Lymphatic System 

The lymphatic system is an essential part of the body’s immune system and serves as a specialized component of the circulatory system. It consists of a network of organs, lymph nodes, lymph ducts, and vessels that work together to produce, transport, and drain lymph from tissues into the bloodstream. The system also includes reticular connective tissue and plays a key role in the production, maturation, and storage of lymphocytes and macrophages, which are critical for the body’s defense mechanisms.

1.0Components of Lymphatic System

1. Lymph 

2. Lymphatic vessels 

• Lymph trunks and ducts 
• Thoracic (left lymphatic) duct 
• Right lymphatic ducts 

3. Lymphatic tissue 

Primary lymphoid organs 

• Bone marrow
• Thymus gland

Secondary Lymphoid Organs

• Spleen 
• Lymph nodes 
• MALT

2.0Lymph

• Excess interstitial fluid drains into the lymphatic capillaries and is known as lymph. 
• This clear, watery fluid is similar to plasma but lacks plasma proteins. Lymph transports these proteins, which seep from capillaries, back into the bloodstream. 
• Additionally, lymph carries bacteria and cellular debris from damaged tissues to lymph nodes, where they are filtered and destroyed. 
• It also contains lymphocytes circulating throughout the lymphatic system, enabling them to monitor various body regions. 
• In the small intestine, fats absorbed by specialized lymphatic capillaries called lacteals give lymph a milky appearance.

Rat of Lymph Flow: About 120 mL of lymph flows into blood per hour. Out of this, about 100 mL/hour flows through the thoracic duct and 20 mL/hour flows through the right lymphatic duct.

Factors Increasing the Flow of Lymph 

Factors Increasing the Flow of Lymph- Flow of lymph is promoted by the increase in

• Interstitial fluid pressure.
• Blood capillary pressure. 
• Surface area of lymph capillary by means of dilatation
• Permeability of lymph capillaries. 
• Functional activities of tissues. 

Composition of Lymph

• Usually, lymph is a clear and colorless fluid. It is formed by 96% water and 4% solids. Some blood cells are also present in lymph.

Functions of Lymph

• Important function of lymph is to return the proteins from tissue spaces into blood. 
• It is responsible for redistribution of fluid in the body. 
• Bacteria, toxins and other foreign bodies are removed from tissues via lymph.
• Lymph flow is responsible for the maintenance of structural and functional integrity of tissue. Obstruction to lymph flow affects various tissues, particularly myocardium, nephrons and hepatic cells. 
• Lymph flow serves as an important route for intestinal fat absorption. This is why lymph appears milky after a fatty meal. 
• It plays an important role in immunity by transport of lymphocytes.

3.0Flow of Lymph Diagram

Lymphatic Capillaries

Lymphatic capillaries are small, thin-walled vessels that are closed at one end. Their primary function is to collect excess interstitial fluid from surrounding tissues and transport it to the venous circulation. They have a larger diameter than other blood capillaries and are lined with endothelial cells.

In addition, lymphatic capillaries in the mucosa of the small intestine, known as lacteals, play a crucial role in absorbing dietary fats and lipid-soluble vitamins. When lymph absorbs triglycerides from the intestinal villi, it forms a milky fluid called chyle. Anchoring filaments attach the endothelial cells to adjacent tissues, ensuring these vessels' stability and proper function.

Lymphatic Vessels 

Lymphatic capillaries converge to form lymphatic vessels, which are thin-walled structures that transport lymph throughout the body. Lined with endothelial cells, lymphatic vessels channel lymph from the capillaries towards larger lymphatic trunks and ducts.

Lymph Trunk and Lymph Ducts 

Lymphatic fluid flows from the lymphatic capillaries into larger lymphatic vessels. These vessels connect with lymph nodes and subsequently converge to form lymph trunks. The five primary lymph trunks are:

• Lumbar Lymph Trunk
• Intestinal Lymph Trunk
• Bronchomediastinal Lymph Trunk
• Subclavian Lymph Trunk
• Jugular Lymph Trunk

Left Lymphatic Duct (Thoracic Duct): This duct, which is the larger of the two, measures approximately 15-18 inches long. It collects lymph from the left side of the head, neck, chest, the left upper limb, and the entire body below the ribs.

Right Lymphatic Duct: This duct drains lymph from the right side of the head and neck, the right upper limb, the right thorax, the right lung, the right side of the heart, and part of the liver.

Lymphatic Tissue/Organs 

• Primary lymphoid organs : where maturation of lymphocytes takes place
• Bone marrow 
• Thymus 
• Secondary Lymphoid Organs: trap antigen and provide sites for mature lymphocytes to interact with that antigen.
• Spleen, and 
• Lymph nodes,
• Various Mucosal Associated Lymphoid Tissues (MALT) .
• BONE MARROW In humans , bone marrow is the site of B-cell origin and development.

4.0Thymus Gland

• The thymus is a small, triangular organ located just behind the sternum and in front of the heart. It consists of glandular epithelial tissue and hematopoietic connective tissue.

Functions:

• T Cell Maturation: It facilitates the development and maturation of T cells, which are crucial for the immune Response.
• Endocrine Function: It acts as an endocrine gland by releasing thymic factors involved in growth and immune system regulation.
• Immune Competence: The thymus releases hormones that contribute to developing immune competence, helping cells respond to foreign substances effectively.

Spleen 

• The largest lymphatic organ, the spleen, is in the left upper abdomen between the stomach's fundus and the diaphragm. 
• It contains both reticular and lymphatic tissues and is typically purplish. Its dimensions vary among individuals but generally measure about 12 cm in length, 7 cm in width, and 2.5 cm in thickness, and it weighs approximately 200 grams. 

Structure:

• The spleen is slightly oval, with its hilum on the lower medial border. 
• The peritoneum covers its anterior surface, encased in a fibroblastic capsule that extends into the organ, forming trabeculae. Within the spleen, the splenic pulp, composed of lymphocytes and macrophages, is situated between the trabeculae.
• Key structures entering and leaving the spleen at the hilum include:
• Splenic Artery: This artery branches off from the coeliac artery.
• Splenic Vein: This vein is a branch of the portal vein.
• Lymph Vessels: These are exclusively efferent.
• Nerves: The spleen contains nerve fibers.
• Blood Flow: Blood entering the spleen moves through sinusoids, which are specialized blood vessels within the organ.

Functions of the Spleen:

• Phagocytosis - The spleen is crucial in breaking down old and abnormal red blood cells. The byproducts, such as bilirubin and iron, are transported to the liver via the splenic and portal veins. Additionally, the spleen phagocytosis other cellular debris, including leukocytes, platelets, and bacteria.
• Blood Storage - The spleen can store up to 350 ml of blood. In Response to sympathetic stimulation, such as during hemorrhage, it can rapidly release this stored blood back into circulation.
• Immune Response - The spleen houses T and B lymphocytes, which become activated when encountering antigens, such as those from infections. During severe infections, lymphocyte proliferation can lead to noticeable changes in the spleen.
• Erythropoiesis - Both the spleen and liver are vital for fetal blood cell production. In adults, the spleen can also produce blood cells in times of significant need.

5.0Lymph Nodes

• Lymph nodes are oval or bean-shaped organs distributed along the length of lymphatic vessels, often grouped. 
• Lymph typically passes through 8-10 nodes before returning to the venous circulation. These nodes vary in size, from as small as pinheads to about the size of an almond.

Structure of Lymph Nodes

• Lymph nodes are oval or bean-shaped organs positioned along the lymphatic vessels. They vary in size from 1 to 25 mm in length and are typically pink. Lymph nodes are distributed throughout the body in clusters, categorized into superficial and deep. Each lymph node is encased in a dense connective tissue capsule, with extensions known as trabeculae extending into the node.
• Internally, the lymph node is divided into two main regions:
• Outer Cortex: Contains densely packed lymphocytes arranged in structures called follicles. Each follicle's outer rim includes T lymphocytes and macrophages.
• Inner Medulla: Features lymphocytes arranged in strands known as medullary rays.
• The stroma of the lymph node, which supports its structure, comprises the capsule, trabeculae, reticular fibers, and fibroblasts. Each lymph node has a concave surface called the hilum. Four or five afferent lymph vessels enter the node, while only one efferent vessel exits, carrying lymph away.

Functions of Lymph Nodes

• Defense Lymph nodes filter lymph as it passes through, removing particulate matter such as bacteria, dead cells, microbes, malignant tumor cells, and inhaled particles. Macrophages and antibodies destroy organic material, although some inorganic particles may remain inside macrophages without causing damage. 
• Incomplete phagocytosis of bacteria can lead to inflammation and enlargement of the lymph nodes.
• Maturation and Proliferation of Lymphocytes Lymph nodes are sites where some lymphocytes complete their maturation process. Activated T and B lymphocytes multiply within the nodes, and antibodies produced by sensitized B lymphocytes are released into the lymph and bloodstream.

6.0MALT

MALT includes Peyer’s patches (in the small intestine), the tonsils, and the appendix, as well as numerous lymphoid follicles within the lamina propria of the intestines and in the mucous membranes lining the upper airways, bronchi, and genital tract.