Study Question Thirteen

• Q13. Describe the changes that occur in the thickness of the tunica externa and tunica media of arteries as they become many smaller arterioles and eventually give rise to many more capillaries.
• Tunica Externa

  • Arteries to Arterioles:
  • Becomes thinner as artery transitions to arteriole.
  • In larger arteries, tunica externa is relatively thick to provide support and protection.
  • Vessel diameter decreases, need for support lessens, so tunica externa thins.
• Tunica Externa

  • Arterioles to Capillaries:
  • Layer becomes less prominent in smaller arterioles.
  • By capillaries, layer is essentially absent, as capillaries are composed of single layer of endothelial cells and lack defined outer connective tissue layer.
• Tunica Media

  • Arteries to Arterioles:
  • Layer mainly of smooth muscle cells and elastic fibers, thick in size.
  • Regulate blood pressure and facilitate blood flow under high pressure.
  • As arteries become arterioles, layer decreases in thickness.
  • Arterioles have a thinner media than larger arteries, but still contain smooth muscle cells.
  • Muscle layer in arterioles vital to controlling vasoconstriction and vasodilation, regulating blood flow to tissues.
• Tunica Media

  • Arterioles to Capillaries:
  • As arterioles become capillaries, layer nearly disappears.
  • In capillaries, no smooth muscle layer at all; vessel wall consists of a single layer of endothelial cells, allows for efficient exchange of gases, nutrients, waste products between blood and surrounding tissues.
• Describe the changes that occur in the thickness of the tunica externa and tunica media of arteries as they become many smaller arterioles and eventually give rise to many more capillaries.

  When arteries shift to arterioles, the thick tunica externa that provided support and protection thins as support is no longer so important. The tunica media is a thick layer composed of smooth muscle cells and elastic fibers, efficient in regulating blood pressure and blood flow under high pressure. This thick tunica media decreases, but functions in vasoconstriction and vasodilation to regulate flow.
• Describe the changes that occur in the thickness of the tunica externa and tunica media of arteries as they become many smaller arterioles and eventually give rise to many more capillaries.


  When arterioles shift to capillaries, the tunica externa becomes less prominent until it is essentially absent in capillaries since capillaries are composed of a single endothelial layer and lack an outer connective layer. The tunica media practically disappears as well, since capillaries lack smooth muscle. Instead, its single layer of endothelial assists in diffusion.