Lymphatic System

Lymphatic System

Introduction

·       The lymphatic system is a vital part of both the circulatory system and the immune system.

·       It consists of lymph (fluid), lymphatic vessels, lymphoid organs, and lymphoid tissues.

·       Its primary functions are:

o   To maintain fluid balance by returning excess interstitial fluid to the bloodstream.

o   To transport dietary lipids and fat-soluble vitamins absorbed in the intestines.

o   To provide immune defense against pathogens through lymphocytes and other immune cells.

The lymphatic system is a one-way drainage system, moving lymph from tissues back to the venous blood circulation.

Lymph

Composition of Lymph

• Water (90–95%) – major component.
• Proteins – smaller than plasma proteins, include albumin, globulins.
• Electrolytes – sodium, potassium, calcium, chloride, bicarbonate.
• Lymphocytes – T cells, B cells, NK cells (responsible for immunity).
• Macrophages and Dendritic cells – for antigen presentation.
• Fats (chylomicrons) – absorbed from small intestine (lymph rich in fat is called chyle).
• Waste products and cell debris from tissues.

Functions of Lymph

1. Fluid balance – Returns excess tissue fluid to venous circulation.
2. Absorption of fats – Lacteals in intestinal villi absorb dietary fats.
3. Immunity – Transports lymphocytes, filters pathogens through lymph nodes.
4. Transport of proteins and large molecules – that cannot enter blood capillaries directly.

Lymph Vessels

Structure

• Lymph Capillaries – Blind-ended, thin-walled, more permeable than blood capillaries. Found in most tissues (except CNS, cartilage, cornea, bone marrow).
• Larger Lymphatic Vessels – Resemble small veins but have thinner walls and more valves to prevent backflow.
• Lymphatic Trunks and Ducts:
• Thoracic Duct – Largest lymphatic vessel, drains lymph from left side of head, neck, thorax, left arm, and both lower limbs into the left subclavian vein.
• Right Lymphatic Duct – Drains right side of head, neck, thorax, and right upper limb into the right subclavian vein.

Flow of Lymph

• Lymph flow is unidirectional (towards the heart).
• Driven by:
• Skeletal muscle contractions (muscle pump).
• Respiratory movements (respiratory pump).
• Valves preventing backflow.
• Rhythmic contraction of lymph vessel smooth muscle.

Lymphatic Organs

1. Primary Lymphoid Organs

These are where lymphocytes are formed and mature.

Thymus

• Location: Behind sternum, in the superior mediastinum.
• Structure: Bilobed organ, with cortex (immature T cells) and medulla (mature T cells, Hassall’s corpuscles).
• Function:
• Site of T-lymphocyte maturation (critical in childhood).
• Produces hormones thymosin, thymopoietin, thymulin – regulate T-cell development.
• Clinical Note: Thymus atrophies after puberty (replaced by fat) but remains immunologically active.

2. Secondary Lymphoid Organs

Sites where lymphocytes are activated and immune responses occur.

Lymph Nodes

• Structure: Small bean-shaped structures located along lymphatic vessels.
• Enclosed by capsule.
• Contains cortex (B-cell follicles) and medulla (T cells, plasma cells, macrophages).
• Afferent lymphatic vessels bring lymph in; efferent vessels take lymph out.
• Functions:

1.               Filter lymph, trapping pathogens, debris, cancer cells.

2.               Site of lymphocyte proliferation and immune activation.

3.               Produce antibodies (plasma cells).

• Clinical Note: Lymphadenopathy = enlargement of lymph nodes in infections or cancer metastasis.

Spleen

• Location: Left upper abdominal quadrant, behind stomach.
• Structure: Encapsulated organ with two types of tissue:
• White pulp – lymphoid tissue around arterioles, rich in lymphocytes (immune function).
• Red pulp – network of sinuses filled with blood, macrophages, and RBCs.
• Functions:

1.               Filters blood (not lymph).

2.               Removes old/abnormal RBCs and platelets.

3.               Acts as a blood reservoir.

4.               Initiates immune responses to blood-borne antigens.

• Clinical Note: Splenectomy → increased susceptibility to infections, esp. by encapsulated bacteria.

MALT (Mucosa-Associated Lymphoid Tissue)

• Scattered lymphoid tissue found in mucosal linings of digestive, respiratory, urinary, and reproductive tracts.
• Includes:
• Tonsils (palatine, pharyngeal, lingual).
• Peyer’s patches in the small intestine.
• Appendix.
• Functions:

1.               Provides first line of defense at mucosal surfaces.

2.               Produces lymphocytes and antibodies against ingested/inhaled pathogens.

Summary of Functions of Lymphatic System

1. Maintains fluid balance.
2. Absorbs and transports fats.
3. Filters and cleanses body fluids.
4. Supports immunity through lymphocytes and antibodies.
5. Removes damaged cells and foreign substances.

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