Arteries: classification and structure

Principles of artery classification

All arteries are classified based on the histological structure of their middle layer — the tunica media. The main criterion is the balance between smooth muscle cells and elastic elements. This ratio changes predictably as blood pressure and blood flow velocity vary.

Three histological types of arteries are distinguished:

• Elastic type (cushioning powerful pressure surges).
• Musculo-elastic type (mixed structure of the vascular wall).
• Muscular type (maintaining the kinetic energy of blood through active contraction).

Elastic-type arteries

This type includes the largest vessels originating directly from the heart, primarily the aorta and pulmonary trunk. They are subjected to maximum blood flow velocity and significant pressure fluctuations. Their main biomechanical task is cushioning the pulse wave: the wall must stretch significantly during systole and elastically return to its original dimensions during diastole, ensuring continuous blood flow.

Features of wall structure (using the aorta as an example):

1. Tunica intima (tunica intima): Includes the endothelium and a massive subendothelial layer formed by loose fibrous connective tissue. This specific layer becomes the site of cholesterol deposition during the development of atherosclerosis. An important distinction is that at the border with the media, there is no continuous elastic membrane, but rather a dense plexus of elastic fibers.
2. Tunica media (tunica media): Forms a powerful elastic framework. The basis consists of 60–70 fenestrated elastic membranes lying concentrically and interconnected by fibers. Smooth myocytes are arranged obliquely between the membranes. Their function here is not pumping blood, but maintaining wall tone and synthesizing extracellular matrix components (collagen, elastin, glycosaminoglycans).
3. Tunica adventitia (tunica adventitia): Represented by loose connective tissue with nerves and collagen fibers. An interesting feature is that the vessels of the vessels (vasa vasorum) penetrate from the adventitia directly into the media to nourish its massive thickness.

Histological view: Elastic elements permeate all three layers. With standard hematoxylin and eosin staining, the membranes do not stain and appear as light wavy bands. Orcein staining selectively reveals elastin, coloring the membranes cherry-red. Glycosaminoglycans of the extracellular matrix are well visualized with toluidine blue (violet-red staining).

Musculo-elastic (mixed)-type arteries

These are transitional vessels, which include major trunk branches departing from the aorta: the carotid, iliac, and subclavian arteries.

Their distinctive morphological features:

• In the tunica intima, beneath the endothelium and a thin subendothelial layer, a clearly defined internal elastic membrane appears (unlike in the aorta).
• In the tunica media, an approximate parity is established: smooth myocytes and elastic structures (fibers and fenestrated membranes) are present in roughly equal amounts.
• In the tunica adventitia, in addition to standard loose connective tissue elements and adipocytes, longitudinal bundles of smooth myocytes are present.

Muscular-type arteries

The vast majority of arteries in our body belong to the muscular type — these are medium- and small-caliber vessels. Their thick muscular wall actively contracts, complementing the work of the heart and maintaining the necessary blood pressure in the peripheral bed.

Wall structure:

• Tunica intima (tunica intima): Consists of endothelium and a very thin subendothelial layer. The key structure is the internal elastic membrane (membrana elastica interna). In histological preparations, it appears as a bright, shiny, convoluted strip. On the glass slide, the inner surface of such arteries often appears folded. This is a fixation artifact: during sample dehydration, smooth myocytes in the media contract sharply, the volume of the media drops, and the non-contractile intima is forced to gather into folds. In a living vessel, this folding is absent.
• Tunica media (tunica media): Is the thickest layer. Almost its entire volume is occupied by powerful circularly directed bundles of smooth myocytes. Collagen and elastic fibers are present here, but against the background of muscles, they are difficult to distinguish. This layer is separated from the adventitia by the external elastic membrane (membrana elastica externa), although it is thinner than the internal one and not visible in every preparation.
• Tunica externa (tunica externa): Built from loose fibrous connective tissue with vasa vasorum and nerves. Unlike the mixed-type vessels, smooth myocytes are completely absent here.

Class and exam questions

What else to study on this topic

• Morphology of arterioles and mechanisms of transition from the arterial bed to capillaries
• Histochemistry of the vascular wall extracellular matrix (glycosaminoglycan staining with toluidine blue)
• Pathomorphology of the subendothelial intimal layer during atherosclerotic plaque formation
• Features of the vasa vasorum system in blood vessels of various types

Other topics in the section "Cardiovascular system: arteries, microvasculature"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.