Stages of Atheroscleroris

The endothelium (delicate layer of cells that lines the inside of an artery, seperating the blood that flows along the artery from the muscular wall) becomes damaged. 
Endothelial damage can occur from a high blood pressure, which would put strain on the layer of cells.
Or it could result from some of the toxins found in the bloodstream from cigarette smoking. 

There is an inflammatory response once the inner lining of the artery is breached. 
The white blood cells leave the blood vessel and move to the artery wall.
These cells accumulate chemicals from the blood, like cholesterol.
A deposit then builds up, called an atheroma. 

There is more build up at the site from Calcium salts and fibrous tissue, which results in the hard swelling of plaque to form on the inner wall of the artery.
the build up of fibrous tissue means that the artery wall loses some of its elasticity. 

The plaque causes the artery to become narrower, which makes it more difficult for the heart to pump blood, therefore leading to a rise in blood pressure.
now there is a positive feedback building up.
The raised blood pressure due to plaques makes it more likely that further plaques will form.