Page 145 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Circulatory system (systema cardiovasculare et Iymphovasculare) 127

Tunica intima (interna)
VetBooks.ir Subendothelial layer
Endothelial cell nucleus
Internal elastic membrane

Tunica media
Nucleus of smooth muscle cell
Tunica adventitia (externa)

6.14 Muscular artery (pig). The endothelium is underlain by a thin subendothelial layer. The meandering inter-
nal elastic membrane is clearly demarcated. In the tunica media, the smooth muscle cells are arranged in a
predominantly circular fashion with some obliquely running fibres. The large number of elastic fibres clearly
distinguishes the connective tissue of the outer tunica adventitia. Haematoxylin and eosin stain (x480).

Collagen fibres in connective tissue
Nucleus of endothelial cell
Erythrocytes
Nucleus of pericyte

6.15 Postcapillary venule in the tongue (horse). The endothelium is surrounded by occasional pericytes and
isolated smooth muscle cells. Expanded postcapillary venules can serve as a blood reservoir. Goldner’s Masson
trichrome stain (x200).

elastic membrane is usually lacking. The tunica media con- The structure of veins is adapted to these roles (Figures
sists of one to three layers of smooth muscle cells. These 6.15 to 6.18 and Table 6.1). A key characteristic is the dis-
join with endothelial cell processes to form a myoendothe- tensibility of the vessel wall, which is 200 times greater
lial complex. Arterioles play a significant role in regulating than that of arteries. Muscle tissue is poorly developed,
peripheral blood pressure and, together with the precapil- while the tunica adventitia is distinctive and relatively thick
lary sphincters, modify the flow of blood in capillary beds. (particularly in large veins).
The continuous endothelium is adjoined externally by
Veins (vena) a thin subendothelial layer supported by loose bundles of
The low-pressure component of the circulatory system elastic and collagen fibres. Longitudinally oriented smooth
begins at the venous side of the capillary bed and extends muscle cells may be present. In place of a distinct internal
to the right atrium of the heart. Its functions include: elastic membrane, there is a thin network of elastic fibres
(rete elastica).
· active metabolism (postcapillary venules), Compared with corresponding arteries the tunica
· storage of blood and media is thin, containing mainly fibres of collagen and
· conduction of blood from the periphery to the heart. elastin. The collagen fibres are arranged in a lattice while

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