Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Cardiovascular System
Blood Vessels
Structure of Blood Vessels

The Structure of a vessel wall, and consequently its Classification into a specific type, depends on hemodynamic conditions (Blood flow velocity, blood pressure, distance of the artery from The Heart, venous Location) and the vessel's function (micrographs 14–18). The largest vessels primarily perform a transport function. Muscular-type Arteries, In addition to conducting blood, regulate blood flow to specific Organs or Tissues. Capillaries ensure Metabolic exchange between the blood within the vascular lumen and the surrounding tissues.

Arteries

Elastic-type arteries (arteriae elastotypica) are large-caliber vessels, such as the aorta and pulmonary artery, which receive blood under high pressure (120–130 mm Hg).

The walls of these vessels consist of the following tunics:

a) the tunica intima, which includes the endothelium, the subendothelial layer (stratum subendotheliale), and a plexus of elastic fibers (plexus fibroelasticus);

b) the tunica media, formed by numerous (40–50) fenestrated elastic membranes interconnected by elastic fibers;

c) the tunica adventitia (externa), composed of loose Connective Tissue containing numerous thick elastic and Collagen fibers.

Mixed-type, or musculo-elastic, arteries (aa. mixtotypicae) include the carotid and subclavian arteries.

The walls of these arteries consist of the following tunics:

a) the tunica intima, formed by endothelium resting on a basal lamina, a subendothelial layer, and an internal elastic membrane;

b) the tunica media, composed of approximately equal proportions of smooth Muscle Cells, helically arranged elastic fibers, and fenestrated elastic membranes. A small number of fibroblasts and collagen fibers are found between the smooth muscle cells and elastic elements;

c) the tunica adventitia, which comprises two layers: an inner layer containing isolated bundles of smooth muscle cells, and an outer layer consisting predominantly of longitudinally and obliquely oriented bundles of collagen and elastic fibers, as well as connective tissue cells. It also contains vasa vasorum and nerve fibers.

Muscular-type arteries (aa. myotypicae). These predominantly include medium- and small-caliber vessels.

The walls of these vessels also consist of three tunics:

1) the tunica intima, comprising endothelium with a basal lamina, a subendothelial layer, and an internal elastic membrane;

2) the tunica media, consisting of spirally arranged smooth muscle cells interspersed with sparse fibroblast-like connective tissue cells, collagen, and elastic fibers;

3) the tunica adventitia, composed of loose connective tissue where the fibers run primarily in oblique and longitudinal directions. Nerves and Blood Vessels are located within this tunic.

Veins

Veins (venae) form the efferent arm of The Vascular System. Low blood pressure (15–20 mm Hg) and slow blood flow velocity determine the relatively weak development of elastic elements and their greater distensibility.

Fibrous-type veins (venae fibrotypicae). These include non-muscular VEINS OF THE dura and pia mater, retinal veins, bone veins, as well as splenic and placental veins.

The endothelial cells lining these veins exhibit more tortuous borders than those in arteries. A distinct tunica media is absent in these vessels. The tunica adventitia is formed by a thin layer of loose connective tissue fused with the surrounding tissues.

Muscular-type veins (venae myotypicae) are subdivided into veins with weakly developed and moderately developed muscular elements. Veins with weakly developed muscular elements include small- and medium-caliber veins (up to 1.0–2.0 mm). These include the tributary veins of the upper torso, neck, face, and the SUPERIOR VENA CAVA.

The walls of these vessels are thinner than those of Arteries of the same caliber, contain fewer muscular elements, and typically appear collapsed in histological preparations. Furthermore, they feature a poorly defined subendothelial layer.

Veins with moderately developed muscular elements include the brachial vein. Its tunica intima forms valve structures. The lining endothelial cells are shorter than those in the corresponding artery. The subendothelial layer consists of longitudinally arranged connective tissue fibers and cells. A network of elastic fibers is located at the boundary between the intima and media. The tunica media is significantly thinner than its arterial counterpart and consists of circularly arranged bundles of smooth myocytes separated by layers of connective tissue. The tunica adventitia is exceptionally well-developed, with dimensions 2 to 3 times greater than the media. Its collagen and elastic fibers run predominantly longitudinally, with occasional isolated muscle cells present.

Veins with strongly developed muscular elements are large veins of the lower torso and lower extremities. They are characterized by well-developed bundles of smooth muscle cells across all three tunics, running longitudinally in the intima and adventitia, and circularly in the media.

The STRUCTURE OF THE femoral vein is most typical of this group of veins.

The tunica intima consists of the endothelium and a subendothelial layer formed by loose Fibrous connective tissue containing longitudinally oriented bundles of smooth muscle cells. The tunica intima forms Valves, which appear as its thin folds. The core of the valve is composed of fibrous connective tissue covered with endothelial cells. On the luminal side of the valve, predominantly elastic fibers lie beneath the endothelium, whereas on the opposite side, collagen fibers are abundant.

The tunica media contains bundles of circularly arranged smooth muscle fibers. Above the Base of the valve, the tunica media thins out, while below the valve attachment site, the muscle bundles interlace, forming a thickening in the venous wall.

The tunica adventitia is formed by fibrous connective tissue, which contains bundles of longitudinally oriented smooth muscle cells.

The structure of the INFERIOR VENA CAVA differs markedly from the veins that empty into it. Its tunica intima and tunica media are relatively poorly developed. The tunica adventitia of the inferior vena cava features numerous longitudinally arranged bundles of smooth muscle cells. The thickness of the tunica adventitia is 6–7 times greater than the combined thickness of the tunica intima and tunica media. Layers of fibrous connective tissue lie between the bundles of smooth muscle cells.

Blood capillaries

Blood capillaries (vasa haemocapillaria) are the most numerous and finest vessels.

The Capillary Wall consists of three very thin layers:

— an inner layer, represented by endothelial cells resting on a basement membrane;

— a middle layer, consisting of pericytes embedded within the basement membrane;

— an outer layer, composed of adventitial cells and fine collagen fibers embedded in an amorphous ground substance.