Structure of Blood Vessel Walls

Overview of Circulation

• Heart to Body: Oxygenated blood pumped from left ventricle into aorta.
• Arteries to Capillaries: Blood travels through arteries, arterioles, and forms capillary beds for exchange with tissues.
• Veins to Heart: Capillaries form venules, unite into veins, and drain through superior and inferior vena cava into right atrium.
• Functions:
  • Arteries: Carry oxygenated blood from heart to peripheries.
  • Capillaries: Site of exchange.
  • Veins: Return deoxygenated blood to the heart.

Arterial Wall Structure

• Composed of three layers known as tunics:
  1. Tunica Intima:
     • Innermost layer.
     • Contains:
       • Endothelium
       • Basement membrane (basal lamina)
       • Subendothelial connective tissue
       • Internal elastic lamina (adds elasticity)
  2. Tunica Media:
     • Middle layer with:
       • Connective tissue
       • Smooth muscle
       • Elastic fibers
     • More elastic in arteries closer to the heart (elastic arteries).
     • More muscular farther from the heart (muscular arteries).
     • External elastic lamina between media and adventitia in larger vessels.
  3. Tunica Externa (Adventitia):
     • Outermost layer.
     • Contains mostly collagen.
     • Vasa vasorum (vessels supplying vessel nutrition) located here.

Arterioles

• Smaller versions of arteries.
• Same layers as arteries but:
  • Thinner walls.
  • No internal/external elastic lamina.
  • Media with fewer smooth muscle layers.
  • Thin adventitia.

Capillaries

• Structure: Super thin walls with only endothelium and basement membrane.
• Types:
  • Continuous Capillaries: Tight junctions; substances move by diffusion or through intercellular clefts.
  • Fenestrated Capillaries: Have pores; found in kidneys, small intestine.
  • Sinusoids: Wide clefts, fenestrations, incomplete basement membrane; found in liver, allow passage of plasma proteins.

Venules

• Formed from capillaries.
• Structure:
  • Intima, media (thinner than arterioles) and adventitia.
  • Few smooth muscle cells, elastic fibers.
  • No internal/external elastic lamina.
• Can have pericytes outside basement membrane.

Veins

• Formed from venules.
• Structure:
  • Larger lumens compared to arteries.
  • No internal/external elastic lamina.
  • Thin media with more connective tissue, less smooth muscle and elastic fibers.
  • Thick externa, may contain vasa vasorum.
  • Lower pressure, more collapsible, can distend easier.
  • Valves to maintain unidirectional flow.

Conclusion

• Arteries: More muscular and elastic, high pressure.
• Veins: More connective tissue, lower pressure.
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