Prepard by : Abdelrahman M. Al-daqqah

Circularity system

Veins

Than the arteries

Classification of the veins

• Basis on their diameter and wall thickness , vein are classify to 3 mail groups :

• 1) large • 2) medium• 3)small

*In certain areas of the body where the structures housing the veins protect them from pressure [retina, meninges, placenta, penis], the veins have little or no smooth muscle in their walls

And the boundaries between the tunica intima and the tunica media of most veins are not clearly distinguishable

Venules and small veins

Venules are similar to but larger than capillaries; larger venules possess smooth muscle cells instead of pericytes

• Postcapillary venules (which receive blood from capillaries) have only an endothelial lining (intima) and lack a smooth muscle media.

• They are surrounded by pericytes, which are undifferentiated mesenchymal cells.

• The basement membrane of the endothelial cells and pericytes may fuse.

• It is at the level of post-capillary venules that white blood cells leave the blood to enter the tissue.

Venules & small veins

Venules & small veins

The endothelium of post-capillary venules is the main site of action for vasoactive agents such as histamine and serotonin which cause extravasation of fluid and WBCs during inflammation or allergic reactions.

*Collecting venules have a thin adventitia in addition to the pericytes surrounding the intima.

*The adventitia consists of longitudinally arranged collagen fibers with a few elastin fibers.

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Venules & small veins

• *Muscular venules have 1-3 layers of smooth muscle surrounding the intima, and an adventitia as described

Venules & small veins

*When looking at a venule which appears to have a media, it is generally not possible to tell if the nuclei of the media belong to smooth muscle cells or pericytes

Venules & small veins

Medium veins

• Are less than 1 cm in diameter

• Tunica intima includes the endothelium & it’s basal lamina & reticular fibers

• Sometime , elastic network surround the endothelium , but these elastic fibers do not form laminae characteristic …

*The tunica media is much thinner relative to that of an artery, and consists mostly of circularly arranged smooth muscle but also contains collagen fibres.

*The tunicas intima and media therefore tend to be less distinct from one another than is the case in arteries.

*The tunica adventitia is usually thicker than the media and is made up mostly of collagen fibres. It may contain longitudinally oriented smooth muscle bundles. (Remember gradations between the vessels of different sizes are continuous.)

The large veins are the venae cavae and portal vein and their tributaries.

The tunica intima consists of the endothelial lining with its basement membrane, a small amount of subendothelial connective tissue and some smooth muscle cells.

It blends in with the tunica media which is relatively thin, and in addition to smooth muscle cells may contain collagen fibres and some fibroblasts (in contrast with the media of arteries).

Large veins

The most distinguishing feature of large veins is the large tunica adventitia.

The adventitia is the thickest layer in large veins and is made of collagen fibres, some elastic fibres and fibroblasts.

Prominent bundles of longitudinally-arranged smooth muscle are a distinguishing feature

This image shows how veins have valves which stop the blood from flowing backwards. They are needed as there is so little pressure in them. Contraction of muscles (AS shown above) also helps the blood to be pushed up the vein

Varicose vein

• Abnormally enlarged tortuous vein .

• Result from :1.Loss of muscle tone .2.Degeneration of vessel walls .3.Valvular incompetence .

• Sites : 1. usually in legs of older person (superficially ). 2. Esophageal varices . 3. Anal canal (Hemorrhoids ).