Arteries: Classification and Structure

Principles of Arterial 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 in response to varying blood pressure and flow velocity.

There are three main histological types of arteries:

• Elastic type (absorb powerful pressure fluctuations).
• Musculoelastic (mixed) type (mixed vessel wall structure).
• Muscular type (maintain the kinetic energy of blood through active contraction).

Elastic Arteries

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

Structural features of the layers (using the aorta as an example):

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

Histological appearance: Elastic elements permeate all three layers. With standard hematoxylin and eosin (H&E) staining, the membranes do not stain well and appear as pale wavy bands. Orcein staining selectively highlights elastic tissue, turning the membranes cherry-red. Glycosaminoglycans of the extracellular matrix are well visualized with toluidine blue (purple-red staining).

Musculoelastic (Mixed) Arteries

These are transitional vessels that include the large major branches originating from the aorta: the common carotid, common iliac, and subclavian arteries.

Their distinctive morphological features:

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

Muscular Arteries

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

Layer structure:

• Tunica intima: Consists of endothelium and a very thin subendothelial layer. A key structure is the internal elastic membrane (membrana elastica interna). In histological sections, it appears as a bright, shiny, convoluted strip. On glass slides, the internal 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 middle layer drops, and the non-contractile intima is forced to fold. In a living vessel, this folding is absent.
• Tunica media: The thickest layer. Almost its entire volume is occupied by powerful circularly arranged bundles of smooth myocytes. Collagen and elastic fibers are present here, but against the background of muscle cells, they are difficult to distinguish. An external elastic membrane (membrana elastica externa) separates this layer from the adventitia, though it is thinner than the internal one and not visible in every preparation.
• Tunica externa (adventitia): Constructed from loose fibrous connective tissue containing vasa vasorum and nerves. Unlike mixed-type vessels, smooth myocytes are completely absent here.