Lymphatic, Immune, and Urinary Systems: Comprehensive Study Notes

Lymphatic System

Main Components of the Lymphatic System

The lymphatic system is a network of tissues and organs that help rid the body of toxins, waste, and other unwanted materials. Its three main components are:

• Lymphatics: The vessels that transport lymph.

• Lymph: The fluid transported within lymphatic vessels.

• Lymph nodes: Small, bean-shaped structures that filter lymph and house immune cells.

Lymphatic Vessels

• Lymphatic capillaries: Small, blind-ended vessels where lymph formation begins. Their minivalves open in response to increased interstitial fluid pressure, allowing fluid to enter; they close when pressure inside is higher, preventing backflow.

• Lacteals: Specialized lymphatic capillaries in the intestinal mucosa that absorb dietary fats.

• Collecting lymphatic vessels: Larger vessels formed by converging capillaries, equipped with valves to prevent backflow.

• Lymphatic trunks and ducts: Trunks drain large areas of the body and empty into two main ducts:

  • Right lymphatic duct: Drains right upper limb, right side of head and thorax.

  • Thoracic duct: Drains the rest of the body.

  Both ducts return lymph to venous circulation at the junction of the internal jugular and subclavian veins.

Lymphoid Cells

• Immune cells:

  • Lymphocytes: Main warriors of the immune system, including B cells (produce antibodies) and T cells (manage immune response, attack infected cells).

  • Macrophages: Phagocytize foreign substances and help activate T cells.

  • Dendritic cells: Capture antigens and deliver them to lymph nodes.

• Supporting lymphoid cells:

  • Reticular cells: Produce the stroma (network) that supports other cell types in lymphoid tissues.

Lymphoid Tissue

• Reticular connective tissue: Forms the structural framework of most lymphoid organs.

• Two types:

  • Diffuse lymphoid tissue: Loosely arranged lymphoid cells and reticular fibers.

  • Lymphoid follicles (nodules): Solid, spherical bodies with tightly packed lymphoid cells.

Lymphoid Organs

• Primary lymphoid organs: Sites of lymphocyte maturation (red bone marrow and thymus).

• Secondary lymphoid organs: Sites where mature lymphocytes first encounter antigens (lymph nodes, spleen, MALT, diffuse lymphoid tissues).

Lymph Nodes

• Functions: Filter lymph and activate the immune system.

• Regions:

  • Cortex: Contains follicles with germinal centers rich in B cells; T cells circulate in deeper cortex.

  • Medulla: Contains medullary cords (B and T cells, plasma cells) and sinuses.

• Circulation: Lymph enters via afferent vessels, percolates through cortex and medulla, and exits via fewer efferent vessels, slowing flow for filtration.

• Buboes: Swollen lymph nodes due to infection.

Spleen

• Main functions: Removes old RBCs, stores platelets, recycles iron, and mounts immune responses.

• Red pulp: Site of RBC destruction.

• White pulp: Contains lymphocytes for immune functions.

MALT (Mucosa-Associated Lymphoid Tissue)

• Tonsils: Trap and remove pathogens entering the pharynx.

• Peyer’s patches: Lymphoid tissue in the small intestine.

• Appendix: Contains lymphoid tissue to destroy bacteria and generate memory lymphocytes.

Thymus

• Main function: Site of T cell maturation.

• Difference: Does not directly fight antigens; lacks B cells; stroma consists of epithelial cells, not reticular fibers.

Immune System

Innate (Nonspecific) Defenses

• First line of defense: Skin and mucous membranes, which provide physical and chemical barriers (e.g., acid mantle, enzymes, mucin, defensins).

• Second line of defense:

  • Phagocytes: Neutrophils and macrophages that engulf pathogens. Steps: adherence, ingestion, phagolysosome formation, digestion, exocytosis.

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

  • Inflammatory response: Redness, heat, swelling, pain, and loss of function. Benefits: prevents spread, disposes of debris, sets stage for repair. Stages: release of chemicals, vasodilation, increased permeability, phagocyte mobilization.

  • Antimicrobial proteins:

    • Interferons: Interfere with viral replication.

    • Complement system: Enhances inflammation and destroys pathogens. Three activation pathways: classical, lectin, alternative. Key components:

      • C3a: Enhances inflammation.

      • C3b: Opsonization (tags pathogens for phagocytosis).

      • MAC (Membrane Attack Complex): Forms pores in pathogen membranes, causing lysis.

  • Fever: Systemic response to infection. Pyrogens reset hypothalamic thermostat. Benefits: inhibits microbes, increases repair rate.

Adaptive (Specific) Defenses

• Characteristics: Specificity, memory, systemic response.

• Two main branches:

  • Humoral immunity: B cells produce antibodies targeting extracellular pathogens.

  • Cellular immunity: T cells target infected or abnormal cells directly.

Antigens

• Complete antigens: Have immunogenicity and reactivity.

• Incomplete antigens (haptens): Reactive only when attached to body proteins.

• Antigenic determinants: Specific regions recognized by lymphocytes.

• Self-antigens (MHC proteins): Mark cells as "self"; crucial for immune recognition.

Lymphocytes

• Two main types: B cells (humoral) and T cells (cellular).

• Life cycle steps:

  1. Origin: Both types originate in red bone marrow.

  2. Maturation: B cells mature in bone marrow; T cells in thymus. Selection ensures self-tolerance (positive: recognize self-MHC; negative: do not react to self-antigens).

  3. Seeding: Lymphocytes populate secondary lymphoid organs.

  4. Antigen encounter and activation: Clonal selection occurs upon antigen binding.

  5. Proliferation and differentiation: Forms effector cells (active fighters) and memory cells (long-term immunity).

Antigen Presenting Cells (APCs)

• Dendritic cells, macrophages, and B lymphocytes present antigens to T cells to initiate immune responses.

Humoral Immune Response

• Activation and differentiation of B cells: Upon antigen binding, B cells become plasma cells (secrete antibodies) or memory cells.

• Primary vs secondary immune response: Primary is slower and weaker; secondary is faster and stronger due to memory cells.

• Active immunity: Produced by exposure to antigen (natural: infection; artificial: vaccination).

• Passive immunity: Antibodies obtained from another source (natural: maternal antibodies; artificial: injection of antibodies).

• Antibodies: Y-shaped proteins with variable regions for antigen binding. Mechanisms: neutralization, agglutination, precipitation, complement activation.

Cellular Immune Response

• Two T cell populations:

  • CD4: Helper, regulatory, or memory T cells.

  • CD8: Cytotoxic or memory T cells.

• MHC proteins:

  • Class I: On all nucleated cells; present endogenous antigens; activate CD8 T cells.

  • Class II: On APCs; present exogenous antigens; activate CD4 T cells.

• Activation of T cells: Requires antigen binding and co-stimulation.

• Effector T cell roles:

  • Helper T cells: Activate B and T cells, stimulate macrophages.

  • Cytotoxic T cells: Destroy infected or abnormal cells.

  • Regulatory T cells: Suppress immune response to prevent overactivity.

Immune System Disorders

• Immunodeficiency: Impaired immune function (e.g., HIV/AIDS).

• Autoimmune disease: Immune system attacks self-tissues (e.g., lupus, rheumatoid arthritis).

• Hypersensitivities: Overactive immune responses (e.g., allergies).

Urinary System

General Components and Functions

• Components: Kidneys, ureters, urinary bladder, urethra.

• Kidney functions: Filter blood, remove wastes, regulate fluid/electrolyte balance, acid-base balance, blood pressure, and produce hormones (e.g., erythropoietin).

Gross and Internal Anatomy of Kidneys

• Supportive tissues: Renal fascia, perirenal fat capsule, fibrous capsule.

• Three internal regions: Cortex (outer), medulla (inner), renal pelvis (central cavity).

• Nephrons: Functional units of the kidney.

  • Renal corpuscle: Glomerulus (capillary tuft) and glomerular (Bowman's) capsule.

  • Renal tubule: Proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule (DCT).

  • Collecting duct: Receives filtrate from multiple nephrons.

  • Classes of nephrons: Cortical (short loops, majority) and juxtamedullary (long loops, concentrate urine).

  • Nephron capillary beds: Glomerulus (filtration), peritubular capillaries (reabsorption), vasa recta (concentrate urine).

  • Juxtaglomerular complex: Regulates blood pressure and filtration; includes macula densa, granular cells, and extraglomerular mesangial cells.

Physiology of the Kidneys

• Filtrate vs urine: Filtrate is the initial fluid filtered from blood; urine is the final product after reabsorption and secretion.

• Three steps of urine formation:

  1. Glomerular filtration: Passive process driven by hydrostatic pressure. Filtration membrane layers: fenestrated endothelium, basement membrane, podocyte foot processes. Pressures:

     • Outward: Hydrostatic pressure in glomerular capillaries (HPgc).

     • Inward: Hydrostatic pressure in capsular space (HPcs), colloid osmotic pressure in capillaries (OPgc).

     Net filtration pressure (NFP): $NFP = HP_{gc} - (HP_{cs} + OP_{gc})$ Glomerular filtration rate (GFR): Volume of filtrate formed per minute. Controlled by intrinsic (renal autoregulation) and extrinsic (nervous, hormonal) mechanisms.

  2. Tubular reabsorption: Returns useful substances to blood. Routes: paracellular (between cells) and transcellular (through cells). Sodium reabsorption is active; water follows by osmosis (obligatory vs facultative). Nutrients and ions are reabsorbed by specific transporters.

  3. Tubular secretion: Moves substances from blood into filtrate for excretion (e.g., H+, K+, drugs).

Urine and Urinary Tract

• Urine composition: 95% water, 5% solutes (urea, creatinine, uric acid, ions).

• Ureters: Three layers—mucosa, muscularis, adventitia—transport urine from kidneys to bladder.

• Urinary bladder: Stores urine; differences in size and position between males and females.

• Urethra: Conducts urine out of body; males have longer urethra and dual function (urine and semen); females have shorter urethra, higher risk of infection. Sphincters (internal and external) control release.

• Micturition: Urination; involves contraction of bladder and relaxation of sphincters. Incontinence is the inability to control urination.

Table: Comparison of Innate and Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Non-specific	Specific to antigens
Memory	None	Develops after exposure
Main Cells	Phagocytes, NK cells	B and T lymphocytes
Response Time	Immediate	Slower (days)

Example: Glomerular Filtration Rate (GFR) Calculation

If HPgc = 55 mmHg, HPcs = 15 mmHg, and OPgc = 30 mmHg:

$NFP = 55 - (15 + 30) = 10 \ mmHg$

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard A&P curriculum.