can be plugged by thrombus or embolus in the lumen
is a solid mass of platelets and/or
fibrin (and other components of blood) that forms locally in a vessel. Thrombi
form when the clotting mechanism is activated. This is supposed to happen
when you are injured. However, it can also occur at the site of an ulcerated atherosclerotic
plaque or wherever the endothelial cells lining the inner surface of an artery
have been damaged.
are cross-sections of the common carotid artery through its bifurcation
into the internal and external carotids and of the smaller internal
carotid. You have already identified plaque in the walls that
narrows the vessels. Can
you find a section in which stringy reddish-brown thrombus further occludes
the lumen? Remember
that thrombus tends to form on the surfaces of plaques where the lining
of the vessel has been damaged.
patient suffered sudden blockage of his right internal carotid artery
and survived for several years although he was extremely disabled
of his left lower face, arm, and leg, loss of sensation on his left
side, vision problems, and left-sided
you find the infarcted brain tissue?
also occurs in places where blood flow is sluggish, enabling clotting factors
to accumulate and giving platelets more opportunity to stick together. Disorders
of blood cells (for instance sickle cell disease) or blood proteins can increase
the chance of thrombus formation and therefore contribute to the risk of ischemic
is most often a piece of a thrombus that has broken free and is carried toward
the brain by the bloodstream. The term thromboembolus is used a lot because
it turns out that most emboli arise from thrombi. However, bits of plaque,
fat, air bubbles, and other material also qualify as emboli. Presumably an
embolus floats along with the flowing blood until it encounters a narrowing
in an artery through which it cannot pass. When the embolus gets stuck, it
blocks the artery. This reduces blood flow to downstream tissues and causes
them to become ischemic.
important sources of emboli to the brain
initially formed within the diseased heart
Regardless of its source, an embolus does its damage by getting stuck in a large
artery or branch and blocking blood flow beyond that point.
For example, the irregular, ineffective contractions of the heart muscle seen
in atrial fibrillation lead to blood pooling in the left atrium and increased
thrombus formation. Bits of these thrombi enter the systemic circulation
as emboli that can travel to any organ, including the brain.
that forms on a heart valve
This occurs more frequently in the left heart, because the mitral and aortic
valves are slammed shut by higher pressures. When the endothelium that usually
covers a valve is damaged, it exposes the underlying tissue which is highly
thrombogenic. A thrombus on a heart valve is sometimes called a vegetation.
This is because it often looks like a branching bush that is attached to the
valve by its main stem.
associated with atherosclerotic plaque formed in extra- or intracranial parts
of a carotid or vertebral artery
A bit of the thrombus breaks off and is carried more distally in the same vessel
by the flowing blood (this is sometimes called artery-to-artery embolism).
The diameters and branching
patterns of the large arteries seem to have a lot to do with where embolic material
tends to travel and where it tends to ultimately lodge. For instance, the
large diameter, gently curving course, and rapid blood flow in the middle cerebral
artery put it at particular risk for embolism--and therefore the regions of brain
that it supplies at risk for embolic stroke. The smaller anterior cerebral
artery, which originates from the internal carotid at a sharper angle, captures
emboli less often--emboli apparently don't corner well!