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Ng H Y, et al.

relatively thicker in arteries than in veins and it spans lamellae possess the elastic recoil characteristics that can
the whole space between the internal and external elastic maintain the continuous and uniform blood flow along
membrane. External elastic membrane is an elastic layer the vascular tube as well as the arterial blood pressure.
that separates tunica media from tunica adventitia. Other In contrast to the tunica media of the large arteries,
constituents of the tunica media include reticular fibers, medium arteries are composed almost entirely of smooth
elastin and proteoglycans which are intercalated between muscle but with little amount of elastin and they contain
layers of smooth muscle cells. Variable amounts of a prominent internal elastic membrane. Some examples
circularly arranged elastic lamellae with fenestrations of muscular arteries are femoral artery, radial artery
are also found in this layer. Unlike tunica adventitia, all and popliteal artery. Small arteries and arterioles can
extracellular components of the tunica intima and media be distinguished from each other by the thickness of
such as elastin, collagen and ground substances are
[11]
synthesized by the smooth muscle cells . The outermost their smooth muscle layers in tunica media. The smooth
layer of the blood vessels is the tunica adventitia, a layer muscle layers in arterioles are only one or two cells
of loose connective tissue with embedded fibroblasts thick whereas those in small arteries may be as many
composed mainly of longitudinal collagenous tissue and as eight to ten cells thick. Capillaries have the smallest
some elastic fibers. This layer plays a role in anchoring diameter amongst all types of blood vessels, each of
the blood vessel to the surrounding tissue and provision them consists of only a single layer of endothelium and
of additional mechanical support [12] . In addition, this their basal lamina. The diameter of their lumen is only
layer is found to be relatively thinner in arteries and large enough to allow one erythrocyte to pass through at
arterioles and is much thicker in venules and veins a time. Unlike other vessels, tunica media and adventitia
as it is the major constituent of their vessel wall. A are absent in the capillaries.
vascular network called vasa vasorum and an autonomic For veins, they usually have thicker tunica adventitia
nervous network called nervi vasorum (vascularis) are as compared to the arteries. They are often being
present in the tunica adventitia of large arteries and classified into four types based on their size, namely
veins. These networks work by supplying nutrients and the venules (postcapillary and muscular), small veins,
oxygen to the walls of the blood vessels and regulating medium veins as well as the large veins. The venules
vasoconstriction as well as vasodilation by sending can be further subdivided into postcapillary venules and
signals to the smooth muscles directly from the nervous muscular venules based on the presence of tunica media
system, respectively. as the former lacks a tunica media in their vascular
The anatomies of arteries and veins are generally quite wall. Postcapillary venules receive blood directly from
similar but can be distinguished by the thickness of the capillaries and are characterized by the presence of
tunica and their cellular and protein compositions of the
layers. Arteries generally have a thicker tunica media and pericytes. Small veins are continuous with the venules.
a thinner tunica adventitia. They are traditionally divided Medium veins usually represent the named veins in our
into three types based on the size and characteristics of bodies and are often accompanied by arteries. Some of
their tunica media, namely the large (or elastic arteries), the examples of medium veins include the radial vein,
the medium (or muscular arteries), small arteries and tibial vein and the popliteal vein. Large veins are veins
arterioles. Some examples of the large arteries include with a diameter larger than 10 mm and they include
the aorta, pulmonary arteries and the brachiocephalic the superior and inferior vena cava. A summary of the
trunk. This type of arteries consists of large amount of characteristics of different types of blood vessels are
elastic lamellae in their vascular walls. These elastic shown in Table 1 and Table 2.
table 1. Characteristics of various types of arteries.

Arteries
vessel large artery (elastic artery) Medium artery (muscular artery) Small artery Arteriole capillary
Diameter > 10 mm 2 – 10 mm 0.1 – 2 mm 10 – 100 μm 4 – 10 μm
tunica intima Endothelium Same as large artery except with Same as large artery Same as large artery Endothelium only
(innermost) Connective tissue prominent internal elastic membrane except with less
Smooth muscle prominent internal
elastic membrane
tunica Media Smooth muscle Smooth muscle Smooth muscle (8–10 Smooth muscle (1–2 None
(middle) Collagen fibers cells thick) cells thick)
Elastic lamellae Lesser elastic tissue Collagen fibers
tunica Adventitia Thinner than tunica media Same as large artery except with Same as medium artery Thin, indistinct None
(outermost) Connective tissue fewer elastic fibers connective tissue
Elastic fibers
International Journal of Bioprinting (2018)–Volume 4, Issue 2 3