Classification of blood vessels based on the structure
- Elastic, Large or conducting arteries
Conduct blood away from the heart.
Important in maintaining constant pressure in arterial system
- Muscular arteries or medium sized arteries
They distribute the blood to the tissues
- Large veins
- Medium sized veins
Wall of the blood vessels
Typically having three concentric layers.
- Tunica intima – innermost layer
- Tunica media – middle layer
- Tunica adventitia (externa) – outermost layer
Histology of elastic arteries
- Tunica intima – thicker than in muscular arteries
. Endothelium rests on thin basal lamina
. Internal elastic lamina may be present between intima & media, but hard to distinguish because of abundant elastin fibres in media.
- Tunica media – contains abundant elastin as concentrically arranged interspersed with smooth muscle fibres.
- Tunica adventitia – thin relative to vessel diameter, contains elastic & type I collagen fibres
,Vasa vasora are seen.
- e.g. – aorta
Histology of Muscular or distributing arteries
- Tunica intima - Contains typical endothelium & subendothelial connective tissue.
Prominent internal elastic lamina appears as wavy, refractile line between intima & media.
- Tunica media - Thick , Up to 40 layers of smooth muscle fibers.
Collagen, elastic fibres & proteoglycan vary.
- Tunica adventitia - Relatively thin & contains mostly collagen fibres
Histology of Large veins
Tunica intima - Well developed & include thick layer of subendothelial connective tissue.
Extensions of intima protrude into lumens of large veins as valves.
- Tunica media – several layers of smooth muscle cells & abundant reticular & collagen fibres. Elastin is sparse.
- Tunica adventitia – best developed in large veins contains abundant collagen & longitudinal bundles of smooth muscle that strengthen vessel to prevent distension.
- Eg.- Vena cava
Histology of Small & medium sized veins
Narrower than large veins & have thin walls.
- Tunica intima -Typical endothelium, less subendothelial tissue. fewer valves than large veins, no internal elastic lamina.
- Tunica media -Thin relative to vessel diameter. Few elastic fibres.
- Tunica adventitia - Relatively thick, but unlike large veins contains little of any muscle.
- e.g.- saphenous, hepatic, portal etc.
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