Chapter 16: The Lymphatic System and Immunity – Anatomy & Physiology Study Notes

Introduction to the Lymphatic System

Overview

The lymphatic system is a network of vessels, cells, and biochemicals that play a crucial role in fluid balance and immune defense. It is closely associated with the cardiovascular system and is essential for maintaining homeostasis and protecting the body from disease.

• Transports excess fluid from interstitial spaces back to the bloodstream.

• Absorbs lipids from the digestive system and transports them to the bloodstream.

• Defends the body against diseases through immune responses.

The term immune system refers to the defense and permanent immunity provided by lymphatic cells against future infections.

Lymphatic Pathways

Structure and Flow

Lymphatic pathways consist of a series of vessels and organs that transport lymph throughout the body.

• Lymphatic capillaries → lymphatic vessels → lymph nodes → lymphatic trunks → lymphatic collecting ducts → subclavian veins in the thorax.

• Lacteals are specialized lymphatic capillaries in the small intestine that absorb dietary fats.

Lymphatic Capillaries

Characteristics

• Microscopic, closed-ended tubes.

• Form networks parallel to blood capillaries.

• Thin-walled, composed of simple squamous epithelium.

• Tissue (interstitial) fluid enters lymphatic capillaries and is then called lymph.

• Lymphatic capillaries merge to form larger lymphatic vessels.

Lymphatic Vessels

Structure

• Walls are similar to veins but thinner.

• Composed of three layers:

  • Endothelial lining (inner)

  • Smooth muscle (middle)

  • Connective tissue (outer)

• Contain semilunar valves to prevent backflow.

• Larger vessels lead to lymph nodes and then to lymphatic trunks.

Lymphatic Trunks and Collecting Ducts

Function and Regions

• Lymphatic trunks drain lymph from vessels and are named for the regions they serve: lumbar, intestinal, intercostal, bronchomediastinal, subclavian, and jugular.

• Lymphatic collecting ducts drain lymph from trunks:

  • Thoracic duct (large)

  • Right lymphatic duct (small)

Lymphatic Drainage

Pathways and Clinical Relevance

• The thoracic duct drains a much larger portion of the body than the right lymphatic duct.

• Both ducts empty into the subclavian veins (thoracic into left, right lymphatic into right).

• Disruption of lymphatic flow (e.g., after cancer surgery) can cause edema.

Summary of Lymphatic Pathway

The flow of lymph follows this sequence:

• Lymphatic capillary → Afferent lymphatic vessel → Lymph node → Efferent lymphatic vessel → Lymphatic trunk → Collecting duct → Subclavian vein

16.2: Tissue Fluid and Lymph

Formation and Composition

• Lymph is tissue fluid that has entered a lymphatic capillary.

• Lymph formation depends on tissue fluid formation.

Tissue Fluid Formation

• Capillary blood pressure filters water and small molecules from plasma.

• Tissue fluid has a similar composition to blood plasma but lacks large plasma proteins.

• Plasma proteins remain in blood plasma to maintain osmotic pressure.

• Equation: $\text{Tissue fluid} = \text{Blood plasma} - \text{Plasma proteins}$

Lymph Formation

• Filtration from plasma normally exceeds reabsorption, leading to net tissue fluid formation.

• Increased hydrostatic pressure in interstitial spaces forces fluid into lymphatic capillaries, forming lymph.

• This process prevents accumulation of excess tissue fluid (edema).

Lymph Flow

Mechanisms

• Lymph inside vessels has low hydrostatic pressure, similar to venous blood.

• Flow is aided by:

  • Contraction of skeletal muscles

  • Respiratory movements (low thoracic pressure, high abdominal pressure during inspiration)

  • Contraction of smooth muscle in larger lymphatic vessels

  • Valves in lymphatic vessels prevent backflow

• Lymphatic flow is highest during physical exercise.

Lymph Function

Roles of Lymphatic Capillaries

• Absorption and delivery of dietary fats to the bloodstream

• Return of small proteins filtered by blood capillaries

• Collection and delivery of excess interstitial fluid

• Delivery of foreign particles to lymph nodes

16.3: Lymphatic Tissues and Organs

Cell Types and Structures

• Lymphatic tissue contains lymphocytes and macrophages.

Mucosa-Associated Lymphoid Tissue (MALT)

• Unencapsulated lymphatic tissue in digestive, respiratory, urinary, and reproductive tracts.

• Tonsils and appendix are composed of lymphatic nodules.

• Peyer's patches are aggregates of lymphatic nodules in the ileum.

Lymphatic Organs

• Encapsulated lymphatic tissue: lymph nodes, thymus, and spleen.

Lymph Nodes

Structure and Function

• Bean-shaped, <2.5 cm long, located along lymphatic vessels.

• Filter pathogens from lymph.

• Contain lymphocytes and macrophages to fight invading pathogens.

Locations

• Found in groups or chains along larger lymphatic vessels.

• Not found in the central nervous system.

• Major locations: cervical, axillary, supratrochlear, inguinal, pelvic, abdominal, thoracic regions.

Functions

• Filter harmful particles from lymph.

• Immune surveillance via macrophages and lymphocytes.

• Centers for lymphocyte production (with red bone marrow).

• Lymphocytes attack pathogens; macrophages engulf and digest foreign substances and debris.

Thymus

Structure and Function

• Soft, bilobed gland in mediastinum, divided into lobules.

• Lobules contain lymphocytes derived from progenitor cells in red bone marrow.

• Most cells are inactive (thymocytes); some mature into functional T lymphocytes (T cells) for immunity.

• Thymosins are hormones produced in the thymus that stimulate T cell maturation.

Developmental Changes

• Large in infancy and early childhood.

• Shrinks at puberty; small in adults.

• In elderly, lymphatic tissue is replaced by adipose and other connective tissues.

Spleen

Structure and Function

• Largest lymphatic organ, located in the upper left abdominal cavity.

• Resembles a large lymph node; contains sinuses filled with blood.

• Contains two tissue types:

  • White pulp: lymphocytes

  • Red pulp: red blood cells, lymphocytes, macrophages

• Filters blood, similar to how lymph nodes filter lymph.

Major Organs of the Lymphatic System

Organ	Location	Function
Lymph nodes	Groups/chains along larger lymphatic vessels	Filter foreign particles, produce lymphocytes, immune surveillance
Thymus	Mediastinum, superior to heart	Houses lymphocytes, differentiates T cells, produces thymosins
Spleen	Upper left abdominal cavity	Filters blood, removes foreign particles, houses lymphocytes and macrophages

16.4: Body Defenses Against Infection

Pathogens and Infection

• Pathogens: Disease-causing agents (bacteria, viruses, protozoa, fungi).

• Infection occurs when pathogens multiply in the body.

Defense Mechanisms

• Innate (nonspecific) defenses: General defenses against many pathogens.

• Adaptive (specific) defenses: Immunity, targeting specific antigens, carried out by lymphocytes.

Types of Innate (Nonspecific) Defenses

Type	Description
Species resistance	Resistance to diseases affecting other species
Mechanical barriers	Skin and mucous membranes prevent pathogen entry
Chemical barriers	Enzymes, interferons, defensins, collectins, complement proteins
Natural killer cells	Lymphocytes that destroy virus-infected and cancer cells
Inflammation	Local response producing redness, swelling, heat, pain
Phagocytosis	Engulfment and digestion of foreign particles
Fever	Elevated body temperature inhibits microbial growth

Species Resistance

• Certain species are resistant to diseases affecting others.

• Lack of receptors or unsuitable environment for pathogen survival.

Mechanical Barriers

• Skin and mucous membranes are the first line of defense.

• Examples: Epidermis sloughing, ciliated epithelium, hair, tears, saliva, urine.

Chemical Barriers

• Enzymes (e.g., pepsin, lysozyme) destroy microorganisms.

• Interferons block viral replication, act against tumors, stimulate phagocytosis.

• Defensins are peptides that disrupt microbial membranes.

• Collectins protect against bacteria, yeast, viruses.

• Complement proteins stimulate inflammation and enhance phagocytosis.

Natural Killer (NK) Cells

• Small population of lymphocytes, distinct from B and T cells.

• Secrete cytolytic substances (perforins) to lyse cell membranes of virus-infected and cancer cells.

• Enhance inflammation.

Inflammation

• Produces local redness, swelling, heat, and pain.

• Walls off infection site and inhibits spread.

Action	Result
Blood vessels dilate	Tissues become red, swollen, warm, painful
White blood cells invade	Pus may form, white blood cells attack pathogens
Tissue fluids containing clotting factors	Clot forms, delays spread of pathogens
Fibroblasts activate	Connective tissue sac may form around infection
Phagocytes are active	Foreign particles and dead cells removed
Cells divide	Newly formed cells replace damaged ones

Phagocytosis

• Removes foreign particles from lymph and blood.

• Phagocytes: neutrophils and monocytes.

• Monocytes become macrophages in tissues.

• Chemotaxis: chemicals from damaged tissue attract phagocytes.

• Mononuclear phagocytic system (reticuloendothelium): monocytes and macrophages throughout the body.

Fever

• Triggered by viral/bacterial infection; lymphocytes secrete interleukin-1 (IL-1), raising thermoregulatory set point.

• IL-1 is an endogenous pyrogen.

• Elevated temperature inhibits microbial growth and increases phagocytic activity.

Adaptive (Specific) Defenses or Immunity

Overview

• Third line of defense: immunity is resistance to specific pathogens or toxins.

• Immune response distinguishes "self" from "non-self" molecules.

• Antigens: non-self molecules that elicit an immune response.

• Adaptive responses are carried out by lymphocytes and macrophages.

Types of Adaptive Defenses

• Cellular immune response: performed by immune cells.

• Humoral immune response: performed by antibodies.

Lines of Defense Against Pathogens

• First line: Mechanical barriers (skin, mucous membranes)

• Second line: Chemical barriers, NK cells, inflammation, phagocytosis, fever

• Third line: Cellular and humoral immune responses