artery

ARTERIOSCLEROSIS AND HYPERTENSION

CHAPTER I.

ANATOMY

With the increased complexity of our modern life comes increased wear and tear on the human organism. "A man is as old as his arteries" is an old dictum, and, like many proverbs, the application to mankind today is, if anything, more pertinent than it was when the saying was first uttered. Notwithstanding the fact that the average age of mankind at death has been materially lengthened—the increase in years amounting to fourteen in the past one hundred years of history—clinicians and pathologists are agreed that the arterial degeneration known as arteriosclerosis is present to an alarming extent in persons over forty years of age. Figures in all vital statistics have shown us that all affections of the circulatory and renal systems are definitely on the increase. "Arterial diseases of various kinds, atheroma, aneurysm, etc., caused 15,685 deaths in 1915, or 23.3 per 100,000. This rate, although somewhat lower than the corresponding ones for 1912 and 1913, is higher than that for 1914, and is very much higher than that for 1900, which was 6.1."

The great group of cases of which cardiac incompetence, aneurysm, cerebral apoplexy, chronic nephritis, emphysema, and chronic bronchitis are the most frequent and important appear as terminal events in which arteriosclerosis has probably played an important part.

Thus, in the sense in which we speak of tuberculosis or pneumonia as a distinct disease, we can not so designate the diseased condition of the arteries.

Arteriosclerosis is not a disease sui generis. It is best viewed as a degeneration of the coats of the arteries, both large and small resulting in several different more or less distinct types.

These types blend one into the other and in the same patient all types may be found. Thus the sclerosis of the arteries is the result of a variety of causes, none of which is definitely known in the sense of a bacterial disease. As we shall see later, one type of arteriosclerosis has a special pathology and etiology, the syphilitic arterial changes.

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.