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Miscellaneous Features of 

Arteries and Veins

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Arteries and Veins

Histological differences between small, mediumand large sized veins

What is Vasa vasorum

Venous Valves

Histological differences between a typical arteryand vein

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Layers of Vessels

Both arteries and veins have are composed of 

the same three layers.

Tunica intima/interna

Tunica media

Tunica adventitia

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Tunica interna

This is the innermost layer and lines the lumen of the blood vessels.

It consists of areloar simple squamous

epithelium and a thin layer of (basementmembrane) to "stick it to the Tunica media.

The simple squamous epithelium provides asmooth surface for the blood to "slide past."

The tunica intima is continuous with theendocardium, makes-up the capillaries and iscollectively referred to as the endothelium.

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Middle Layer

Tunica media

This is the middle layer. It is made

of smooth muscle and elastic fibers. It is

responsible for vasodialation and

vasoconstriction of the blood vessels. It is also

the target tissue for Vasopressin (ADH)

Hormone and plays into blood pressure.

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Superficial Layer

Tunica adventitia

This is the most superficial of the layers. It

is made of dense irregular CT with lots of 

collagen fibers running in all directions for

strength in many different directions.

In many of the slide preps, you can see the

collagen fibers as squiggly lines in this layer.

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Large Veins Medium Veins Small Veins

Tunica intima Endothelial lining

with basement

membrane,

subendothelialconnective tissue

and smooth muscle

cells

Endothelium and a

thin subendothelial

layer, smooth

muscle cells

Endothelium

surrouned by

pericytes,

undifferentiatedmesenchymal cells

Tunica media Thin, consists of  

smooth muscle, aswell as collagen

fibers and

fibroblasts

Thin, consists of 

circularly arrangedsmooth muscle ,

collagen fibres

Lacks a smooth

muscle media

Tunica adventitia Large, thickest

layer,, made of 

collagen fibers,elastic fibers and

fibroblasts

Thinner than large

veins, mostly

collagen fibers, maycontain

longitudinally

arranged collagen

fibers

Longitudinally

arranged collagen

and elastin fibres;muscular venules

contain 1-3 layers

of smooth muscles

Valves No Yes Yes

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Differences between arteries and veinsArteries Veins

Tunica media is relatively thicker than in

veins

Tunica media is relatively thin, consisting

of smooth muscle fibers. May contain

collagen fibers and fibroblasts

Tunica adventitia l layer is lesser than in

veins

Tunica adventitia composed solely of 

longitudinally arranged collagen fibrers, is

more thicker than in arteries. Large

arteries contain many smooth muscle

layers

Valves absent except semilunar valves of 

pulmonary artery and aorta

Valves present to prevent return of waste

materials to tissues

Lumen is relatively narrower than in veins

Lumen is regular

Lumen is wider in vein than arteries

Lumen is irregular

Smooth muscle layer, elastic fiber layer is

more thicker in arteries to provide

pressure for pulsatile movements of heart

for blood flow

Smooth muscle and elastic fiber layer is

less than arteries

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Venous Valves

The valves are formed by the loose pocket-

shaped folds of tunica intima, which extend

into lumen of the vein

The opening of pocket point into direction of 

flow of blood towards the heart

One to three but usually two pockets form the

valves

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Valve structure

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How valve prevents backflow of blood

Blood flowing towards heart will pass thepockets.

If the flow reverses, blood will fill the pockets

which will occlude the lumen of the vein andprevent the return of blood into the part of thevein preceding the valve.

The ability of the valves to prevent backflow

depends to some extent on the state of contraction (tone) of the smooth muscle in thewall of the vein.

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Vasa vasorum

A network of small blood vessels that supply

larger vessels with blood

Literally means vessels of the vessel

Both oxygenated and deoxygenated blood are

carried to and from larger vessels by these tiny

blood vessels.

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Why is vasa vasorum needed by large

blood vessels?

Because the large veins and arteries are by

necessity so thick, their outer and middle cell

layers cannot be adequately nourished

without this additional network of blood

vessels