Lymphatic System: Functions and Pathways

• The lymphatic system includes cells and biochemicals traveling in lymphatic vessels, forming a fluid-circulating network closely tied to the cardiovascular system.

Lymphatic System Functions

• Transports excess interstitial fluid back to the bloodstream from interstitial spaces.
• Absorbs and transports lipids from the digestive system to the bloodstream via lacteals (specialized lymphatic capillaries).
• Defends against diseases, which contributes to overall immunity called the "immune system".

Lymphatic Pathways

• Lymphatic capillaries collect interstitial fluid which becomes lymph, flow through lymphatic vessels to lymph nodes, then to larger lymphatic vessels, lymphatic trunks, and collecting ducts, finally draining into the subclavian veins in the thorax.

Lymphatic Capillaries

• Microscopic, closed-ended tubes forming networks alongside blood capillaries.
• They have thin walls made of simple squamous epithelium.
• Interstitial fluid enters, becoming lymph.

Lymphatic Vessels

• Similar to veins, but thinner, comprising endothelial lining, smooth muscle/elastic fiber middle layer, and connective tissue outer layer.
• Semilunar valves ensure one-way flow.
• Larger vessels carry lymph to lymph nodes and lymphatic trunks.

Lymphatic Trunks and Collecting Ducts

• Lymphatic trunks, such as lumbar, intestinal, intercostal, bronchomediastinal, subclavian, and jugular trunks, drain lymph from lymphatic vessels.
• Lymphatic collecting ducts receive lymph from trunks.
• The thoracic duct, the larger of the two, drains the majority of the body and starts as the cisterna chyli, emptying into the left subclavian vein.
• The right lymphatic duct drains the upper right portion of the body, emptying into the right subclavian vein.

Tissue Fluid and Lymph

• Lymph is tissue fluid that has entered a lymphatic capillary. Lymph formation depends on tissue fluid formation
• Tissue fluid is formed when capillary blood pressure filters water and small molecules from the plasma.
• Tissue fluid contains water, dissolved nutrients, gases, and hormones, but lacks large plasma proteins, which remain in blood plasma to maintain osmotic pressure.
• Tissue fluid is essentially blood plasma minus plasma proteins.
• Filtration from plasma exceeds reabsorption, leading to tissue fluid formation, increasing hydrostatic pressure, and forcing fluid into lymphatic capillaries as lymph, preventing edema.
• Edema can occur if lymphatic drainage is obstructed.

Lymph Flow

• Lymph within vessels has low hydrostatic pressure, like venous blood.
• Skeletal muscle contraction compresses lymphatic vessels, moving lymph.
• Respiratory processes create pressure changes that aid lymph flow from abdomen to thorax.
• Smooth muscle contraction in larger vessels assists lymph flow.
• Valves prevent backflow.
• Lymphatic flow is highest during exercise.
• Lymphatic capillaries absorb dietary fats in the small intestine, return small proteins to the bloodstream, collect excess interstitial fluid, deliver fluid to the bloodstream, and transport foreign particles to lymph nodes.
• Flap-like valves between cells of lymphatic capillaries allow easy entry of tissue fluid.

Lymphatic Tissues and Lymphatic Organs

• Lymphatic tissue contains lymphocytes and macrophages.
• Mucosa-associated lymphoid tissue (MALT) is unencapsulated lymphatic tissue in the digestive, respiratory, urinary, and reproductive tracts.
• Tonsils, the appendix, and Peyer's patches (in the ileum) are examples of lymphatic nodules or aggregates.
• Lymphatic organs, such as lymph nodes, the thymus, and the spleen, consist of encapsulated lymphatic tissue.
• Lymph nodes are typically bean-shaped.