2. Arteries can be plugged by thrombus or embolus in the lumen
A thrombus 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.
These 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.
This patient suffered sudden blockage of his right internal carotid artery and survived for
several years although he was extremely disabled by paralysis of his left lower face, arm, and
leg, loss of sensation on his left side, vision problems, and left-sided neglect. Can you find
the infarcted brain tissue?
Thrombus formation 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 stroke.
An embolus 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.
Three important sources of emboli to the brain
• Thrombus that initially formed within the diseased heart
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.
• Thrombus 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.
• Thrombus 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).
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.
