Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Y. B. Chaykovsky 2008

Special Part
Cardiovascular system, systema cardiovasculare – Venous system
Veins – Structure of veins

According to their histological Structure, the wall of Veins consists of three tunics (see Fig. 2):

1. The inner layer, tunica interna s. intima;

2. The middle layer, tunica media;

3. The outer layer, tunica externa s. adventitia.

Unlike Arteries, veins feature a poorly developed elastic component in their walls, which is attributed to low Blood pressure (15-20 mmHg) and a slow rate of blood flow (approximately 10 mm/s in organ veins). Based on Structural Features of their walls, veins are classified into non-muscular and muscular types. Non-muscular veins include those of the dura and pia mater, the retina, bones, Spleen, and Placenta. Muscular veins, in turn, are subdivided into: veins with poorly developed muscular elements—small and medium veins (1-2 mm); veins with moderately developed muscular elements—medium-diameter veins (such as the brachial vein); and veins with strongly developed muscular elements—VEINS OF THE lower torso and extremities. These are characterized by the presence of smooth Muscle Cells in all three tunics: oriented longitudinally in the inner and outer layers, and circularly in the middle layer.

The inner tunic of the venous wall is formed by the endothelium and a subendothelial layer composed of fibers and Connective Tissue cells. Many veins contain Valves (Fig. 59 A), which are thin folds (duplications) of the inner tunic that prevent backflow and direct blood toward The Heart. Each valve typically has two cusps, though single- or triple-cusp valves also occur. Experiments have shown that these valves can withstand pressures of 2 to 3 atmospheres. Valves are most commonly found near the junctions of venous tributaries (ostial valves). They are most numerous in the veins of the limbs, especially the lower extremities, and in Body Cavities. Valves are absent or scarce in the vv. azygos, hemiazygos, and intercostal veins. They are completely absent in the umbilical veins, Pulmonary veins, portal Venous system, veins of the bones, Vertebral Column, ovarian veins, hepatic, renal, and uterine veins, as well as in the trunks of the superior and INFERIOR VENA CAVA and cerebral veins.

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Fig. 59. General diagram of the human venous system

Fig. 59 A. Structure of veins. A — external contours of a vein. B — Cytology/practical/54.html">Longitudinal section of a vein (venous valves). C — arterio-venous anastomoses and the direction of BLOOD FLOW IN Blood Vessels

The outer tunic of the venous wall is formed by loose Fibrous connective tissue. The Blood supply to the venous wall is provided by branches of adjacent arteries. The innervation of veins is carried out by branches of somatic nerves, which also contain autonomic fibers. The veins of body cavities receive innervation from autonomic nerve plexuses.

The veins (venae [phlebs]) of The Human Body are integrated into three venous systems:

1. Veins of the pulmonary (lesser) Circulation;

2. Veins of the cardiac circulation;

3. Veins of the systemic (greater) circulation.

The veins of the lesser circulation form major venous collectors—the superior and inferior vena cava (v. cava superior and v. cava inferior), while the veins of the cardiac circulation form the coronary sinus (sinus coronarius). All major veins empty into the right atrium of the heart.