Blood Vessel Wall Layers – Study Notes

Learning Outcomes

• Describe the three concentric layers (tunics) that form the wall of a typical blood vessel.
• State the primary function of each tunic.
• Define and distinguish between the processes of vasoconstriction and vasodilation.

Structural Layers of Blood Vessels

• All blood vessels—except the very smallest—share the same basic wall plan:
  • Three tunics wrap around a central, blood‐filled space called the lumen.
  • Arrangement (from innermost to outermost):
  1. Tunica intima
  2. Tunica media
  3. Tunica externa (aka tunica adventitia)
• The three layers differ in thickness, tissue composition, and functional emphasis depending on vessel type (arteries, capillaries, veins).

Tunica Intima (Innermost Layer)

• In direct, “intimate” contact with the blood in the lumen.
• Key components
  • Endothelium
  • Simple squamous epithelium.
  • Continuous with the endocardium lining the heart.
  • Closely fitted, flattened cells create a slick, low-friction surface → promotes smooth blood flow.
  • Subendothelial layer (present only in vessels with diameter > 1\ \text{mm})
  • Basement membrane + loose connective tissue.
  • Provides structural support for the endothelium.
• Primary function: minimize friction while providing a selectively permeable, anti-thrombogenic surface.

Tunica Media (Middle Layer)

• Composition
  • Mostly circularly arranged smooth muscle cells.
  • Interlaced sheets of elastin.
• Regulation
  • Controlled by sympathetic vasomotor nerve fibers (autonomic nervous system).
  • Various circulating/local chemicals (e.g., nitric oxide, endothelins) fine-tune tone.
• Functional actions
  • Vasoconstriction: smooth muscle contracts → lumen diameter ↓.
  • Vasodilation: smooth muscle relaxes → lumen diameter ↑.
• Physiological significance
  • Small changes in diameter cause large changes in resistance → critical for regulating blood pressure & regional blood flow.
  • Bulkiest layer in arteries, reflecting their primary role in maintaining and distributing pressure generated by the heart.

Tunica Externa / Adventitia (Outermost Layer)

• Composition
  • Loosely woven collagen fibers → protect, reinforce, and anchor the vessel to neighboring structures.
  • Nerve fibers & lymphatic vessels permeate the layer.
  • In larger veins: additional network of elastic fibers for stretch-recoil.
• Vasa vasorum ("vessels of the vessels")
  • Tiny blood vessels within the tunica externa of large vessels.
  • Supply oxygen and nutrients to the outer half of the vessel wall—the inner half is nourished directly by blood in the lumen.
• Overall role: external support and integration with surrounding tissues.

Vasoconstriction vs. Vasodilation

• Vasoconstriction
  • Trigger: sympathetic stimulation or vasoconstrictor chemicals.
  • Response: smooth muscle contraction → ↓ lumen diameter.
  • Result: ↑ vascular resistance, ↑ blood pressure, ↓ local flow.
• Vasodilation
  • Trigger: reduced sympathetic tone or vasodilator chemicals (e.g., NO).
  • Response: smooth muscle relaxation → ↑ lumen diameter.
  • Result: ↓ vascular resistance, ↓ blood pressure (systemically), ↑ local flow.

Nervous & Chemical Control

• Innervation
  • Branch: sympathetic division of the autonomic nervous system.
  • Nerve fibers penetrate primarily the tunica media.
• Effectors
  • Vascular smooth muscle cells.
• Outcome
  • Contraction produces vasoconstriction; relaxation produces vasodilation.

Vessel Categories & Relative Dimensions (Illustrative Hierarchy)

• Arteries
  • Elastic (conducting) artery → Muscular (distributing) artery → Arteriole.
• Capillaries
  • Microscopic, single-cell-layer walls (only tunica intima).
• Veins
  • Venule → Vein (capacitance vessel).
• Text notes mention indicative diameters & wall thicknesses (e.g., D \approx 20\ \text{mm},\ T \approx 1.0\ \text{mm} for an elastic artery) but stress that figure sizes are not to scale and were enlarged for clarity.

Self-Check Questions & Key Answers

• Q: Which branch of the autonomic nervous system innervates blood vessels?
  • A: The sympathetic branch.
• Q: Which tunic is directly innervated, and what are the effectors?
  • A: Tunica media; the effectors are smooth muscle cells.
• Q: What happens to vessel diameter when vascular smooth muscle contracts, and what is this process called?
  • A: Diameter decreases; the process is vasoconstriction.

Key Terminology

• Lumen: Central, blood-containing space inside a vessel.
• Endothelium: Simple squamous epithelium lining the lumen.
• Subendothelial layer: Connective tissue supporting endothelium in larger vessels.
• Vasomotor fibers: Sympathetic nerves controlling vessel diameter.
• Vasa vasorum: Small vessels that feed the external walls of large vessels.
• Capacitance vessels: Alternate term for veins due to their blood-reservoir function.