Blood Vessel Structure and Types

Blood Vessel Structure

• The circulatory system consists of blood vessels, including the heart, a folded tube forming during embryonic development.
• Blood vessels are layered:
  • Tunica intima: Innermost layer of endothelial cells, basement membrane, and sub-endothelial connective tissue.
  • Tunica media: Middle layer, mainly smooth muscle fibers. Elastic fibers are prominent in some arteries, less in others and veins. The heart's myocardium is composed of cardiac muscle fibers.
  • Tunica adventitia: Outermost layer of connective tissue. Larger arteries have small arteries supplying their walls (vasa vasorum), also called coronary circulation in the heart.

Artery Types

• Muscular/Distributing Arteries: Distribute blood to organs.
  • Thick Tunica media with many smooth muscle layers.
  • Smooth muscle controls blood flow. Examples: "fight or flight" relaxes smooth muscles in arteries supplying vital organs (heart, skeletal muscles), contracting those to less urgent areas.
• Elastic/Conducting Arteries: Large arteries connected to the heart (like the aorta).
  • Multiple layers of elastic fibers in the Tunica media.
  • Elastic layers dampen pulsatile blood flow from the heart for continuous blood flow.

Arterioles, Venules, Veins

• Arterioles: Small arteries (<0.5mm).
  • Similar structure to arteries, but only 1-5 smooth muscle layers in the Tunica media.
  • Some have an internal elastic lamina, but no external.
• Venules: Small veins accompanying arterioles.
  • Thin walls compared to lumen size, easily compressed.
• Veins: Thinner walls than arteries due to lower blood pressure.
  • Tunica adventitia is the thickest layer.
  • Valves prevent backflow towards the heart.

Distinguishing Arteries and Veins

• Arteries:
  • Circular, uniform shape.
  • Thick walls with a prominent Tunica media.
  • Internal and external elastic laminae.
• Veins:
  • Often flattened, easily compressed due to thinner walls.
  • Thin Tunica media compared to the Tunica adventitia.
  • No internal elastic lamina.
  • May have valves.

The Heart Wall

• Endocardium: Inner lining.
  • Endothelial cells and sub-endothelial connective tissue.
  • Contains Purkinje fibers, specialized cardiac muscle for conduction.
• Myocardium: Thick muscular layer.
  • Cardiac muscle fibers: branched, striated, centrally located nuclei.
  • Intercalated discs: specialized junctions (gap junctions and desmosomes) for communication and contraction.
• Epicardium: Outer layer.

Capillaries

• Capillaries: Tiny blood vessels with only a Tunica intima.
  • Facilitate substance exchange between blood and interstitial fluid.

Key Facts

• Internal Elastic Lamina: Elastic fibers between Tunica intima and Tunica media.
• External Elastic Lamina: Elastic fibers between Tunica media and Tunica adventitia.
• Vasa Vasorum: Small arteries supplying larger blood vessel walls.
• Intercalated Discs: Specialized junctions in cardiac muscle for communication & prevention of separation during contraction.
• Purkinje Fibers: Modified cardiac muscle fibers conducting electrical impulses in the heart.
• Nervous System: Nerves accompany blood vessels to regulate blood flow.