University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2019
Views:

305

Pages:

8
Author:

folclord7m

Veins 31 Venules and Small Veins Their walls are similar to capillaries. Venules (post capillary) have thin endothelium surrounded by pericyte and reticular fibres. In larger venules, pericytes are replaced by scattered smooth muscles. In small veins, smooth muscles form a continuous layer. The endothelium in venules of lymphoid organs is rather cuboidal (HEV) and functions in lymphocyte segregation and recognition. 32 Medium Veins Less than 1 cm in diameter. Drain most of the body. Occasionally the endothelium is surrounded by elastic fibres but no IEL. Tunica media: Smooth muscles are loosely organized with collagen fibres and fibroblasts in between. Tunica adventitia: collagen and elastic bundles are longitudinally arranged with few smooth muscles. 33 Large Veins Include: SVC, IVC and veins forming them and their major tributaries, pulmonary and portal veins Tunica intima: similar to medium veins but with thick subendothelial tissue. Tunica media: poorly develop in most of the large veins. Tunica adventitia: well developed. 34 35 Tunica adventitia: well developed LM of the wall of the inferior vena cava. The lumen (*) is lined by an attenuated intima (TI). A few layers of circular smooth muscle cells occupy the thin media (TM). The adventitia (TA), the thickest layer, contains longitudinal bundles of smooth muscle (SM) interspersed with collagen fibers, as well as vasa vasorum (arrow). 55×. H&E 36 Special Features of Certain Veins Cerebral, meningeal veins, dural venous sinuses, retinal veins, penile erectile tissue and veins of the bones: have no definite media. Veins of the gravid uterus, umbilical vein, some mesenteric veins and limbs are rich in smooth muscles. Adventitia of SVC, IVC and pulmonary veins contain cardiac muscle close to the heart. 37 The end 38