Cardiovascular System – Capillaries & Lymphatics

Capillaries: Structure & Function

• Exchange site between blood and tissues → requires:
  • Very thin wall: single endothelial layer + basement membrane (no tunica media/adventitia)
  • Large total cross-sectional area → ↓ flow velocity, ↑ exchange time
  • Smooth, slow blood flow regulated by precapillary sphincters (smooth muscle rings)

Types of Blood Capillaries

• Continuous (most common)
  • Endothelial cells joined by tight junctions; few intercellular clefts
  • Diameter ≈ $8\text{–}10\,\mu\text{m}$
  • Found in skeletal, cardiac muscle, skin, CNS
• Fenestrated
  • Endothelium perforated by fenestrae (pores) → ↑ permeability
  • Diameter ≈ $8\text{–}10\,\mu\text{m}$
  • Examples: glomerulus (kidney), small intestine, endocrine glands
• Sinusoidal (discontinuous)
  • Large fenestrae + gaps in basement membrane → “very leaky”
  • Diameter ≈ $30\text{–}40\,\mu\text{m}$; irregular lumen
  • Locations: liver, bone marrow, spleen, adrenal cortex

Capillary Bed Hemodynamics

• Metarteriole + thoroughfare channel create vascular shunt
• Precapillary sphincters open/close to match tissue metabolic demand
• Exchange pathways:
  • Direct diffusion through endothelial cell
  • Passage via intercellular clefts (continuous)
  • Passage via fenestrae (fenestrated)
  • Passage through large gaps (sinusoidal)

Lymphatic System: Key Functions

• $1$ Drain excess interstitial fluid & plasma proteins back to blood
• $2$ Filter foreign material; immune surveillance of lymph
• $3$ Initiate immune responses (antibodies, activated lymphocytes)
• $4$ Absorb dietary fats in intestine → transport as chyle to blood

Lymphatic Vessel Structure

• Begin as blind-ended, large-diameter lymph capillaries in tissues
• Very thin wall: endothelial cells on incomplete basement membrane
• Greater permeability than blood capillaries; admit proteins, pathogens, fat droplets
• Collecting vessels:
  • Thin smooth muscle layer, numerous bicuspid valves → prevent backflow
  • Converge into trunks → ducts
• Intestinal lacteals drain fat-laden lymph → cisterna chyli

Major Lymphatic Pathways

• Right lymphatic duct → right subclavian vein (drains RUQ: right head, thorax, arm)
• Thoracic duct (larger) begins at cisterna chyli; empties into left subclavian vein (drains rest of body)

Regional Lymph Nodes

• Cervical, axillary, inguinal clusters filter lymph from head/neck, upper limb/thorax, lower limb/pelvis respectively
• Structure: multiple afferent vessels, capsule, cortex (B-cells), medulla, single efferent vessel at hilum
• Clinical note: breast lymphatics (axillary nodes) common route for metastatic spread

Distinguishing Features: Arteries vs Veins vs Capillaries vs Lymphatics

• Artery: thick tunica media, no valves, round lumen, RBC present
• Vein: thin media, thick adventitia, valves, RBC present
• Capillary: single endothelial layer, basement membrane, diameter ≈ $8\,\mu\text{m}$ (RBC width)
• Lymphatic vessel: very thin wall, valves, no RBCs, may contain fat droplets (chyle)