The Vessel Wall

The walls of the arteries and veins have three layers called tunics (figs. 20.2 and 20.3):

1. The tunica externa (tunica adventitia1) is the outermost layer. It consists of loose connective advent = added to

Lumen Tunica intima Endothelium Basement

Tunica media

Tunica media

Tunica Intima Endothelium

Tunica intima Endothelium Basement membrane

Internal elastic lamina Tunica media External elastic lamina Tunica externa

Tunica intima Endothelium

Basement membrane Tunica media Tunica externa

Tunica intima Endothelium Basement membrane Valve

Tunica media

Tunica externa

Basement membrane

Capillary

Figure 20.2 The Structure of Arteries and Veins. Why are elastic laminae found in arteries but not in veins?

Lumen

Tunica intima Endothelium Basement membrane Collagenous tissue

Tunica media

Tunica externa Vasa vasorum Nerve

Tunica intima Endothelium Basement membrane

Internal elastic lamina Tunica media External elastic lamina Tunica externa

Tunica intima Endothelium Basement membrane Valve

Tunica media

Tunica externa

Tunica intima Endothelium

Basement membrane Tunica media Tunica externa

Basement membrane

Capillary

Figure 20.2 The Structure of Arteries and Veins. Why are elastic laminae found in arteries but not in veins?

Saladin: Anatomy & I 20. The Circulatory System: I Text I © The McGraw-Hill

Physiology: The Unity of Blood Vessels and Companies, 2003

Form and Function, Third Circulation Edition

750 Part Four Regulation and Maintenance

750 Part Four Regulation and Maintenance

Neurovascular Anatomy And Blood Flow
Figure 20.3 A Neurovascular Bundle. A small artery, small vein, and nerve traveling together in a common sheath of connective tissue.

tissue that often merges with that of neighboring blood vessels, nerves, or other organs. It anchors the vessel and provides passage for small nerves, lymphatic vessels, and smaller blood vessels. Small vessels called the vasa vasorum2 (VAY-za vay-SO-rum) supply blood to at least the outer half of the wall of a larger vessel. Tissues of the inner half of the wall are thought to be nourished by diffusion from blood in the lumen.

2. The tunica media, the middle layer, is usually the thickest. It consists of smooth muscle, collagen, and sometimes elastic tissue. The smooth muscle is responsible for the vasoconstriction and vasodilation of blood vessels.

3. The tunica intima (tunica interna), the inner layer, is exposed to the blood. It consists of a simple squamous endothelium overlying a basement membrane and a sparse layer of fibrous tissue. The endothelium acts as a selectively permeable barrier to blood solutes, and it secretes vasoconstrictors and vasodilators to be considered later. It also provides a smooth inner lining that normally repels blood cells and platelets. However, platelets may adhere to a damaged endothelium. During inflammation, leukocytes also adhere loosely to it by means of cell-adhesion molecules produced by the endothelial cells (see chapter 21).

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Responses

  • alyx
    What is an artery, vein, and nerve traveling together called?
    7 years ago
  • rowan
    What vasodilator is secreted by the tunica intima?
    5 years ago

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