Histology of blood vessels
Tubes of various sizes , carrying blood from the heart through the tissues
and back to the heart again ,and thin- walled channels conveying lymph
,from the extensive circulatory system . Blood vessels are encountered in all
tissues except epithelium ,some parts of the body are not supplied by
lymphatic vessels . Blood vessels walls have considerable elasticity and
constrict when the heart stops beating . materials carried by the blood are
passed through the thin walls of the smallest vessels into the tissues.
Capillaries ,pre capillaries
Approximately 90% of all blood vessels are capillaries and pre capillaries.
Capillaries are simple tubes of endothelium connecting arteries and veins.
All vessels and even the heart , begin in the embryo as capillaries , adding
layers of connective tissues and muscle as they developed capillaries are
small their inside diameter is approximately that of an erythrocyte 8 to10 µ
or little wider the capillary wall consist of thin endothelial cells are ovoid
and stain rather lightly with hematoxylin they resemble the light nuclei of
fibrocytes in the surrounding connective tissue in the surrounding connective
tissue .the inner surface of the cells are in contact with blood .the outer
surfaces rest on a basal lamina forming a continuous layer between the
endothelium and surrounding loose fibrous connective tissue which contain
its usual cells –fibrocytes ,macrophages , lymphocytes. The structure of
capillary wall differs regionally in response to requirements for passage of
fluid and solutes from the blood to intestinal spaces . thickness of the walls
varies contacts between endothelial cells are relatively loose in many
locations.
Capillary permeability
This term refers to the facility with which materials such as oxygen
,nutriments,products of cellular metabolism and hormones ,dissolved in
blood plasma and tissue fluid ,are transported across the capillary wall.The
net work of capillaries between the smallest arteries and veins is called the
capillary bed . the density of capillary bed varies in different tissues .
The next larger vessels is called precapillaries have afew smooth muscle
cells in their walls. these and the arterioles are the vessels that contract and
regulate the flow of blood into capillaries .
Arteries
All arteries large enough to be seen without magnification have walls
consisting of three coats ( layer )
1- the tunica intima is the endothelial lining with basal lamina and some
connective tissue beneath it .
2-the middle coat or tunica media ,is formed by smooth muscle and
connective tissue principally elastic fibers arranged in circular fashion .
3- the external coat or tunica adventitia (tunica externa) it is fibrous
connective tissue with other structures in it arranged longitudinally . the
three coats may be separated by elastioc membranes ,internal and external.
Muscular arteries these are sometimes called distributing arteries they can
regulate blood flow to different regions by contraction
The smallest arteries
,called arterioles are members of the muscular artery class the tunica intima
consist of endothelium ,basal , and an internal elastic membrane that may be
incomplete in the smallest arterioles in cross sections the elastic membrane
appears as acorrugated , highly refractive line throwing the endothelium into
folds which bulge in to the lumen.
The tunica media is a distinct muscular coat . the adventitia is about one
third or one fourth as thick as muscular coat and merges with surrounding
connective tissue although afew elastic fibers appear in the wall of the
arteriols ,no external elastic membrane can be seen the smallest arterioles
merge with pre capillaries .
large and medium sized muscular arteries
the structure of large and medium sized muscular arteries is consist of the
tunica intima which formed by endothelium .basal lamina and a sub
endothelial layer of fibrous connective tissue the internal elastic membrane
is a well –developed fenestrated membrane .the muscle fibers of the tunica
media are spirally arranged and may form 20-40 layers elastic fibers are
interspersed among muscle fibers and a few collagenous and reticular fibers
are demonstrable the tunica adventitia consist of fibrous connective tissue
with longitudinal orientation where smooth muscle occurs in it fibers are
arranged longitudinally lymphatic vessels are not seen in tunica media of the
arteries but occure in the adventitia. muscular arteries are supplied with
nerve fiber both motor fibers from the autonomic ganglia and sensory fibers
from cranial and spinal nerves