Brent Cornell

    
Blood Clots

Clotting (haemostasis) is the mechanism by which broken blood vessels are repaired when damaged

• Clotting functions to prevent blood loss from the body and limit pathogenic access to the bloodstream when the skin is broken

There are two key components of a blood clot – platelets and insoluble fibrin strands

• Platelets undergo a structural change when activated to form a sticky plug at the damaged region (primary haemostasis)
• Fibrin strands form an insoluble mesh of fibres that trap blood cells at the site of damage (secondary haemostasis)

Components of a Blood Clot  (Scanning Electron Microscopy)

    
Coagulation Cascade

The process by which blood clots are formed involves a complex set of reactions collectively called the coagulation cascade

• This cascade is stimulated by clotting factors released from damaged cells (extrinsic pathway) and platelets (intrinsic pathway)

The coagulation cascade involves many intermediary steps, however the principal events are as follows:

• Clotting factors cause platelets to become sticky and adhere to the damaged region to form a solid plug
• These factors also initiate localised vasoconstriction to reduce blood flow through the damaged region
• Additionally, clotting factors trigger the conversion of the inactive zymogen prothrombin into the activated enzyme thrombin
• Thrombin in turn catalyses the conversion of the soluble plasma protein fibrinogen into an insolube fibrous form called fibrin
• The fibrin strands form a mesh of fibres around the platelet plug and traps blood cells to form a temporary clot
• When the damaged region is completely repaired, an enzyme (plasmin) is activated to dissolve the clot

Simplified Coagulation Cascade

    
Coronary Thrombosis

Coronary thrombosis is the formation of a clot within the blood vessels that supply and sustain the heart tissue (coronary arteries)

• Occlusion of a coronary artery by a blood clot may lead to an acute myocardial infarction (heart attack)

Blood clots form in coronary arteries when the vessels are damaged as a result of the deposition of cholesterol (atherosclerosis)

• Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis)
• The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress)
• The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall
• As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques
• If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow
• If the thrombus is dislodged it becomes an embolus and can cause a blockage in a smaller arteriole

Causes and Consequences of Coronary Thrombosis