Lymphatic System

Lymphatic System Overview

• Key components of the lymphatic system include:

  • Thymus

  • Liver

  • Tonsils

  • Spleen

  • Lymph vessels

21.1 What is Lymph?

• Characteristics of lymph:

  • Formed from interstitial fluid that leaves blood capillaries and is not reabsorbed.

  • Fluid enters lymphatic capillaries, becoming lymph.

  • Components of lymph:

  • Water

  • Dissolved solutes

  • Small amounts of protein

  • Immune cells

  • May contain cell debris, microorganisms, and cancer cells.

21.1 Lymphatic Capillaries

• Definition: Small, closed-ended vessels that absorb interstitial fluid.

• Characteristics:

  • Interspersed around most blood capillaries.

  • Absent in avascular tissues and red bone marrow.

  • Present around the brain (meninges) and in the glymphatic system (glia-dependent).

  • Larger than blood capillaries and lack a basement membrane.

  • Walls built from overlapping endothelial cells.

  • Flaps between cells allow fluid entry, but prevent exit.

  • Anchoring filaments maintain position to nearby structures.

  • Lacteals: lymphatic capillaries in the gastrointestinal tract that absorb lipid-soluble substances.

21.1 Lymph Movement Through Lymphatic System

• Movement of lymph into lymphatic capillaries:

  • Hydrostatic pressure of interstitial fluid pushes lymph into the capillary.

  • Anchoring filaments prevent vessel collapse.

  • Pressure of lymph inside the vessel closes intercellular openings (“flaps”).

  • Lymph travels through successively larger vessels:

  • Lymphatic capillaries

  • Lymphatic vessels

  • Lymphatic trunks

  • Lymphatic ducts

  • Back to blood circulation.

• Clinical significance: role in metastasis of cancer.

21.1 Lymphatic Vessels

• Overview:

  • Fed by lymphatic capillaries and situated adjacent to arteries and veins.

  • Composed of three tunics: intima, media, and externa.

  • Include valves to prevent pooling and backflow.

  • Do not have a pump; lymph movement relies on:

  • Skeletal muscles and respiratory pumps.

  • Pulsatile movement of nearby arteries.

  • Rhythmic contraction of smooth muscle in larger lymph vessels.

  • Some lymphatic vessels connect to lymph nodes for filtration.

21.1 Lymphatic Trunks

• Description:

  • Larger lymphatic vessels supplied by lymphatic vessels.

  • Types of trunks:

  • Jugular trunks: drain lymph from head and neck.

  • Subclavian trunks: drain upper limbs, breasts, and superficial thoracic wall.

  • Bronchomediastinal trunks: drain deep thoracic structures.

  • Intestinal trunks: drain abdominal structures.

  • Lumbar trunks: drain lower limbs, abdominopelvic wall, and pelvic organs.

21.1 Lymphatic Ducts

• Overview:

  • Largest lymphatic vessels, fed by lymphatic trunks.

  • Two ducts:

  • Right lymphatic duct:

    • Near right clavicle, drains:

    • Right side of head and neck

    • Right upper limb

    • Right side of thorax.

  • Thoracic duct:

    • Largest, with a saclike cisterna chyli at its base.

    • Receives lipid-rich chyle from GI tract.

    • Drains lymph from the left side of the head and neck, left upper limb, left thorax, abdomen, and both lower limbs.

21.1 Clinical Significance: Lymphedema

• Cause: Accumulation of interstitial fluid due to interference with lymph drainage (often from blockage).

• Symptoms include swelling and pain in affected areas.

• Effects may include interference with wound healing and increased infection risk.

• Common causes:

  • Trauma

  • Parasitic infections of lymph vessels

  • Tumors

  • Radiation therapy scarring

  • Surgical removal of lymph nodes.

• Treatments include compression garments, exercise, and massage.

• Example of extreme condition: Elephantiasis, caused by parasitic worm infection.

21.2 Overview of Lymphoid Structures

• Primary lymphoid structures:

  • Involved in formation and maturation of lymphocytes.

  • Red bone marrow

  • Thymus

• Secondary lymphoid structures:

  • Do not produce lymphocytes but house them and are sites of immune response initiation.

  • Lymph nodes

  • Spleen

  • Tonsils

  • Lymphatic nodules

  • MALT (mucosa-associated lymphoid tissue).

21.3 Red Bone Marrow

• Location: Between trabeculae of spongy bone, found in the flat bones of the skull, ribs, sternum, vertebrae, ossa coxae, and heads of humerus and femur.

• Function: Site of hematopoiesis (production of blood’s formed elements), including T-lymphocytes and B-lymphocytes.

• Immature T-lymphocytes, known as thymocytes, migrate to the thymus to complete maturation.

21.3 Thymus

• Involvement: Maturation of T-lymphocytes.

• Location: Mediastinum.

• Characteristics:

  • Larger in children than in adults.

  • Grows until puberty, then regresses and is gradually replaced by adipose tissue.

  • Consists of two lobes, each surrounded by a connective tissue capsule, subdivided into lobules by trabeculae.

  • Cortex contains immature T-lymphocytes; medulla contains mature T-lymphocytes.

21.4 Secondary Lymphoid Structures

• Function: House lymphocytes and other immune cells within a reticular connective tissue matrix.

• Classification:

  • Lymphoid organs: complete capsule of dense irregular connective tissue (e.g., spleen and lymph nodes).

  • Aggregates of lymphoid nodules: incomplete capsule or absent capsule (e.g., tonsils, MALT).

Lymph Nodes Structure

• Description of lymph node:

  • Cortex: contains lymphoid nodules and germinal centers.

  • Medulla: contains medullary sinuses and cords.

  • Flow of lymph through afferent vessels into the node and out through efferent lymphatic vessels.

Clinical Significance: Cancer and Other Diseases

• Malignant neoplasm may present as non-tender, enlarged lymph nodes, potentially accompanied by night sweats, fever, and weight loss.

• Types of lymphoma:

  • Hodgkin Lymphoma: prevalent in young adults and those over 60.

  • Non-Hodgkin Lymphoma: more common overall.

• Palpation:

  • Enlarged, tender lymph nodes may indicate infection (e.g., mononucleosis).

  • Infections like the common cold typically cause swelling in cervical nodes, whereas cancer may cause hard, non-tender enlargement.

21.4b Spleen

• Description:

  • Largest lymphoid organ, located in the left upper abdominal quadrant, lateral to the left kidney, and behind the stomach.

  • Identified by its hilum, where blood vessels and nerves enter.

• Functions:

  • Surrounded by a connective tissue capsule.

  • Trabeculae extend from the capsule into the spleen, dividing it into red and white pulp areas.

21.4c Tonsils

• Classification:

  • Secondary lymphoid structures involved in immune surveillance of inhaled and ingested materials.

• Features:

  • Contain tonsillar crypts that trap material and lymphoid nodules, some with germinal centers.

• Types of tonsils:

  • Pharyngeal tonsil (adenoids when enlarged, located in nasopharynx)

  • Palatine tonsils (located in posterolateral oral cavity)

  • Lingual tonsils (located along the posterior one-third of the tongue).

21.4 MALT (Mucosa-Associated Lymphoid Tissue)

• Prominent in the small intestine, particularly in the ileum with Peyer's Patches.

• Role: Involved in immune response in mucosal areas, aiding protection against ingested pathogens.