Biomedical Engineering Reference
In-Depth Information
5.11.2 Platelet Activation/Thromboembolism
Another condition that typically occurs in parallel with cardiovascular diseases is
enhanced platelet activation. Platelets can become activated after exposure to a short dura-
tion high-intensity shear stress or other altered shear stresses. Activated platelets express
negative phospholipids on the exterior leaflet of their cell membrane, release vasoactive
compounds, release cytokines, and release growth factors. The products that platelets
release upon activation have the effect of activating more platelets, inducing a chain reac-
tion that activates many platelets. Due to all of these processes, it is possible for platelets
to begin to aggregate onto the endothelial cell wall and to other platelets, if the activated
platelets are not removed from the circulation.
Platelet aggregates are basically small blood clots that are termed a thrombus. A throm-
bus can act like a plaque because it blocks the blood flow and reduces the cross-sectional
area of the blood vessel. At this point, it is possible for fibrinogen to be cleaved into fibrin
and adhere to activated platelets within the aggregate. This increases the inter-cellular
strength of the thrombus, making it harder to remove it from the circulation. As fibrin
adheres to the thrombus, red blood cells and platelets can be caught within the growing
thrombus. When the thrombus becomes larger, the shear forces that act on it also increase
(due to the acceleration of blood around the thrombus). If the shear forces exceed the
strength of the inter-cellular/inter-molecular bonds, a portion of the thrombus can break
off and enter the circulation. A portion of the thrombus that breaks off of the main throm-
bus is termed a thromboembolus or embolus (pl. emboli; in general, emboli can be any
abnormal solid mass flowing within the cardiovascular system).
Emboli that enter the circulation can have many devastating effects on the patient.
Emboli have size and mass associated with them. As they pass through the circulatory sys-
tem, it is possible that emboli with different sizes become lodged in different sections of
the cardiovascular system. This will cease the blood flow to that particular region and
depending on the region, it can cause death. If the emboli become stuck in the cerebral cir-
culation, a stroke can occur. If they become stuck in the cardiac circulation, a heart attack
or myocardial infarction can occur. In other instances, free-flowing emboli can recruit
more platelets to them and potentially adhere back to a damaged endothelial cell wall.
Whatever the outcome of the embolus is, it can have devastating effects within the body.
5.11.3 Aneurysm
An aneurysm (
Figure 5.23
) is a bulge in the wall of an artery that is typically caused by
a weakening of the muscles within the vascular wall. Because arteries carry blood under
high pressure, they are more susceptible to aneurysm formation. This typically occurs
when a patient has high blood pressure, which exceeds the viscoelastic mechanical proper-
ties of the blood vessel wall. The bulge in the blood vessel looks like a bubble within the
vascular system. If the vessel wall has weakened to such a great extent that it cannot main-
tain the blood pressure any longer, the aneurysm ruptures. This typically occurs in all
aneurysms which have developed for a long period of time, because as the wall begins to
weaken, it cannot withstand the pressure forces and weakens further. As the blood vessel
wall continues to weaken, the bulge in the blood vessel becomes larger and the likelihood
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