Lymphatic System

Overview of the Lymphatic System

The lymphatic system is a network of vessels, tissues, and organs ( provided structural basis of immune system by housing phagocytic cells and lymphocytes) that help maintain fluid balance (returns fluids leaked from blood vessels back to blood) , protect the body from infection, and facilitate the absorption of dietary fats. It consists of three main components: lymphatic vessels, lymph, and lymph nodes.

• Lymphatics: The network of lymphatic vessels that transport lymph throughout the body.

• Lymph: The fluid that circulates within the lymphatic system (vessels), derived from interstitial fluid.

• Lymph Nodes: Small, bean-shaped structures that filter lymph and house immune cells. (cleanse lymph)

Lymphatic Vessels

Lymphatic vessels offer one-way system, ensuring lymph (fluid in vessels) flows only towards the heart). Includes lymphatic capillaries and larger lymphatic vessels.

• Lymphatic Capillaries:

- open-ended vessels that weave between tissue cells and blood capillaries

- similar to blood capillaries, but they are more permeable and can take up larger molecules & particles (ex: proteins, cell debris, pathogens, & cancer cells).

- increased permeability comes from two specialized structures minivalves & collage fibers?

Minivalves: formed from endothelial cells overlap loosely, are anchored by collgen filaments to matric

• Microscopic, blind-ended vessels that absorb interstitial fluid. Their minivalves open in response to increased interstitial pressure, allowing fluid to enter.

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

• Collecting Lymphatic Vessels: Larger vessels that receive lymph from capillaries and transport it toward lymphatic trunks.

• Lymphatic Trunks and Ducts: Trunks drain large regions of the body and empty into two main ducts:

  • Right Lymphatic Duct: Drains lymph from the right upper limb, right side of the head, and thorax into the right subclavian vein.

  • Thoracic Duct: Drains lymph from the rest of the body into the left subclavian vein.

Lymphoid Cells

• Immune Cells:

  • Lymphocytes: Main warriors of the immune system, including B cells (produce antibodies) and T cells (mediate cellular immunity).

  • Macrophages: Phagocytic cells that engulf pathogens and present antigens.

  • Dendritic Cells: Specialized antigen-presenting cells that activate lymphocytes.

• Supporting Lymphoid Cells:

  • Reticular Cells: Produce the reticular fiber stroma that supports other cells in lymphoid organs.

Lymphoid Tissue

• Reticular Connective Tissue: The main tissue type in lymphoid organs, providing a supportive framework.

• Types of Lymphoid Tissue:

  • Diffuse Lymphoid Tissue: Loosely arranged lymphoid cells and reticular fibers found in most body organs.

  • Lymphoid Follicles (Nodules): Densely packed lymphoid cells and reticular fibers, often with germinal centers for B cell proliferation.

Lymphoid Organs

• Primary Lymphoid Organs: Sites of lymphocyte maturation (red bone marrow and thymus).

• Secondary Lymphoid Organs: Sites where mature lymphocytes encounter antigens (lymph nodes, spleen, MALT, diffuse lymphoid tissue).

Lymph Nodes

• Functions: Filter lymph and activate the immune system.

• Regions:

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

  • Medulla: Contains medullary cords with B cells, T cells, and plasma cells.

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

• Buboes: Swollen lymph nodes, often due to infection.

Spleen

• Main Functions: Filters blood, recycles old red blood cells, stores platelets and monocytes, and initiates immune responses to blood-borne antigens.

• Red Pulp vs White Pulp:

  • Red Pulp: Site of RBC destruction and blood filtration.

  • White Pulp: Contains lymphocytes for immune function.

MALT (Mucosa-Associated Lymphoid Tissue)

• Tonsils: Trap and remove pathogens entering the pharynx.

• Peyer's Patches and Appendix: Protect the digestive tract from pathogens.

Thymus

• Main Functions: Site of T cell maturation; most active during childhood.

• Differences: Does not directly fight antigens; lacks B cells; atrophies with age.

Immune System

Innate (Nonspecific) Defenses

• First Line of Defense: Skin and mucous membranes act as physical barriers; secrete chemicals (e.g., acids, enzymes, defensins) that inhibit or destroy microorganisms.

• Second Line of Defense:

  • Phagocytes: Include neutrophils and macrophages; engulf and digest pathogens in a series of steps: adherence, ingestion, phagosome formation, lysosome fusion, and digestion.

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

  • Inflammatory Response: Local response to injury; signs include redness, heat, swelling, pain, and loss of function. Stages: release of inflammatory chemicals, vasodilation, increased permeability, phagocyte mobilization.

  • Antimicrobial Proteins:

    • Interferons: Proteins released by virus-infected cells to protect neighboring cells.

    • Complement System: Group of plasma proteins that enhance immune responses. Activated via three pathways: classical, lectin, and alternative. Key components: C3a (inflammation), C3b (opsonization), MAC (membrane attack complex for cell lysis).

  • Fever: Systemic response to infection; pyrogens reset the hypothalamic thermostat. Benefits include inhibiting pathogen growth and speeding tissue repair.

Adaptive (Specific) Defenses

• Characteristics: Specificity, memory, and systemic response.

• Two Main Branches:

  • Humoral Immunity: Mediated by B cells producing antibodies targeting extracellular pathogens.

  • Cellular Immunity: Mediated by T cells targeting infected or abnormal cells directly or indirectly.

• Antigens:

  • Complete Antigens: Have immunogenicity and reactivity.

  • Incomplete Antigens (Haptens): Not immunogenic unless attached to a carrier.

  • Antigenic Determinants: Specific regions recognized by lymphocytes.

  • Self-Antigens (MHC Proteins): Major histocompatibility complex proteins mark cells as "self."

• Lymphocytes:

  • B Cells: Mature in bone marrow; produce antibodies.

  • T Cells: Mature in thymus; mediate cellular immunity.

  • Life Cycle Steps:

    1. Origin (bone marrow)

    2. Maturation (positive selection for self-MHC recognition; negative selection eliminates self-reactive cells)

    3. Seeding in secondary lymphoid organs

    4. Antigen encounter and activation (clonal selection)

    5. Proliferation and differentiation (effector and memory cells)

• Antigen Presenting Cells (APCs): Dendritic cells, macrophages, and B lymphocytes present antigens to T cells.

Humoral Immune Response

• Activation and Differentiation of B Cells: Upon antigen binding, B cells differentiate into plasma cells (antibody producers) and memory cells.

• Primary vs Secondary Immune Response: Primary is slower and weaker; secondary is faster and stronger due to memory cells.

• Active vs Passive Humoral Immunity:

  • Active: Body produces antibodies (naturally by infection, artificially by vaccination).

  • Passive: Antibodies are received (naturally via placenta/milk, artificially via injection).

• Antibodies:

  • Basic Structure: Y-shaped molecules with variable regions for antigen binding.

  • Defensive Mechanisms: Neutralization, agglutination, precipitation, complement activation.

Cellular Immune Response

• T Cell Populations:

  • CD4 Cells: Helper, regulatory, or memory T cells; recognize antigens on MHC II.

  • CD8 Cells: Cytotoxic or memory T cells; recognize antigens on MHC I.

• MHC Proteins:

  • Class I: On all nucleated cells; present endogenous antigens; crucial for CD8 activation.

  • Class II: On APCs; present exogenous antigens; crucial for CD4 activation.

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

• Effector T Cell Roles:

  • Helper T Cells: Activate B cells, other T cells, and macrophages.

  • Cytotoxic T Cells: Destroy infected or abnormal cells.

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

Immune System Disorders

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

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

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

Urinary System

Overview and General Functions

The urinary system removes metabolic wastes, regulates fluid and electrolyte balance, and maintains acid-base homeostasis. Its main components are the kidneys, ureters, urinary bladder, and urethra.

• Kidneys: Filter blood, form urine, regulate blood pressure, and produce hormones.

• Ureters: Transport urine from kidneys to bladder.

• Urinary Bladder: Stores urine.

• Urethra: Conducts urine out of the body.

Gross and Internal Anatomy of the Kidneys

• Supportive Tissues: Renal fascia, perirenal fat capsule, and fibrous capsule protect and anchor the kidneys.

• Three Internal Regions: Cortex (outer), medulla (inner), and renal pelvis (central collecting region).

Nephrons

• Renal Corpuscle:

  • Glomerulus: Capillary tuft for filtration.

  • Glomerular (Bowman's) Capsule: Surrounds the glomerulus and collects filtrate.

• Renal Tubule:

  • Proximal Convoluted Tubule (PCT): Major site of reabsorption.

  • Nephron Loop (Loop of Henle): Descending and ascending limbs for water and solute reabsorption.

  • Distal Convoluted Tubule (DCT): Further reabsorption and secretion.

• Collecting Duct: Receives filtrate from multiple nephrons; final urine concentration occurs here.

• Classes of Nephrons:

  • Cortical Nephrons: Located mostly in the cortex; short loops.

  • Juxtamedullary Nephrons: Long loops extend deep into the medulla; important for concentrating urine.

• Nephron Capillary Beds: Glomerulus (filtration), peritubular capillaries (reabsorption), vasa recta (concentration gradient maintenance).

• Juxtaglomerular Complex: Regulates blood pressure and filtrate formation; includes macula densa, granular cells, and extraglomerular mesangial cells.

Urinary Physiology

• Filtrate vs Urine: Filtrate is the initial fluid filtered from blood; urine is the final excreted 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, and podocyte filtration slits.

  2. Tubular Reabsorption: Selective movement of substances from filtrate back into blood via paracellular (between cells) and transcellular (through cells) routes. Sodium reabsorption occurs across basolateral and apical membranes. Water reabsorption can be obligatory (always occurs) or facultative (regulated by hormones).

  3. Tubular Secretion: Transfer of substances from blood into filtrate; important for eliminating wastes and regulating pH.

• Pressures Involved in Filtration:

  • Outward Pressure: Hydrostatic pressure in glomerular capillaries (pushes fluid out).

  • Inward Pressures: Hydrostatic pressure in capsular space and colloid osmotic pressure in capillaries (oppose filtration).

  • Net Filtration Pressure (NFP): Determines the amount of filtrate formed.

• Glomerular Filtration Rate (GFR): Volume of filtrate formed per minute. Controlled by intrinsic (renal autoregulation) and extrinsic (nervous and endocrine) mechanisms.

Urine and Urinary Tract Anatomy

• Urine Composition: 95% water, 5% solutes (urea, creatinine, ions, etc.).

• Ureters: Three layers: mucosa, muscularis, and adventitia.

• Urinary Bladder: Stores urine; differs in position and size between males and females.

• Urethra: Contains internal and external sphincters; longer in males than females.

• Micturition: Process of urination; involves voluntary and involuntary muscle control. Incontinence is the inability to control urination.

Table: Comparison of Innate and Adaptive Immunity

Table: Types of Humoral Immunity

Additional info: Academic context and explanations have been expanded for clarity and completeness. Tables have been logically constructed to summarize key comparisons and classifications.