First of all the artery wall (specifically the endothelium) becomes damaged. This can be caused by high blood pressure, ageing, high levels of cholesterol or triglycerides (fatty acids) and/or cigarette smoking.
Due to the inflamed artery wall, white blood cells enter so that they can fight off any infections. However, the white blood cells (attached to the damaged endothelium) accumulate chemicals - especially cholesterol.
Caption: : These hugely contribute to the narrowing of the lumen
Atherosclerosis - stage 5
Calcium salts and fibrous tissue accumulate now which result in a hard plaque forming. The plaque means that the artery wall becomes hardened therefore losing some of its elasticity, This is bad as the artery wall needs to be elastic so that it can return to its original shape if it is widened by the passing of blood. With age the artery wall loses elasticity however atherosclerosis hugely accelerates and exaggerates the problem. The lumen naturally becomes smaller which raises blood pressure further, only escalating the problem.
Caption: : This diagram shows how an artery needs to stretch. If it cannot stretch during systole then blood pressure is raised as there is less space for the blood to flow through
Atherosclerosis - stage 6
The raising blood pressure and narrowing lumens of arteries proceed to cause a dangerous positive feedback system to build up. Plaque leads to rising blood pressure and rising blood pressure leads to more plaques....By this stage some symptoms may begin to emerge; angina and other pains are commonly associated with atherosclerosis.
Once the arteries have become narrower and blood pressure is raised then more arteries become damaged and when a part of the body is damaged the process of blood clotting begins. The clotting is vital to fix any breaks in blood vessels which prevents further damage and infections whilst reducing blood loss. The problem is that the process of blood clotting in atherosclerosis only makes things a lot worse.
Platelets come into contact with the damaged endothelium and become sticky (attaching to the exposed collagen in the artery wall) in order to seal the wound. They all stick together as well and this forms a platelet plug. The platelets emit chemicals which activate more platelets until there are lots of these inside the artery.
Caption: : Blue circle shows where the platelets are collecting and sticking to the exposed collagen
Blood clotting - stage 3
If blood comes into contact with the damaged blood vessel wall (extremely likely) then a cascade of chemical changes happens which results in a blood clot. Firstly, the protein thromboplastin is released from damaged tissue and from the platelets.
If there are a lot of vitamin K and calcium ions present in the blood then the following chemical changes will go ahead: thromboplastin activates an enzyme which converts the protein prothrombin into an enzyme thrombin.
Caption: : The fibrin mesh has formed around the red blood cells and platelets here
The insoluble fibrin forms a mesh to hold the platelets together and traps red blood cells in order to form the clot however this blood clot often blocks the entire artery and as a result the blood flow is stopped or disrupted. This can lead to a stroke or a heart attack.