Lymphatic system

Capillary Dynamics

  • Exchange of Gases and Nutrients

    • Gases and nutrients move through diffusion.

    • As diffusion occurs, bulk fluid flows also occur.

  • Hydrostatic Pressure (HP)

    • Defined as the force exerted by a fluid pressing against a wall or membrane.

    • In capillaries, Hydrostatic Pressure of Capillaries (HPC) = capillary blood pressure = filtration pressure.

    • HPC is greater at the arterial end (35 mmHg) than at the venous end (16 mmHg).

    • The force pushing HPC is opposed by the hydrostatic pressure of the interstitial fluid (HPIF), which is 0 mmHg (due to excess fluid being reclaimed by the lymphatic vessels).

    • Thus, the formula is:

    • extNetHP=extHPC−extHPIFext{Net HP} = ext{HPC} - ext{HPIF}

    • At the arterial end, extNetHP=35−0=35extmmHgext{Net HP} = 35 - 0 = 35 ext{ mmHg} (fluid out).

  • Osmotic Pressure (OP)

    • Created by the presence of large, non-diffusible molecules (plasma proteins) in a fluid. These molecules draw water towards them.

    • Osmotic Pressure of Capillaries (OPC) = 26 mmHg; OP of Interstitial Fluid (OPIF) = 5 mmHg.

  • Fluid Dynamics in Capillaries

    • At the arterial end, fluid leaves because Net HP > Net OP (pressure differential of 14 mmHg).

    • At the venous end, some fluid is reclaimed because Net OP > Net HP (pressure differential of 5 mmHg).

    • Net fluid loss: 1.5 mL/min.

    • Excess fluid becomes interstitial fluid and is picked up by the lymphatic system to return to the vascular system.

    • If not, the body would deplete plasma within a day.

The Lymphatic System

Components and Functions

  • Lymphatic Vessels

    • Transport excess fluid from interstitial spaces back to the venous system.

  • Lymphoid Tissues and Organs

    • House phagocytic cells (lymphocytes) that cleanse lymph and play a role in body defense.

Inflammation

  • Definition

    • Inflammation occurs when body tissues are injured due to physical trauma, infections (bacterial, viral, fungal), chemical damage, or heat.

  • Importance of Inflammatory Response

    • Prevents/limits the spread of damaging agents to adjacent tissues.

    • Disposes of cellular debris and pathogens.

    • Sets the stage for the repair process.

  • Chemical Release in Inflammation

    • Injured cells, phagocytes, lymphocytes, and mast cells release inflammatory chemicals leading to:

    1. Vasodilation of arterioles, increasing blood to the area (causing heat and redness).

    2. Increased Permeability of local capillaries, allowing more fluid to enter interstitial spaces (causing edema and pain due to excess fluid pressure).

  • Functions of Inflammation

    • Dilutes harmful substances.

    • Brings in oxygen and nutrients for repair.

    • Allows entry of clotting proteins.

    • Clotting proteins form a gel-like mesh, isolating the injured area and preventing the spread of harmful substances.

  • Phagocyte Action

    • Neutrophils arrive first, followed by monocytes which become macrophages.

    • Phagocytes and other cells are drawn to the injury site via positive chemotaxis due to leukocytosis-inducing factors released by injured cells.

    • Endothelial cells release cell adhesion molecules (CAMS) called selectins, leading to neutrophil margination (sticking to endothelial wall) and diapedesis (squeezing through walls).

Body Defenses: Nonspecific and Specific Defenses

Defense Systems

  1. Nonspecific (Innate) Defense System

    • Responds immediately to foreign substances.

    • Includes:

    • External body membranes (skin and mucosa).

    • Chemical and cellular defenses (inflammation, phagocytes, antimicrobial proteins).

  2. Specific (Adaptive) Defense System (Immune System)

    • Mounts an attack against specific foreign substances.

    • Key cells include macrophages and lymphocytes.

    • Acts against microorganisms, cancer cells, and transplanted tissues.

Nonspecific Defense System
A. Surface Membrane Barriers

  • Skin and Mucous Membranes

    • Epidermis - acts as a physical barrier, resistant to weak acids and bases.

    • Acidic skin secretions (acid mantle) prevent bacterial growth.

    • Mucous membranes - trap bacteria in respiratory and digestive passages.

B. Nonspecific Cellular and Chemical Defenses

  1. Phagocytes

    • Main phagocytes are macrophages (from monocytes) and neutrophils.

    • Engulf foreign particles through phagocytosis.

    • Macrophages present antigens and can be wandering or fixed.

    • Opsonization - coating of foreign substances with proteins to facilitate adhesion to phagocytes.

  2. Natural Killer (NK) Cells

    • Non-specific defense, located in blood and lymph.

    • Can lyse cancer and virus-infected cells before adaptive immune response.

    • Release cytolytic enzymes (perforins) and other chemicals enhancing immune response.

Immune System and Lymphatic Pathways

Cells and Organs Involved in Immunity
A. Lymphocytes

  • T Cells

    • Activated T cells manage immune response and can directly attack antigens.

  • B Cells

    • Produce plasma cells that produce antibodies, binding and inactivating antigens.

B. Lymphoid Tissue

  • Reticular Connective Tissue

    • Contains macrophages and lymphocytes in clusters and spaces.

C. Lymphoid Organs

  1. Lymph Nodes

    • Clustered along lymph vessels, filter lymph, and monitor antigens.

    • More afferent lymphatic vessels than efferent vessels for thorough cleaning.

  2. Tonsils

    • Partially encapsulated lymph nodules found in the pharynx; help to trap pathogens.

  3. Peyer’s Patches

    • Aggregations in the ileum of small intestine to defend against ingested pathogens.

  4. Spleen

    • Largest lymphatic organ, filters blood, removes bacteria, and recycles red blood cells.

    • Contains white pulp (lymphocytes) and red pulp (red blood cells and macrophages).

Lymphatic Pathways

  • Fluid leaves capillaries at arterial end; most is reclaimed at venule end.

  • Lymphatic Capillaries

    • Blind-ended vessels; excess interstitial fluid enters here.

    • Present throughout the body except in teeth and bones.

    • Once fluid enters capillaries, it is called lymph.

  • Lacteals

    • Specialized capillaries in intestinal villi that transport fatty lymph, known as chyle.

  • Pathway: Lymphatic capillaries → afferent lymphatic vessels → lymph nodes (filter lymph) → efferent vessels → lymphatic trunks → ducts (right lymphatic duct and thoracic duct).

  • Thoracic Duct

    • Collects lymph from the left side of the head, neck, thorax, abdomen, and legs; empties into venous system at left subclavian vein and left internal jugular vein.

  • Right Lymphatic Duct

    • Collects lymph from the right side of the head, neck, thorax; empties into right subclavian vein and right internal jugular vein.

Lymph Transport

  • Pumpless Transport

    • Lymph transport is low pressure and dependent on several mechanisms:

    1. Skeletal muscle contraction.

    2. Actions of breathing.

    3. Smooth muscle contraction in lymph vessel walls.

    4. Valves prevent backflow.

  • Increased physical activity increases lymph flow, hence it’s important to immobilize infected areas to prevent spreading bacteria.

Antimicrobial Proteins and Fever

  • Antimicrobial Proteins

    • Enhance non-specific defenses by directly attacking microorganisms or preventing their reproduction.

A. Complement Proteins

  • Group of at least 20 plasma proteins circulating in an inactive state that, when activated, amplify the inflammatory response, kill bacteria by cell lysis, and enhance phagocytosis.

B. Interferons

  • Proteins released by virus-infected cells; diffuse to nearby cells, stimulating production of antiviral proteins such as PKR, inhibiting viral replication.

    • Interferons play an anti-cancer role by activating macrophages and mobilizing NK cells.

C. Fever

  • Definition: Abnormally high body temperature caused by pyrogen release from leukocytes and macrophages.

    • Moderate fever is beneficial as it sequesters nutrients from bacteria and speeds up healing processes.

    • High fever is dangerous as it can denature proteins.

Specific Body Defenses (Immunity)

Aspects of Immune System

  • Metabolically expensive; must be primed by initial exposure to an antigen; is antigen-specific and systemic; produces memory cells that recognize and mount a strong attack against previously encountered pathogens.

Types of Immunity

  1. Antibody-Mediated Immunity (Humoral Immunity)

    • Involves antibodies produced by B lymphocytes, circulating in blood and lymph, marking antigens for destruction.

  2. Cell-Mediated Immunity (Cellular Immunity)

    • Involves T lymphocytes. Defense can be direct (lysing cells) or indirect (releasing chemicals that enhance inflammation and activate other lymphocytes or macrophages).