Lymphatic System Notes

Lymphatic System Overview

Functions of the Lymphatic System

• Transport and House Immune Cells: The lymphatic system plays a critical role in housing lymphocytes (T cells and B cells) and other immune cells, which are essential for immune responses.
• Return Excess Fluid to Blood: It helps to return excess interstitial fluid from body tissues back to the bloodstream, thus maintaining blood volume and fluid balance in the body.

Components of the Lymphatic System

• Lymph: This is the fluid transported within lymph vessels, which contains water, dissolved solutes, and occasionally proteins, cell debris, pathogens, or cancer cells.
• Lymph Vessels: These are the vessels that carry lymph throughout the body. They include lymphatic capillaries, vessels, trunks, and ducts.
• Lymphoid Tissues and Organs: These include primary structures such as red bone marrow and thymus (where lymphocytes are formed and matured), and secondary structures such as lymph nodes, spleen, and tonsils that house immune cells and initiate immune responses.

Characteristics of Lymph

• Lymph is derived from interstitial fluid that leaves the blood capillaries but is not reabsorbed.
• Interstitial fluid enters lymphatic capillaries where it is termed lymph, enabling it to be transported back into the bloodstream.

Lymphatic Capillaries

• Structure: Small, closed-ended vessels that intersperse around blood capillaries. They have overlapping endothelial cells which allow fluid to enter but not exit, and anchoring filaments prevent collapse.
• Absence in Certain Areas: They are not present in avascular tissues, red marrow, spleen, and the central nervous system.
• Lacteals: Specialized lymphatic capillaries in the gastrointestinal tract responsible for absorbing lipid-soluble substances.

Movement of Lymph

• Lymph enters capillaries due to hydrostatic pressure from interstitial fluid. This pressure is complemented by skeletal muscle contractions and respiratory pumps, which help to propel lymph through progressively larger vessels (lymphatic capillaries → vessels → trunks → ducts) back into the bloodstream.

Cancer Metastasis

• Cancer cells can utilize the lymphatic system to spread to other parts of the body. This process is known as metastasis, where cells migrate from the primary tumor site to form secondary tumors in different locations.

Lymphatic Vessels

• Structure: Composed of three tunics akin to veins and equipped with valves to prevent backflow. They lack a dedicated pump, relying instead on surrounding muscular movements and arterial pulsations to facilitate lymph movement.
• Trunks and Ducts: Larger vessels feed into trunks (e.g., jugular, subclavian) and two major ducts—right lymphatic duct and thoracic duct—return lymph to venous circulation.

Lymphedema

• A condition resulting from impaired lymphatic drainage, causing swelling due to fluid accumulation. It can lead to pain and increased risk of infections.
• Common causes include trauma, infections, tumors, or conditions like elephantiasis.

Lymphoid Structures

• Primary Structures: Red bone marrow (where T and B lymphocytes are formed) and thymus (where T lymphocytes mature).
• Secondary Structures: Include lymph nodes and spleen, which serve as sites for immune response initiation and the housing of lymphocytes.

Red Bone Marrow

• Involved in hematopoiesis—the formation of blood cells. It is primarily found in flat bones and the ends of long bones where lymphocytes begin their development.

Thymus

• Located in the mediastinum, larger in children and regresses post-puberty, replaced by adipose tissue. It is essential for T lymphocyte maturation with distinct regions (cortex and medulla) housing different maturity levels of T cells.

Secondary Lymphoid Structures

• Comprising lymph nodes, spleen, tonsils, and mucosa-associated lymphoid tissue (MALT), these structures serve as sites where lymphocytes interact with pathogens and initiate immune responses.

Lymph Nodes

• Serve as filters for lymph, located throughout the body; they have afferent vessels bringing lymph in and efferent vessels allowing lymph to exit.
• Components of Lymph Nodes: Include lymphoid nodules (B cells) in the outer cortex and medullary cords (T cells and macrophages) in the inner medulla. They are critical for monitoring lymph for pathogens or foreign substances.

Spleen

• The largest lymphoid organ, filtering blood instead of lymph, it consists of red pulp (monitors blood, removes old or damaged cells) and white pulp (houses lymphocytes and immune response initiation).

Tonsils

• Positioned in the throat, they sample pathogens from ingested or inhaled substances. They have crypts that trap particles and harbor lymphoid nodules essential for immune surveillance.

Clinical Notes

Lymphoma

• A malignant neoplasm originating from lymphoid structures, characterized by painless swollen lymph nodes along with other systemic symptoms.

Splenectomy

• Removal of the spleen can occur due to various medical reasons but can increase vulnerability to infections.

Tonsillitis

• Inflammation of the tonsils, typically leading to surgical removal in cases of chronic infection.