Thrombosis or intravascular blood clotting refers to coagulation of blood inside the blood vessels. Normally, blood does not clot in the blood vessel because of some factors which are already explained. But some abnormal

conditions cause thrombosis.

Causes of Thrombosis

1. Injury to blood vessels

During infection or mechanical obstruction, the endothelial lining of the blood vessel is damaged and it

initiates thrombosis.

2. Roughened endothelial lining

In infection, damage or arteriosclerosis, the endothelium becomes rough and this initiates clotting.

3. Sluggishness of blood flow

Decreased rate of blood flow causes aggregation of platelets and formation of thrombus. Slowness of blood flow occurs in reduced cardiac action, hypotension, low metabolic rate, prolonged confinement to bed and immobility of limbs.

4. Agglutination of RBCs

Agglutination of the RBCs leads to thrombosis. Agglutination of RBCs occurs by the foreign antigens or

toxic substances.

5. Toxic thrombosis

Thrombosis is common due to the action of chemical poisons like arsenic compounds, mercury, poisonous mushrooms and snake venom.

6. Congenital absence of protein C

Protein C is a circulating anticoagulant, which inactivates factors V and VIII. Thrombosis occurs in the absence of this protein. Congenital absence of protein C causes thrombosis and death in infancy.

Complications of Thrombosis

1. Thrombus

During thrombosis, lumen of blood vessels is occluded. The solid mass of platelets, red cells and/or clot, which obstructs the blood vessel, is called thrombus. The thrombus formed due to agglutination of RBC is called agglutinative thrombus.

2. Embolism and embolus

Embolism is the process in which the thrombus or a part of it is detached and carried in bloodstream and

occludes the small blood vessels, resulting in arrests of blood flow to any organ or region of the body. Embolus is the thrombus or part of it, which arrests the blood flow. The obstruction of blood flow by embolism is common in lungs (pulmonary embolism), brain (cerebral embolism) or heart (coronary embolism).

3. Ischemia

Insufficient blood supply to an organ or area of the body by the obstruction of blood vessels is called ischemia. Ischemia results in tissue damage because of hypoxia (lack of oxygen). Ischemia also causes discomfort necrosis.

4. Necrosis and infarction

Necrosis is a general term that refers to tissue death caused by loss of blood supply, injury, infection,

inflammation, physical agents or chemical substances.  Infarction means the tissue death due to loss of

blood supply. Loss of blood supply is usually caused by occlusion of an artery by thrombus or embolus and sometimes by atherosclerosis . Area of tissue that undergoes infarction is called

infarct. Infarction commonly occurs in heart, brain, lungs, kidneys and spleen.

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