Fig 018 Blood Circulation

Blood Vessels

The blood vessels are composed of three layers (or tunics). Different types of vessels display functional variations in the thickness and composition of these layers.

• Tunica intima
  • Endothelium - simple squamous epithelium joined by tight junctions
  • Subendothelial connective tissue
  • Internal elastic lamina - more prominent in arteries
    • Fenestrated sheets of elastic fibers
• Tunica media - typically the thickest layer in arteries
  • Concentric layers of smooth muscle cells
  • Various amounts of elastic and collagen fibers are interspersed between the smooth muscle cells
    • These fibers are produced by the smooth muscle cells (not fibroblasts)
  • External elastic lamina - only present in large arteries
    • Fenestrated sheets of elastic fibers
• Tunica adventitia - typically the thickest layer in veins
  • Longitudinally arranged dense connective tissue
  • In large veins, bundles of smooth muscle cell are present

Nutrients reach the cells of the vascular wall by diffusion from the lumen. In large vessels, small blood vessels (vasa vasorum) in the tunica adventitia supply the outer layers.

Arteries

As arterial size decreases, the relative amount of elastic fibers within their walls decreases and the relative amount of smooth muscle cells increases.

• Large arteries / Elastic arteries / Conducting arteries
  • Tunic intima (TI) - endothelium and subendothelial connective tissue
  • Tunica media (TM) - large number (40 to 70 in normal adults) of sheets of elastic fibers (i.e., elastic laminae)
  • Tunica adventitia (TA) - thin layer of dense irregular connective tissue
• Medium arteries / Muscular arteries / Distributing arteries
  • Tunica intima (TI) - endothelium, subendothelial connective tissue, and a wavy band of elastic fibers called the internal elastic lamina
  • Tunica media (TM) - up to 40 layers of smooth muscle cells (an external elastic lamina is present in larger arteries
  • Tunica adventitia (TA) - thin layer of dense irregular connective tissue
• Arteriole / Resistance vessels
  • Tunica intima (TI) - endothelium and an indistinct internal elastic lamina (absent in small arterioles)
  • Tunica media (TM) - 1 to 2 layers of smooth muscle cells
  • Tunica adventitia (TA) - loose irregular connective tissue

Veins

The tunica adventitia is usually the thickest layer in veins..

• Large Veins (closest to the heart)
  • Tunica intima (TI) - endothelium and a thin layer of subendothelial connective tissue
  • Tunica media (TM) - a few layers (3 to 8) of smooth muscle cells with indistinct elastic laminae
  • Tunica adventitia (TA) - thickest layer of dense irregular connective tissue
• Medium Veins
  • Tunica intima (TI) - endothelium, subendothelial connective tissue, and an internal elastic lamina
  • Tunica media (TM) - a few layers of smooth muscle cells
  • Tunica adventitia (TA) - thickest layer of dense irregular connective tissue
• Venules
  • Tunica intima (TI) - endothelium and sparse subendothelial connective tissue
  • Tunica media (TM) - a few layers of smooth muscle cells
  • Tunica adventitia (TA) - thickest layer with large amounts of collagen fibers