13.02.1 Thrombus Formation

On the arteriolar side to the circulation, white/arterial thrombus form, and the whiteness is due to the presence of platelets, which are white cells. When there is damage to blood vessel wall, this can lead to bleeding. In order to repair blood vessels, platelets stick to the damaged area, and then platelet-platelet aggregation occurs at the site. Normal platelet aggregation is good, as it is involved in the repair process. However, if the platelet aggregation is excessive, this may occlude the areas of slower arterial blood flow. This leads to more local bleeding, and the capture of red blood cells can lead to a red thrombus around the white thrombus. The problem caused is local ischaemia, with the tissues normally serviced by the blood vessel no longer receiving oxygen or nutrients. If the artery is a coronary artery supplying the blood to the heart, and the artery has excessive atherosclerosis associated with platelet-platelet aggregation, this leads to transient ischaemic attacks (typical angina). Typical angina progresses to be unstable, and then leads to myocardial infarction. In the brain, if the carotid artery is similarly affected, this leads to stroke.

On the venous side, red or venous thrombus form, with the redness due to the red blood cells. The blood flow is slower in the veins, and this accounts for the differences in the thrombus. Whereas arteriolar thrombi are layered, blood clot on the venous side, are a mix of platelet-platelet aggregation, fibrin, and red blood cells. Venous thrombi are easily detached, and move in the blood stream to smaller veins, which they block. The problems caused are that thrombi from the calf/deep vein can move around the circulation to cause embolization of the pulmonary arteries, which can be deadly.