depicted
in the above diagram). There are two primary disorders, hemophilia A and
hemophilia B. Hemophilia A results from low levels of factor VIII and
hemophilia B results from low levels of factor IX. Low levels of virtually
any of the factors in the above diagram (with the exception of factor
XII) will result in an inability to form blood clots; this will result
in some degree of excess bleeding.
To understand the problems associated with excess clotting, consider
the goal of the primary functions of blood in the body. Blood contains
numerous cells and proteins and those components all have vital roles
in the human body. However, the primary function of blood is to deliver
oxygen throughout the body, which is accomplished by red blood cells.
Blood receives its oxygen supply from the lungs and is then transported
from the heart in blood vessels called arteries to the rest of the body.
Once the blood has delivered its oxygen, it is returned to the heart in
blood vessels called veins.
Arterial Thrombotic Disease:
Clots (thrombi) that form in arteries can limit and/or completely block
the supply of blood (and thus oxygen) to a portion of the body. Lack of
oxygen to a tissue is referred to as ischemia. Prolonged ischemia can
lead to tissue death, referred to as infarction. This process can occur
anywhere in the body. For example, a heart attack (myocardial infarction)
occurs when the blood flow that provides oxygen to the heart is blocked.
A stroke (cerebrovascular accident) occurs when the blood flow that provides
oxygen to the brain is blocked. In both cases, there are numerous risk
factors that can predict a susceptibility to these disorders. Many of
these are not modifiable such as age and sex. There are other risk factors
that can be changed: smoking, obesity, inactive (sedentary) lifestyle,
uncontrolled high blood pressure (hypertension) and uncontrolled high
Venous Thrombotic Disease:
Clots that form in veins can block the flow of blood back to the heart.
Again, these clots can form in veins throughout the body, but one of the
most common places is in the deep veins of the legs. When a clot forms
in one of these veins (called a deep vein thrombosis or DVT), it can cause
redness, swelling, warmth and pain. Portions of this clot can break off
from the main clot, becoming an embolus (an embolus is any foreign object
traveling in the blood stream). The embolus travels in the blood stream
and can travel through the heart and into the lung, giving rise to a clot
in the lung (pulmonary embolus). A pulmonary embolus blocks blood flow
to the lungs, preventing adequate oxygenation of the blood. As with strokes
and heart attacks, pulmonary emboli can be small and asymptomatic or large
Every year, approximately 250,000 patients develop a deep venous thrombosis
(DVT's). Of these patients, between 30-50% develop a pulmonary embolus,
some of which can be fatal
Secondary Disease States Affecting Clotting Processes:
There are a number of diseases that can alter the clotting process; although
these are not the main focus of this website, it is important to briefly
mention a few as these can interact with other disorders, thus increasing
the possibility of forming a clot. Many types of cancer, auto-immune diseases
(for example lupus), post-operative states (especially following knee
and hip surgery) and any condition which results in prolonged immobiliation.
Thrombophilic states can increase the likelihood of clotting problems,
as is discussed on the WebPages specific to the disorders. However, even
without any known thrombophilic state or risk factor, one can develop
arterial and/or venous thrombi. There are numerous factors and disorders
that can predispose one to forming clots and it has been only recently
that physicians and scientists have begun to understand such disorders.
It is quite likely that further disorders remain to be discovered which
may provide further insight into how and why unwanted clots form and ultimately
how they can be treated and/or prevented.