Thrombi are classified into two major groups depending on their location and the relative amount of platelets and red blood cells. The two major groups are: •
Arterial or white thrombi (characterized by predominance of platelets) •
Venous or red thrombi (characterized by predominance of red blood cells).
Microclots In the
microcirculation consisting of the very small and smallest blood vessels, the
capillaries, tiny thrombi (microthrombi) known as microclots can obstruct the flow of blood in the capillaries. Microclots are small clumps of blood that form within the circulation, possibly as a result of a larger thrombus breaking down into smaller pieces or more likely by accretion. They can be a cause for concern as they can lead to blockages in small vessels and restrict blood flow, leading to tissue damage and potentially causing
ischemic events. This can in turn lead to a form of chronic ischaemia-reperfusion injury and to the generation of autoantibodies. Because of their amyloid nature they are somewhat resistant to thrombolytic agents, which, along with the presence of certain other proteins, explains their persistence. Evidence based on the proteomes of such microclots implies that the macroclots formed in other diseases should also be amyloid in character; this has been shown for
ischaemic stroke. Microclots can cause a number of problems particularly affecting the
alveoli in the
lungs of the
respiratory system, resulting from reduced oxygen supply. Microclots have been found to be a characteristic feature in severe cases of
COVID-19, and in
long COVID. Fibrinaloid microclots can be induced directly via the addition of
SARS-CoV-2 spike protein to 'healthy' plasma, and the fact that the amyloidogenic potential of the spike variant is related to its virulence provides a strong indication that the microclots are on the
aetiological pathway of long COVID. The fibrinaloid microclots also provide a ready explanation for other phenomena such as
Postural Orthostatic Tachycardia Syndrome (POTS), atrial fibrillation, and
fibromyalgia. Fibrinaloid microclots are easily measured using techniques such as fluorescence microscopy and flow cytometry ('flow clotometry').
Mural thrombi Mural thrombi form and adhere on the inner wall of a large
blood vessel or
heart chamber, often as a result of blood stasis. They are most commonly found in the
aorta, the largest
artery in the body, more often in the
descending aorta, and less often in the
aortic arch or
abdominal aorta. They can restrict blood flow but usually do not block it entirely. Mural thrombi are usually found in vessels already damaged by
atherosclerosis. A mural thrombus can affect any heart chamber. When found in the
left ventricle it is often a result of a heart attack complication. The thrombus in this case can separate from the chamber, be carried through arteries and block a blood vessel. They appear grey-red with alternating light and dark lines (known as
lines of Zahn) which represent bands of white blood cells and red blood cells (darker) entrapped in layers of
fibrin. ==Cause==