Blood clots

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A thrombus, or blood clot, is the final product of the blood coagulation process in hemostasis. It is achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral coagulation system (i.e. clotting factors). A thrombus is physiologic in cases of injury, but pathologic in case of thrombosis.

Specifically, a thrombus is a blood clot in an intact blood vessel. A thrombus in a large blood vessel will decrease blood flow through that vessel. In a small blood vessel, blood flow may be completely cut-off resulting in the death of tissue supplied by that vessel. If a thrombus dislodges and becomes free-floating, it is known as an embolus.

Some of the conditions in which blood clots develop include atrial fibrillation (a form of cardiac arrhythmia), heart valve replacement, a recent heart attack, extended periods of inactivity (see Deep vein thrombosis), and genetic or disease-related deficiencies in the blood's clotting abilities. Estrogen administered as part of a hormone replacement therapy can also cause Deep vein thrombosis, especially in patients over 40. For this reason, dosages are kept as low as possible and administration methods are used to minimize this risk, such as sublingual (under the tongue) or transdermal (patches) delivery strategies.

Preventing blood clots reduces the risk of stroke, heart attack and pulmonary embolism. Heparin and warfarin are often used to inhibit the formation and growth of existing blood clots, thereby allowing the body to shrink and dissolve the blood clots through normal methods.

A thrombus differs from a haematoma by:

  • Having high haematocrit (the ratio of the volume occupied by packed red blood cells to the volume of the whole blood as measured by a hematocrit)
  • Being non-laminar
  • Being soft and friable
  • Having an absence of circulation

Virchow's Triad describes the conditions necessary for thrombus formation:

1) Changes in vessel wall morphology (e.g. trauma, atheroma) 2) Changes in blood flow through the vessel (e.g. valvulitis, aneurysm) 3) Changes in blood composition (e.g. leukaemia, hypercoagulability disorders)

Disseminated intravascular coagulation (DIC) involves widespread microthrombi formation throughout the majority of the blood vessels. This is due to excessive consumption of coagulation factors and fibrinolysis using all of the body's available platelets and clotting factors. The end result is ischaemic necrosis of the affected tissue/organs and spontaneous bleeding due to the lack of clotting factors. Causes are septicaemia, acute leukaemia, shock, snake bites or severe trauma. Treatment involves the use of fresh, frozen plasma to restore the level of clotting factors in the blood.


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