Cardiovascular Flashcards
What is haemostasis
A physiological process which is initiated when there is damage to a blood vessel. It involves the rapid formation of a solid plug at the site of injury.
It is protective and stops loss of blood from the circulation.
What are the constituents of a haemostatic plug
Platelets, fibrin and RBCs
Explain the process of haemostasis
(1) Endothelial injury leads to adhesion and aggregation of platelets.
(2) Platelets adhere to collagen by vWF and RBCs become enmeshed with the platelets, resulting in the formation of a lose platelet plug
(3) At the same time, exposure of tissue factor initiates the coagulation cascade = formation of fibrin
Fibrin stablises the loose platelet plug
What is the role of the fibrinolytic system
It ensures that the haemostatic plug does not become too big- limits it to the site of injury.
Endothelial injury also initiates fibrinolysis (as well as haemostasis) and it results in plasmin formation - which breaks down fibrin to soluble products.
What is thrombosis
This occurs when there is an inappropriate (ie. pathological) activation of haemostasis which overwhelms the capacity of the fibrinolytic system, resulting in the formation of a solid plug = thrombus.
What are the constituents of a thrombus
The same as a haemostatic plug - platelets, fibrin and RBCs
What is a clot composed of
RBCs and fibrin (no platelets)
Explain the difference between clots and thrombi
(1) Composition Thrombus = RBC, fibrin, platelets Clot = RBC, fibrin (2) Location T = forms within the CVS C = forms outside the CVS (3) Blood T = forms in flowing blood (life) C = forms in stationary blood (during/after life)
What are the 3 components of Virchow’s triad
3 predisposing factors to thrombus formation
(1) Endothelial injury
(2) Abnormal blood flow
(3) Hyper-coagulability
What can cause endothelial injury
- Atherosclerosis
- Vasculitis
- Direct Trauma = heat / chemical injury
What can cause abnormal blood flow
Turbulence:
- Atherosclerosis
- Artificial valves, stents
Stasis:
- Post-op / trauma
- Congestive cardiac failure
- Immobility
- Pelvic obstruction (mass)
- Aneurysms
What can cause hypercoagulability
Too many blood cells: - Erythrocytosis - Thrombocytosis Coagulation factor defects: - Hereditary = factor V Leiden, protein C/S deficiency - Acquired = OCP, malignancy, pregnancy
What is the most important risk factor for thrombosis in an artery
Atherosclerosis (results in endothelial injury and turbulence)
What is the most important risk factor for thrombosis in a vein
Stasis and hyper-coagulability
List 3 main ways in which thrombi can cause disease
- Partial occlusion of the vessel at the site of thombosis
- Complete occlusion at the site of thrombosis
- Embolism to distant site
What are the complications of thrombosis
Partial or complete occlusion results in ischaemia - can progress to infarction.
What is embolism
The occlusion of a vessel by undissolved material that is transported in the blood stream.
(In clinical practice most emboli are thromboemboli)
List the types of emboli
- Thrombi
- Fat/bone marrow
- Air
- Amniotic fluid
- Tumour
- Septic emboli (eg. infective endocarditis)
- Atheromatous debris
Where will emboli originating in the venous system occlude
A pulmonary artery
Symptoms and signs include swollen calf; PE - results in a pulmonary infarct
Where will emboli originating in the arterial system occlude
A systemic artery e.g. mesenteric artery, cerebral artery, renal artery
What is atherosclerotic plaque stability determined by
The balance between:
(A) inflammatory cells = destabilise by making MMPs that digest the fibrous cap
Can also cause the SMC in the intima to under go apoptosis
(B) smooth muscle cells = protective as they produce the fibrous cap stabilising the plaque
Also make TIMPs which inhibit MMPs
What are the features of a stable atherosclerotic plaque
- contains few inflammatory cells and large numbers of SMCs
- thick fibrous cap which is resistant to rupture
- grow slowly = gradual stenosis
What are the features of an unstable ‘vulnerable’ plaque?
- inflammatory cells (foam cells) > smooth muscle cells
- Thinner fibrous cap, which is more prone to rupture = may result in thrombosis/embolism
List 3 main mechanisms by which atherosclerosis causes disease
(1) Gradual enlargement of a stable plaque leading to luminal stenosis and reduced blood flow through the artery
(2) Sudden rupture of a vulnerable plaque
(3) Aneurysm formation