||1. Tunica interna a.k.a. tunica
tunica interna is the innermost lining of the
blood vessel. It is lined by endothelium,
which is almost always simple squamous
epithelium, and its underlying connective
tissue. In arteries a layer of elastic tissue
called the internal elastic membrane
marks the outer boundary of the tunica interna.
||2. Tunica media
tunica media is the middle layer. It consists of
concentric sheets of smooth muscle cells
supported by varying amounts of connective
contraction of the smooth muscles cells is
controlled by the autonomic nervous system,
hormones and local chemicals. Contraction of the
smooth muscles of the media is called
vasoconstriction and results in a decrease
in the size of the lumen and a decrease in blood
flow. Relaxation of the smooth muscles is
vasodilation which has the opposite effects.
Arteries sometimes have an outer layer of
elastic tissue called the external elastic
membrane. When present this marks the
boundary between the tunica media and
||3. Tunica externa a.k.a. tunica
tunica externa is the connective tissue that
forms the outermost layer of the vessel. The
fibers of this layer often blends into the
surrounding tissues and stabilizes the position
of the vessel.
thick-walled vessels blood vessels supply oxygen
and nutrients to the outer layers. These vessels
are called vasa vasorum.