Cardiovascular Diseases 3: Atherosclerosis and Embolism

What is cholesterol?

Cholesterol is produced at the liver, but is also consumed when we eat meat and dairy products. In order to test the levels of cholesterol in an individual, a serum lipid test is undergone. The ideal result is less than 5.2mmol/L. When this test is undergone, three types of cholesterol are found in the blood. These are:
  • Triglycerides (0.1-1.9g/L)
  • HDL cholesterol (>0.1mmol/L)
  • LDL cholesterol (<3.2mmol/L)
Most of the cholesterol in our system is carried by LDL (low density lipoproteins), with the smaller portion being carried by HDL (high density lipoproteins). HDL is the more favourable transport system, however, as it transports cholesterol to the liver from tissues for catabolism into excretable bile salts. LDL carries cholesterol to cells for the synthesis of cell membranes and steroids, which is not ideal in large quantities.

What is atherosclerosis?

Atherosclerosis is when there is a build up of plaque in the arteries. Risk increases with age. An atheroma forms over many years, e.g. 40 years, and by then it becomes an old atheroma (see below). After this point, the consequences of myocardial infarct, cerebral infarct, gangrene of extremities and aneurysm begin.

NOTE: thrombosis is a build up of blood, i.e. blood clot, in an artery and an embolism is a moving obstruction, which can be plaque or blood based.

The formation of an atheroma is described below:
  1. Endothelial injury results from predisposing causes: hyperlipidemia, hypertension, smoking, homocysteine, viruses or immune reactions
  2. Lipids accumulate due to the disrupted endothelium and there is increased capillary wall permeability → monocyte + platelet adhesion and monocyte emigration (out of capillary wall into interstitial space)
  3. There is an influx of smooth muscle cells from the sub-endothelial space and macrophages are activated
  4. A young atheroma forms, where smooth muscle cells and macrophages ingest lipids, causing inflammation
  5. An old atheroma forms, meaning smooth muscle proliferates, collagen deposition occurs and extracellular lipid deposits form

As the total serum cholesterol level is increased, the risk of coronary heart disease also increases. High levels of cholesterol pose a threat for vascular problems, e.g. gallstones.

Causes: turbulent flow, thrombotic cap, erosion/ulceration, fissure/haemorrhage

Consequences: myocardial infarct, cerebral infarct, gangrene of extremities, abdominal aortic aneurysm, thrombus formation and weakening of a vessel wall (e.g. in the aorta), ischemia and infarction (e.g. in coronary and cerebral arteries)

What is an embolism?

There a multiple types and shapes and forms of embolism, a common one being thromboembolism, a moving blood clot, and pulmonary thromboembolism, which is a thromboembolism that exists in the pulmonary circuit. Embolism is a consequence of an atheroma that has broken away from the vessel wall.

But then... arteriosclerosis?

Arteriosclerosis is associated with hypertension and diabetes. It is caused by the narrowing of the vessel lumen, leading to an ischemic injury (tissue lacks sufficient blood flow). It only occurs in small arteries and arterioles.

So, high cholesterol and blood pressure → atherosclerosis in either large or medium sized arteries, and hypertension and diabetes → arteriosclerosis in small arteries

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