BLOOD VESSELS II

Histology 2016-2017
Department of Anatomy &Histology: Dr.Rajaa Ali
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BLOOD VESSELS II
CAPILLARIES
GENERAL FEATURES
• Arterioles break up into small blood vessels called capillaries.
• Capillaries are often referred to as exchange vessels, because they are involved in exchange of gases, nutrients and metabolites between blood and tissue.
• Tissues with high metabolic rates have abundant capillary network (e.g. kidney, liver, cardiac muscle).

STRUCTURE
The lumen of a typical capillary is about 7–9 ?m wide (equal to the diameter of an erythrocyte) and is lined by endothelial cells, which are two or three in number on cross section of vessel and form its tunica intima.
The margin of endothelial cells are held together by tight and gap junctions. Numerous pinocytotic vesicles are seen in the cytoplasm. They are involved in transporting material across the endothelial lining in either direction.
Pericytes or adventitial cells are occasionally seen within the basement membrane of the endothelium constituting the tunica media. These cells contain contractile fi laments in the cytoplasm and can transform into other cells.
A thin layer of collagen fi bres that surround the capillaries form the tunica adventitia.
Capillaries are divided into following three types depending on the nature of the endothelium: