Bearing in mind that arteriosclerosis (called by some "arteriocapillary fibrosis," by others "atherosclerosis") is not a true disease, it may, for convenience be defined as a chronic disease of the arteries and arterioles, characterized anatomically by increase or decrease of the thickness of the walls of the blood vessels, the initial lesion being a weakening of the middle layer caused by various toxic or mechanical agencies. This weakness of the media leads to secondary effects, which include hypertrophy or atrophy of the inner layer—and not infrequently hypertrophy of the outer layer—connective tissue formation and calcification in the vessels, and the formation of minute aneurysms along them. The term arteriocapillary fibrosis has a broader meaning, but is a cumbersome phrase, and conveys the idea that the capillary changes are an essential feature of the process, whereas these are for the most part secondary to the changes in the arteries. The veins do not always escape in the general morbid process, and when these are affected the whole condition is sometimes called vascular sclerosis or angiosclerosis.
Upon the anatomical structure of the arteries depends, as a rule, the character and extent of the arteriosclerotic lesions. For the clear comprehension of the process, it is necessary to keep in mind the essential histological differences between the aorta and the larger and smaller branches of the arterial tree.
The vascular system is often likened to a central pump, from which emanates a closed system of tubes, beginning with one large distributing pipe, which gives rise to a series of tubes, whose number is constantly increasing at the same time that their caliber is decreasing in size. From the smallest of these tubes, larger and larger vessels collect the flowing blood, until, at the pump, two large trunks of approximately the same area as the one large distributing trunk empty the blood into the heart, thus completing the circle. This is but a rough illustration, and, while possibly useful, takes into account none of the vital forces which are constantly controlling every part of the distributing system.
General Structure of the Arteries
The aorta and its branches are highly elastic tubes, having a smooth, glistening inner surface. When the arteries are cut open, they present a yellowish appearance, due to the large quantity of elastic tissue contained in the walls. The elasticity is practically perfect, being both longitudinal and transverse. The essential portion of any blood vessel is the endothelial tube, composed of flat cells cemented together by intercellular substance and having no stomata between the cells. This tube is reinforced in different ways by connective tissue, smooth muscle fibers, and fibroelastic tissue. Although the gradations from the larger to the smaller arteries and from these to the capillaries and veins are almost insensible, yet particular arteries present structural characters sufficiently marked to admit of histological differentiation.
The whole vascular system, including the heart, has an endothelial lining, which may constitute a distinct inner coat, the tunica intima, or may be without coverings, as in the case of the capillaries. The intima (Fig. 1) consists typically of endothelium, reinforced by a variable amount of fibroelastic tissue, in which the elastic fibers predominate. The tunica media is composed of intermingled bundles of elastic tissue, smooth muscle fibers, and some fibrous tissue. The adventitia or outer coat is exceedingly tough. It is usually thinner than the media, and is composed of fibroelastic tissue. This division into three coats is, however, somewhat arbitrary, as in the larger arteries particularly it is difficult to discover any distinct separation into layers.
Fig. 1.—Cross section of a large artery showing the division into the three coats; intima, media, adventitia. The intima is a thin line composed of endothelial cells. The wavy elastic lamina is well seen. The thick middle coat is composed of muscle fibers and fibroelastic tissue. The loose tissue on the outer (lower portion of cut) side of the media is the adventitia. (Microphotograph, highly magnified.)
The muscular layer varies from single scattered cells, in the arterioles, to bands of fibers making up the body of the vessel in the medium-sized arteries and veins.