Atherosclerotic causes

The inner lining of the arteries (the epitheliumThe outer layer of cells covering the open surfaces of the body, both over external surfaces and lining hollow structures.) can be damaged by a number of factors: high bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure, high levels of fatty substances called lipoproteins in the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid., or chemical toxins from cigarette smoke, for example.[1]

This damage to the epitheliumThe outer layer of cells covering the open surfaces of the body, both over external surfaces and lining hollow structures. means that it no longer acts as a protective barrier, so that several substances are able to pass through it from the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid.. These substances make up the harmful plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth.. For example, lipoproteins and white bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. cells pass through the abnormal epitheliumThe outer layer of cells covering the open surfaces of the body, both over external surfaces and lining hollow structures. and infiltrate the arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood..

One type of white bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. cellThe basic unit of all living organisms., called a T-cellA kind of lymphocyte, a type of white blood cell that fights infection., brings about inflammationThe body’s response to injury., and also stimulates the production of increased numbers of smooth muscleA type of muscle responsible for actions in the body that we are not aware of; for example, the muscles that constrict blood vessels. cells. Smooth muscleTissue made up of cells that can contract to bring about movement. cells create a tough fibrous substance at the site.[1] All of these factors contribute to the developing plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth..

Ongoing inflammationThe body’s response to injury., with more invasion by lipoproteins and white bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. cells, means that the plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. continues to grow.[1]

Some plaques are more unstable than others and so are more likely to rupture, in other words, to break apart. If this happens, the inner contents come into contact with bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flowing through the arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood..

The contents of the ruptured plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. act as a sort of framework for the formation of a bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. clot, or thrombusA blood clot.. This clot may partially or completely block the arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. so that bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow to the heart is restricted.[1]

If an arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. is completely blocked by a ruptured plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth., this may cause a heart attackThe death of a section of heart muscle caused by an interruption in its blood supply. Also called a myocardial infarction., the death of a section of heart muscleTissue made up of cells that can contract to bring about movement.. If the bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow to the heart is reduced, but not blocked entirely, either by a clot or by plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. itself, this may not be enough to cause a heart attackThe death of a section of heart muscle caused by an interruption in its blood supply. Also called a myocardial infarction.; in this case, the person may have chest pain, called anginaA central chest pain caused by insufficient oxygen supply to the heart., or he or she may feel nothing at all.[1]

Not all types of plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. are prone to rupture; in fact, some atherosclerotic plaques may be quite stable. Sometimes, stable plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. is called 'hard plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth.', while unstable plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. is called 'soft plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth.'.[1]

Hard plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. both restricts bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow in an arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. and makes its walls less flexible, so the arteryA blood vessel that carries blood away from the heart. Apart from the pulmonary artery and umbilical artery, all arteries carry oxygenated blood. is less able to open up to increase bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow at times of higher demand, such as during exercise. This may lead to anginaA central chest pain caused by insufficient oxygen supply to the heart.. Soft plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. is more prone to rupture. Although not all ruptured plaques cause symptoms,[1] soft plaqueAny flat, raised patch; for example, a raised patch on the skin, fatty deposit in the inner wall of an artery, or layer over the surface of a tooth. is responsible for most heart attacks.

References: 
  1. Subclinical atherosclerosisDisease leading to fatty deposits in the inner walls of the arteries, which reduce and may eventually obstruct blood flow.: what it is, what it means and what we can do about it. Toth PP. Int J Clin Pract 2008;62(8):1246-54.