Circulatory system histology - venous sys by : Abdelrahman Al-daqqa , second year medical st ,, An-najah University

9. Classification of the veins
• Basis on their diameter and wall thickness
, vein are classify to 3 mail groups :
• 1) large
• 2) medium
• 3)small

10. *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

12. Venules and small veins
Venules are similar to but larger than capillaries; larger
venules possess smooth muscle cells instead of pericytes

13. Venules & small veins
• 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.

14. 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.

15. Venules & small veins
*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.

16. Venules & small veins
• *Muscular venules have 1-3 layers of
smooth muscle surrounding the
intima, and an adventitia as described

17. 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

21. 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 …

22. *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.)

24. Large veins
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).

25. 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

32. 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

33. 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 ).