**2. Background**

The role of the circulation is to service the needs of the tissues. It includes transporting nutrients to thecells of the body and waste products away from the cells of the body. And finally transporting hormones from one part of the body to another. Naturally, circulation maintains an appropriate environment in all the tissue fluids of the body for optimal survival and function of the cells.

Systemic circulation, pulmonary circulation, peripheral circulation, etc. have some details to maintain the blood flow. The arteries are the large conductive vessels that transport blood under high pressure to the tissues. The arterioles are the last small branches of the arterial system. And, the capillaries are where the exchange of fluid, electrolytes, nutrients, hormones, and other substances occurs. And, of course veins, venules, collect blood from the capillaries. Our main subject is atherosclerosis of arterial vessels.

We have to learn the normal state before discussing the pathological status. The normal artery wall is consisting of intima, media, and adventitia. Lumen is lined by a monolayer of endothelial cells that overlies smooth muscle cells. The inner layer of smooth muscle cells, known as the intima, is circumscribed by the internal elastic lamina. The media layer is between the internal elastic lamina and external elastic lamina. The media is another layer of smooth muscle cells. Outside the external elastic lamina is an adventitia part that is rarely populated by cells and microvessels of the vasa vasorum.

Atherosclerosis is a progressive disease of medium- and large-sized arteries characterized by focal intimal lesions called atheromas or atherosclerotic plaques that protrude into vessel lumen and eventually leading to various complications [2]. There are several diseases led by atherosclerosis: coronary artery disease, peripheral artery disease, and carotid artery disease. These are real threats for mortality and morbidity in the developed countries.
