Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2008

Special Part
Lymphatic system (systema lymphaticum [systema lymphoideum]) - General Overview
Components of the Lymphatic System - Lymphatic Vessels

Lymphatic vessels, or vasa lymphatica, receive Lymph formed within lymphatic capillaries. Functionally, the boundary between a lymphatic capillary and a lymphatic vessel is marked by the Location OF THE first semilunar valve. Lymphatic vessels consist of distinct segments characterized by constrictions corresponding to THE POSITION OF vascular Valves, and dilations between them. This gives lymphatic vessels a beaded appearance and ensures the unidirectional flow of lymph. The valves of lymphatic vessels are paired, semilunar folds (cusps) of the tunica intima (or valve segment) positioned opposite each other (Fig. 101 B). In intra-organic lymphatic vessels, the valves are sometimes not semilunar in shape, but rather resemble a Microscope Diaphragm (O. O. Sushko). Bundles of Connective Tissue fibers from the outer layer of the lymphatic vessel penetrate into the thickness of the valve. The segment of a lymphatic vessel located between two valves is termed a lymphangion (H. Mislin, 1967; A. V. Borisov, 1995).

Thus, from a Structural and functional perspective, lymphatic vessels are composed of individual structural units known as lymphangions. Each lymphangion Functions as the "lymphatic Heart" of higher vertebrates (O. O. Sushko, 1966). The number of valves in lymphatic vessels is significantly greater than in Veins. In the subcutaneous lymphatic Vessels of the lower limb, extending from the digits to the inguinal Lymph Nodes, there are 80–100 valves, whereas in the superficial lymphatic vessels of the upper limb, extending from the digits to the axillary lymph nodes, there are 60–80 valves (R. Sappey)—averaging about one valve per centimeter. The presence of valves confirms that lymph flow in lymphatic vessels is unidirectional (Bartels' "law of conduction"). However, this law does not hold true in all cases. Under pathological conditions, lymphatic vessels become significantly dilated due to stasis, valve insufficiency develops, and lymph flows in a retrograde direction in such instances. This phenomenon explains the retrograde metastasis of malignant tumors.

*Lymphatic vessels were discovered by Gaspare Aselli (G. Aselli) on July 22, 1622, in the center of the new culture (Milan).

The wall of small lymphatic vessels is formed by a thin layer of connective tissue and endothelium. In wider vessels, ranging in diameter from 0.2 to 0.5 mm, the wall consists of three tunics: the inner (tunica intima), the middle (tunica media/muscular coat), and the outer connective tissue tunic (tunica adventitia).

In the lymphatic vessels of the lower limbs, the middle (muscular) tunic is better developed than in the vessels of the upper limbs, HEAD, and neck. The walls of large lymphatic vessels contain Blood Vessels that supply them, known as vasa vasorum lymphaticorum, as well as nerves, nn. vasorum lymphaticorum.

Lymphatic vessels are subdivided into intra-organic vessels, which drain lymph from capillary networks located within Organs, and extra-organic vessels, which convey lymph from organs toward the lymph nodes.

Lymphatic vessels that bring lymph to the lymph nodes are called afferent lymphatic vessels, or vasa afferentia, whereas vessels emerging from the hilum of a lymph node are termed efferent lymphatic vessels, or vasa efferentia.

In certain organs, intra-organic lymphatic vessels anastomose with one another to form wide-meshed plexuses of diverse architecture. By connecting with each other, lymphatic vessels form lymphatic ducts. The lymphatic vessels of the soma are further subdivided into superficial and deep. Superficial lymphatic vessels are located beneath the Skin and accompany superficial veins and nerves. Deep lymphatic vessels course within neurovascular bundles.