Page 111 - Headlines Histology2024_Neat

• The basal lamina is discontinuous or absent. Macrophages are usually
associated with their walls.

THE VENOUS SYSTEM

• The primary function of the venous system is to return blood from the
capillaries to the heart.

• Veins are classified according to their size and structure into:
Venules
Post-capillary

• They are endothelial tubes slightly wider than capillaries (about 30 ?m in
diameter). Pericytes and little connective tissue surround them.

• Functions
• Exchange like capillaries.
• They are the main sites at which white blood cells leave or enter the

circulation.
Collecting venules

• The post-capillary venules drain into collecting venules. They are slightly
larger with a greater number of pericytes.

Muscular venules (small veins)

• The tunica intima consists of endothelium rests on basal lamina. The internal
elastic lamina is absent.

• The tunica media consists of one or two layers of smooth muscles.
• The tunica adventitia is a thin connective tissue layer continuous with the

surrounding connective tissue.
• The muscular venules are similar to arterioles except;
• They have a wide diameter relative to the thickness of the wall.
• The lumen is usually collapsed and filled with blood.

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