Lymphatic System

Primary and Secondary Lymphatic Organs

The lymphatic system is composed of organs and tissues that help defend the body against infection and maintain fluid balance.

• Primary lymphatic organs: Sites where lymphocytes mature and become immunocompetent.

  • Red bone marrow: Site of B cell maturation and origin of all lymphocytes.

  • Thymus: Site of T cell maturation.

• Secondary lymphatic organs: Sites where mature lymphocytes encounter antigens and initiate immune responses.

  • Lymph nodes

  • Spleen

  • MALT (Mucosa-Associated Lymphoid Tissue): Includes tonsils, Peyer's patches, and the appendix.

Functions of Lymphatic Vessels

• Return excess interstitial fluid to the bloodstream.

• Transport dietary lipids from the digestive tract (via lacteals).

• Facilitate immune responses by transporting lymphocytes and antigen-presenting cells.

Structure and Distribution of Lymphatic Vessels

• Lymphatic capillaries: Blind-ended, highly permeable vessels found throughout most tissues (except CNS, bone marrow, and avascular tissues).

• Larger lymphatic vessels: Resemble veins but have thinner walls and more valves.

• Distribution: Lymphatic vessels are widespread, paralleling veins in the body.

Source of Lymph and Mechanisms of Lymph Transport

• Source: Lymph is derived from interstitial fluid that enters lymphatic capillaries.

• Transport mechanisms:

  • Skeletal muscle contraction

  • Pressure changes during breathing

  • Valves to prevent backflow

  • Pulsations of nearby arteries

  • Smooth muscle contraction in vessel walls

Structure and Cellular Population of Lymphoid Tissue

• Reticular connective tissue: Main structural framework.

• Cell types:

  • Lymphocytes (B and T cells)

  • Macrophages

  • Dendritic cells

  • Reticular cells

Structure and Function of Lymph Nodes

• Structure: Bean-shaped, encapsulated organs with cortex (containing follicles) and medulla (containing cords).

• Function: Filter lymph and activate the immune system.

Comparison: Spleen vs. Lymph Nodes

MALT (Mucosa-Associated Lymphoid Tissue)

• Definition: Lymphoid tissues in mucous membranes throughout the body.

• Major components: Tonsils, Peyer's patches (small intestine), appendix.

Thymus: Structure and Function

• Structure: Bilobed organ in the mediastinum; prominent in children, atrophies with age.

• Function: Site of T cell maturation; does not directly fight antigens.

Immune System

Role of the Immune System

The immune system protects the body from pathogens, removes dead or damaged cells, and recognizes and eliminates abnormal cells.

Innate vs. Adaptive Immunity

Three Lines of Defense

1. First line: Surface barriers (skin, mucous membranes)

2. Second line: Innate internal defenses (phagocytes, NK cells, inflammation, antimicrobial proteins, fever)

3. Third line: Adaptive defenses (B and T cells, antibodies)

Surface Membrane Barriers

• Physical barriers: Skin, mucous membranes

• Chemical barriers: Acid mantle, enzymes, mucin, defensins

Phagocytosis, Natural Killer Cells, and Fever

• Phagocytosis: Engulfment and destruction of pathogens by macrophages and neutrophils.

• Natural killer (NK) cells: Destroy virus-infected and cancerous cells by inducing apoptosis.

• Fever: Elevated body temperature inhibits pathogen growth and enhances immune reactions.

Inflammatory Process

• Triggered by tissue injury or infection.

• Cardinal signs: Redness, heat, swelling, pain, loss of function.

• Functions: Isolate pathogens, recruit immune cells, promote healing.

Antimicrobial Substances

• Interferons: Inhibit viral replication.

• Complement proteins: Enhance phagocytosis, lyse pathogens, promote inflammation.

• Defensins: Antimicrobial peptides produced by epithelial cells.

Aspects of Adaptive Immunity

• Specificity: Targets specific antigens.

• Systemic response: Not limited to infection site.

• Memory: Mounts stronger attacks on previously encountered antigens.

Antigens and Adaptive Defenses

• Antigen: Any substance that can provoke an immune response.

• Antigens are recognized by lymphocytes, triggering adaptive immunity.

B and T Lymphocytes: Origin, Maturation, and Function

• Origin: Both arise from hematopoietic stem cells in bone marrow.

• Maturation: B cells mature in bone marrow; T cells mature in thymus.

• Function: B cells mediate humoral immunity (antibody production); T cells mediate cellular immunity (direct cell attack, regulation).

Immunocompetence and Self-Tolerance

• Immunocompetence: Ability to recognize a specific antigen.

• Self-tolerance: Ability to avoid attacking the body's own cells.

• Developed during lymphocyte maturation.

Antigen-Presenting Cells (APCs)

• Examples: Dendritic cells, macrophages, B cells.

• Role: Process and present antigens to T cells to initiate adaptive responses.

Clonal Selection of B Cells, Plasma Cells, and Memory Cells

• Clonal selection: Activation and proliferation of B cells upon antigen binding.

• Plasma cells: Produce antibodies.

• Memory cells: Provide long-term immunity.

Antibodies: Structure and Classes

• Structure: Y-shaped molecules with variable (antigen-binding) and constant regions.

• Classes:

Cellular Immunity: T Cell Activation and Clonal Selection

• T cells are activated by antigen presentation and co-stimulation.

• Clonal selection leads to proliferation of effector and memory T cells.

Types and Functions of T Cells

• Helper T cells (CD4+): Activate B cells, cytotoxic T cells, and macrophages.

• Cytotoxic T cells (CD8+): Destroy infected or abnormal cells.

• Regulatory T cells: Suppress immune responses to prevent autoimmunity.

• Memory T cells: Provide long-term immunity.

Categories of Immune Disorders

Respiratory System

Location, Structure, and Function of Respiratory Organs

• Nose: Filters, warms, and moistens air; houses olfactory receptors.

• Paranasal sinuses: Lighten skull, warm and moisten air.

• Pharynx: Passageway for air and food; houses tonsils.

• Larynx: Voice production; routes air and food.

• Trachea: Windpipe; conducts air to bronchi.

• Bronchi: Branch into lungs; conduct air to alveoli.

Protective Mechanisms of the Respiratory System

• Mucus traps particles.

• Cilia move mucus toward the pharynx.

• Cough and sneeze reflexes expel irritants.

• Alveolar macrophages remove debris.

Respiratory Membrane: Structure and Function

• Composed of alveolar epithelium, capillary endothelium, and fused basement membranes.

• Thin barrier (< 0.5 μm) for efficient gas exchange.

Respiratory Passageways (Descending Order)

1. Nose/nasal cavity

2. Pharynx

3. Larynx

4. Trachea

5. Primary bronchi

6. Secondary (lobar) bronchi

7. Tertiary (segmental) bronchi

8. Bronchioles

9. Terminal bronchioles

10. Respiratory bronchioles

11. Alveolar ducts

12. Alveoli

Gross Structure of Lungs and Pleurae

• Lungs: Paired, spongy organs divided into lobes (3 right, 2 left).

• Pleurae: Double-layered serous membranes (parietal and visceral pleura) that reduce friction and compartmentalize lungs.

Pulmonary Function Tests

• Measure lung volumes and capacities (e.g., tidal volume, vital capacity).

• Assess airflow and gas exchange efficiency.

• Diagnose obstructive and restrictive lung diseases.

Atmospheric vs. Alveolar Air Composition

• Alveolar air has higher CO2 and water vapor, lower O2 than atmospheric air due to gas exchange and humidification.

External and Internal Respiration

• External respiration: Gas exchange between alveoli and blood.

• Internal respiration: Gas exchange between blood and tissues.

Oxygen and Carbon Dioxide Transport in Blood

• Oxygen: Mostly bound to hemoglobin; small amount dissolved in plasma.

• Carbon dioxide: Transported as bicarbonate ion (HCO3-), bound to hemoglobin, or dissolved in plasma.

Key equation for CO2 transport:

Factors Affecting Oxygen Loading and Unloading

• Increased temperature, decreased pH, increased BPG, and increased PCO2 promote oxygen unloading (Bohr effect).

Neural Controls of Respiration

• Respiratory centers in the medulla and pons regulate rate and depth of breathing.

• Chemoreceptors monitor CO2, O2, and pH levels.

Hyperpnea vs. Hyperventilation

• Hyperpnea: Increased ventilation in response to metabolic need (e.g., exercise); PCO2 remains stable.

• Hyperventilation: Excessive ventilation beyond metabolic need; leads to decreased PCO2 (hypocapnia).

Respiratory Disorders: Causes and Consequences

Laboratory Exercises: Lymphatic and Respiratory Systems

Exercise 35: Lymphatic System & Immune Response

• Locate and identify primary and secondary lymphatic organs.

• Identify basic structure and cellular population of lymphoid tissue.

• Identify general location, histological structure, and functions of lymph nodes.

• Identify location and histological structure of spleen and thymus.

• Locate and identify major components of MALT.

Exercise 36 & 37: Anatomy & Physiology of the Respiratory System

• Identify structures of the upper and lower respiratory system.

• Identify structures in the conducting and respiratory zones.

• Identify histologic structure of trachea and lung tissue microscopically.

• Identify membranes that enclose the lungs.

Additional info: Students are encouraged to use anatomical models and histology slides for identification and review.