BLOOD VESSEL

Classification of blood vessels based on the structure

1. Arteries
   • 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
2. Veins
   • Large veins
   • Medium sized veins

Wall of the blood vessels

Typically having three concentric layers.

1. Tunica intima – innermost layer
2. Tunica media – middle layer
3. 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. Mostly collagen.
• e.g.- saphenous, hepatic, portal etc.