Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Organs of Hematopoiesis and the Immune System. The Lymphatic System
Peripheral Organs of the Immune System
The Lymphatic System

The Lymphatic system comprises lymphatic capillaries (Lymph capillaries) branching throughout Organs and Tissues, Lymphatic vessels, trunks, and ducts. Lymph Nodes are situated along the pathways of lymphatic vessels (Fig. 76, see color insert).

The function of the lymphatic system, the most crucial defense system in The Human Body, is to remove metabolic products dissolved and suspended in tissue fluid from organs and tissues, filtering them through biological filters—the lymph nodes. Substances that cannot penetrate the Blood through capillary walls are absorbed into the lymph capillaries along with tissue fluid. These include coarse-dispersed Proteins, debris from dead Cells, dust particles that have entered the body, microbial bodies, and their metabolic products, which are trapped in the lymph nodes, recognized by lymphocytes, and destroyed by macrophages. The tissue fluid absorbed into the lymph capillaries, along with the substances it contains, is called lymph.

Lymph (from Lat. lympha—clear Water) is a transparent or slightly opalescent alkaline fluid of low viscosity that consistently contains varying numbers of lymphocytes and other cells. The Biochemical Composition of lymph and its cellular content depend on the Structure and Functional state of the organ or tissues from which the lymph drains. In tumor diseases, Cancer cells may penetrate the lymph capillaries along with tissue fluid. In individuals weakened by illness, these cells can become trapped in the lymph nodes, multiply there, and form secondary tumors known as metastases.

Lymphatic capillaries serve as the initial link, the roots of the lymphatic system. They are present in all organs and Tissues of the human body, except for the Brain AND SPINAL cord and their Meninges, the Eyeball, the Inner ear, the epithelium of the Skin and mucous membranes, Cartilage, the parenchyma of the Spleen, Bone Marrow, and the Placenta. Lymphatic capillaries have a larger diameter than blood capillaries (up to 0.2 mm) and feature blind-ending pouches and dilations (lacunae) at confluence sites. Interconnecting with one another, lymphatic capillaries run in various directions to form closed networks. They are located between the Structural and functional units of organs (bundles of Muscle fibers, groups of glands, renal corpuscles, hepatic lobules). The villi of the Small Intestine contain wide, blind-ending capillaries (lymphatic sinuses) that empty into the lymphatic network of the organ's mucosa. These sinuses play a major role in the absorption of digestive products. The walls of lymphatic capillaries are formed by a single layer of endothelial cells, lacking a basement membrane and pericytes. Consequently, the endothelium is in direct contact with the intercellular matrix, facilitating the easy penetration of particles between endothelial cells into the lumen of the lymph capillaries. By merging with one another, the capillaries give rise to lymphatic vessels.

Lymphatic vessels differ from capillaries by their larger diameter, the presence of three coats in their walls—endothelial, muscular, and outer Connective Tissue (adventitia)—as well as numerous Valves, which give lymphatic vessels a characteristic beaded appearance. Lymphatic vessels originating from Internal Organs and Muscles typically accompany Blood Vessels and are called deep lymphatic vessels. Superficial lymphatic vessels lie in the subcutaneous adipose tissue, formed by the lymphatic capillaries of the skin and underlying tissues.

Located along the pathways of lymph flow, lymph nodes lie adjacent to blood vessels, most frequently Veins. Depending on the Location OF THE LYMPH NODES AND the direction of lymph flow from organs, regional groups of lymph nodes are distinguished (from Lat. regio—region). These groups are named either after the region where they are situated (e.g., submandibular, inguinal, lumbar, axillary) or after the major vessel near which the nodes lie (celiac, superior mesenteric, jugular). Groups of lymph nodes located beneath the skin are called superficial, while those situated deeper are termed deep.

Efferent lymphatic vessels emerging from lymph nodes HEAD toward the next groups of lymph nodes lying along the outflow pathway or toward collecting lymphatic ducts and trunks. Lymph from the right upper limb collects into the right subclavian trunk; from the right half of the Head and Neck, into the right jugular trunk; and from the Organs of the right thoracic cavity and its walls, into the Right lymphatic duct. These three large lymphatic vessels empty into the right venous angle, formed by the junction of the right subclavian and internal jugular veins. Lymph from the left upper limb and the left half of the head and neck drains via the left subclavian and jugular trunks, which empty into the left venous angle—the confluence site of the left subclavian and internal jugular veins. Lymph from the lower half of the body (below the Diaphragm) and the organs of the left thoracic cavity and its walls is collected by the Thoracic duct, the largest vessel of the lymphatic system. The thoracic duct empties into the left venous angle.

The thoracic duct forms in the retroperitoneal adipose tissue at the level of the XII thoracic to II lumbar vertebrae by the confluence of the right and left lumbar trunks. In its initial section, the thoracic duct is fused with the right crus of the diaphragm; consequently, as it follows the respiratory Movements of the diaphragm, it is compressed and expanded, which facilitates the propulsion of lymph upward toward the VEINS OF THE neck. The thoracic duct enters the thoracic cavity through the aortic hiatus of the diaphragm. The total length of the thoracic duct is 30–41 cm. A paired valve is present at the opening of the duct, preventing blood from veins from entering it. 7–9 valves are located along the course of the duct. The walls of the thoracic duct contain a well-developed middle (muscular) coat formed by smooth muscle cells, the contractions of which aid in propelling the lymph.

The lymphatic Vessels of the lower limb are divided into superficial and deep. Superficial lymphatic vessels are formed by the lymph capillary networks of the skin and subcutaneous tissue and run toward the popliteal and superficial inguinal lymph nodes. Deep vessels are formed by the lymph capillaries of muscles and joints. They travel alongside the major Blood vessels of the leg and thigh and head toward the deep inguinal lymph nodes. Numerous anastomoses interconnect the superficial and deep lymphatic vessels.

The inguinal lymph nodes are located in the region of the femoral triangle. They receive lymph from the lymphatic vessels of the lower limb, external genitalia, skin of the lower anterior abdominal wall, and gluteal region. The efferent lymphatic vessels of these nodes run toward the external iliac lymph nodes, which lie in the pelvic cavity along the course of the external iliac artery and vein.

Visceral and parietal lymph nodes are located within the pelvic cavity and along its walls, receiving lymph from adjacent organs and pelvic walls. From the external and internal iliac lymph nodes, lymphatic vessels proceed to the common iliac lymph nodes, which lie adjacent to the common iliac artery and vein. The efferent lymphatic vessels of the right and left common iliac lymph nodes lead to the lumbar lymph nodes situated near the Abdominal Aorta and INFERIOR VENA CAVA.

Lymph Nodes in the Abdominal cavity are likewise divided into parietal and visceral. Parietal (lumbar) lymph nodes are situated predominantly on the posterior abdominal wall. The number of lumbar lymph nodes reaches up to 40; they receive lymph from the lower limbs, pelvic and abdominal walls, and pelvic and abdominal organs. The efferent lymphatic vessels of the lumbar lymph nodes give rise to the lumbar lymphatic trunks. Visceral lymph nodes of the abdominal cavity are located near organs and unpaired visceral Branches of the Abdominal Aorta (superior and inferior mesenteric Arteries, celiac trunk, hepatic, gastric, and splenic arteries). Lymph from regional nodes of The Stomach, Pancreas, Liver, and Kidneys drains into the celiac lymph nodes. The efferent lymphatic vessels of the celiac nodes proceed to the lumbar nodes. Superior mesenteric lymph nodes (ranging from 60 to 400) are located in the mesentery of the small intestine along the branches of the superior mesenteric artery and tributaries of the corresponding veins. These nodes receive lymph from the small intestine, and their efferent lymphatic vessels pass to the lumbar lymph nodes. Colic nodes serve as the regional nodes for the Large Intestine. Their efferent lymphatic vessels also head toward the lumbar lymph nodes.

In the thoracic cavity, much like the abdominal cavity, there are parietal lymph nodes located along the cavity walls and visceral nodes lying close to the organs. Parietal lymph nodes (parasternal, intercostal, etc.) receive lymph from the thoracic walls, diaphragm, Pleura, Pericardium, mammary gland, and diaphragmatic surface of the liver. The efferent lymphatic vessels of these nodes either run directly to the thoracic duct or pass through the mediastinal lymph nodes. Visceral lymph nodes (mediastinal, esophageal, tracheobronchial) receive lymph from the organs of the thoracic cavity. The lymphatic vessels of the lung lead to the bronchopulmonary nodes.

Intrapulmonary bronchopulmonary nodes lie within the Lungs near the lobar Bronchi at their branching points. Extrapulmonary bronchopulmonary nodes are situated around the main bronchi, near the pulmonary arteries and veins. The efferent lymphatic vessels of the bronchopulmonary and tracheobronchial lymph nodes, which drain lymph from the lungs, empty into the thoracic duct and the right lymphatic duct.

Lymph from the tissues and organs of the head and neck drains into lymph nodes arranged in groups at the border of the head and neck (occipital, parotid, retropharyngeal, submandibular, submental). The efferent vessels of these nodes proceed to the cervical lymph nodes, which also receive lymphatic vessels from the organs of the neck. In the neck, a distinction is made between superficial lymph nodes lying near the External Jugular Vein and deep nodes located predominantly near the Internal jugular vein. The efferent lymphatic vessels of these nodes form the jugular trunk, which empties into the corresponding venous angle.

Lymph from the upper limb drains via superficial and deep lymphatic vessels into the cubital and axillary lymph nodes. The cubital lymph nodes, located in the cubital fossa near blood vessels, receive superficial and deep lymphatic vessels of the hand and forearm. The efferent lymphatic vessels of these nodes head toward the axillary nodes. The axillary lymph nodes are situated in the adipose tissue of the axillary fossa around the neurovascular bundle. The axillary nodes receive superficial and deep lymphatic vessels from the upper limb, thoracic walls, and mammary gland. The efferent vessels of the axillary lymph nodes form 2–3 large trunks that accompany the subclavian vein (subclavian lymphatic trunk) and empty into the venous angle or (occasionally) into the subclavian vein.

Review and Self-Assessment Questions:

1. Name the Functions of the lymphatic system in the human body.

2. Which links (component parts) make up the lymphatic system? What functions does each link perform?

3. What lymphatic ducts and trunks do you know, and where do they empty?

4. Describe where the thoracic lymphatic duct originates and where it empties.

5. Name the groups of lymph nodes in the human body familiar to you. Where are these lymph nodes located?