The document discusses the structure of blood vessels, including the three layers - tunica interna, tunica media, and tunica externa. It describes the key components of each layer and how they vary across the different types of blood vessels - arteries, arterioles, capillaries, venules and veins. The structures are adapted to facilitate the transport and exchange of blood, nutrients and waste throughout the body.

1. STRUCTURE OF
BLOOD
VESSELS
-BY RICHA
(M.SC. 1ST YEAR)

2. INTRODUCTION
The cardiovascular system contributes to homeostasis of our body
systems by transporting and distributing blood throughout the body to
deliver materials like oxygen, nutrients, and hormones, etc.
The structure involved in these important tasks are the blood vessels,
which form a closed system of tubes that carries blood away from the
heart, transport it to the tissues of the body, and then returns it to the
heart. The left side of the heart pumps blood through an estimated
100,000 km of blood vessels. The right side of the heart pumps blood
through the lungs, enabling blood to pick up oxygen and unload carbon
dioxide.

3. BLOOD VESSELS
The blood vessels are the components of the
circulatory system that transport throughout the human
body. These vessels transport blood cells, nutrients,
and oxygen to the tissues of the body. They also take
waste and carbon dioxide away from the tissues.

4. BASIC STRUCTUTRE OF BLOOD
VESSELS
• The wall of a blood vessel consists of three tunics, of different tissues: an
endothelial inner lining, a middle layer consisting of smooth muscle and elastic
connective tissue, and a connective tissue outer covering.
• From innermost to outermost, the three structural layers of a generalized
blood vessel are the tunica interna (intima), tunica media, and tunica externa.
• Subtle modifications of this basic design account for the five different types of
blood vessels and the structural and functional differences among the various
vessel types. It will be easier to learn the structures of the various vessels if
you remember that structural variations are correlated to differences in function
throughout the cardiovascular system.

5. TUNICA INTIMA
 The tunica interna (intima innermost) forms the inner lining of a blood
vessel and is in direct contact with the blood as it flows through the
lumen, or interior opening, of the vessel.
 Its innermost layer is called endothelium, which is continuous with the
endocardial lining of the heart. The endothelium is a thin layer of
flattened cells that lines the inner surface of the entire cardiovascular
system.
 Endothelial cells are active participants in a variety of vessel related
activities, including physical influences on blood flow, secretion of locally
acting chemical mediators that influence the contractile state of the
vessel’sassistance with capillary permeability.

6. CONT.
In addition, their smooth luminal surface facilitates efficient blood
flow by reducing surface friction.
Also provide resilience for stretching and recoil.
The outermost part of the tunica interna, which forms the
boundary between the tunica interna and tunica media, is the
internal elastic lamina (lamina thin plate). The internal elastic
lamina is a thin sheet of elastic fibers with a variable number of
window-like openings that give it the look of Swiss cheese. These
openings facilitate diffusion of materials through the tunica interna
to the thicker tunica media.

8. TUNICA MEDIA
The tunica media (media middle) is a layer composed of
muscular and connective tissue. This layer displays the
greatest variation among the different vessel type.
 In most vessels, it is a relatively thick layer comprised
mainly of smooth muscle cells and substantial amounts of
elastic fibers.
 The primary role is to regulate the diameter of the lumen
wall. As you will learn in more detail shortly, the rate of blood
flow through different parts of the vascular network is
regulated by the extent of smooth muscle contraction in the
walls of particular vessels.

9. CONT.
Muscle contraction in particular vessel types is crucial in the regulation of blood
pressure. In addition to regulating blood flow and blood pressure, smooth muscle
contracts when an artery or arteriole is damaged (vascular spasm) to help limit loss of
blood through the injured vessel if it is small. Smooth muscle cells also produce the
elastic fibers within the tunica media that allow the vessels to stretch and recoil under
the applied pressure of the blood.
 The external elastic lamina, forms the outer part of the tunica media and separates the
tunica media from the outer tunica externa. Sympathetic fibers of the autonomic
nervous system innervate the smooth muscle of blood vessels. An increase in
sympathetic stimulation typicallystimulates the smooth muscle to contract, squeezing
the vessel wall and narrowing the lumen. Such a decrease in the diameter of the
lumen of a blood vessel is called vasoconstriction. In contrast, when sympathetic
stimulation decreases, in the presence of certain chemicals (such as nitric oxide, lactic
acid), or in response to the pressure of blood, smooth muscle fibers relax. The
resulting increase in lumen diameter is called vasodilation.

10. TUNICA EXTERNA
o The outer covering of a blood vessel, the tunica externa (externa
outermost), consists of elastic and collagenous fibers.
o It ranges in size from a thin connective tissue wrapping to the thickest
layer of the blood vessel.
o The tunica externa contains numerous nerves and, especially in larger
vessels, tiny blood vessels that supply the tissue of the vessel wall.
These small vessels that supply blood to the tissues of the vessel are
called vasa vasorum, or vessels to the vessels.
o They are easily seen on large vessels such as the aorta.

13. ARTERIES
Arteries were found empty at death, in ancient times they
were thought to contain only air. Like other blood vessels,
the wall of an artery has three layers, but the tunica
media may be thicker or more elastic as outlined in the
following discussion.
 Due to their plentiful elastic fibers, arteries normally have
high compliance, which means that their walls stretch
easily or expand without tearing in response to a small
increase in pressure.

14. ELASTIC ARTERIES
 Elastic arteries are the largest arteries in the body, ranging
from the garden hose sized aorta and pulmonary trunk to the
finger-sized branches of the aorta. They have the largest
diameter among arteries, but their vessel walls
(approximately one-tenth of the vessel’s total diameter) are
relatively thin compared to the overall size of the vessel.
 These vessels are characterized by well-defined internal and
external elastic laminae, along with a thick tunica media that
is dominated by elastic fibers, the elastic lamellae.

15. MUSCULAR ARTERIES
Medium-sized arteries are called muscular arteries
because their tunica media contains more smooth
muscle and fewer elastic fibers than elastic arteries.
Thus, muscular arteries are capable of greater
vasoconstriction and vasodilation to adjust the rate
of blood flow.
E.g., radial artery and splenic artery.

16. ANASTOMOSIS
Most tissues of the body receive blood from more than one artery.
The union of the branches of two or more arteries supplying the
same body region is called an anastomosis.
Anastomoses between arteries provide alternative routes for
blood to reach a tissue or organ. If blood flow stops momentarily
when normal movements compress a vessel, or if a vessel is
blocked by disease, injury, or surgery, then circulation to a part of
the body can continue. The alternative route of blood flow to a
body part through an anastomosis is known as collateral
circulation. Anastomoses may also occur between veins and
between arterioles.

17. ARTERIOLES
 Literally meaning “small arteries,” arterioles are abundant
microscopic vessels that regulate the flow of blood into the
capillary networks of the body’s tissues.
 The approximately 400 million arterioles have diameters that
range in size from 15 m to 30 m. The wall thickness of
arterioles is one-half of the total vessel diameter.
 Arterioles have a thin tunica interna with a thin internal elastic
lamina containing small pores that disappears at the terminal
end.

18. CONT.
 The tunica media consists of one to two layers of smooth muscle
cells having a circular (rather than longitudinal) orientation in the
vessel wall. The terminal end of the arteriole, the region called the
metarteriole, tapers toward the capillary junction. At the
metarteriole–capillary junction, the most distal muscle cell forms
the precapillary sphincter, which monitors the blood flow into the
capillary; the other muscle cells in the arteriole regulate resistance
(opposition to blood flow).
 Since arterioles play a key role in regulating blood flow from
arteries into capillaries by regulating resistance, they are known as
resistance vessels. In a blood vessel, resistance is due mainly to
friction between blood and the inner walls of blood vessels.

19. CONT.
• When blood vessel diameter is smaller, the friction is greater,
so there is more resistance. Contraction of arteriolar smooth
muscle causes vasoconstriction, which further increases
resistance and decreases blood flow into capillaries supplied
by that arteriole. By contrast, relaxation of arteriolar smooth
muscle causes vasodilation, which decreases resistance and
increases blood flow into capillaries. A change in arteriole
diameter can also affect blood pressure.
• Vasoconstriction of arterioles increases blood pressure, and
vasodilation of arterioles decreases blood pressure.

20. CAPILLARIES
Capillaries (capilluslittle hair), the smallest of blood vessels,
have diameters of 5–10mm, and form the “U-turns” that
connect the arterial outflow to the venous return.
Since red blood cells have a diameter of 8 m, they must often
fold upon themselves in order to pass single file through the
lumens of these vessels.
The flow of blood from a metarteriole through capillaries and
into a postcapillary venule (a venule that receives blood from
a capillary) is called the microcirculation (microsmall) of the
body.

21. CONT.
Body tissues with high metabolic requirements, such as muscles, the
brain, the liver, the kidneys, and the nervous system, use more O2 and
nutrients and thus have extensive capillary networks. Tissues with lower
metabolic requirements, such as tendons and ligaments, contain fewer
capillaries. Because capillary walls are composed of only a single layer
of endothelial cells and a basement membrane, a substance in the
blood must pass through just one cell layer to reach the interstitial fluid
and tissue cells.
However, when a tissue is active, such as contracting muscle, the entire
capillary network fills with blood. Throughout the body, capillaries
function as part of a capillary bed, a network of 10–100 capillaries that
arises from a single metarteriole. In most parts of the body, blood can
flow through a capillary network from an arteriole into a venule as
follows:

22. 1. CAPILLARIES:
In this route, blood flows from an arteriole into capillaries and
then into venules (postcapillary venules). When the precapillary
sphincters are relaxed (open), blood flows into the capillaries;
when precapillary sphincters contract (close or partially close),
blood flow through the capillaries ceases or decreases.
Typically, blood flows intermittently through capillaries due to
alternating contraction and relaxation of the smooth muscle of
metarterioles and the precapillary sphincters. This intermittent
contraction and relaxation, which may occur 5 to 10 times per
minute, is called vasomotion.

24. 2. THOROUGHFARE
CHANNEL:
The proximal end of a metarteriole is surrounded by
scattered smooth muscle fibers whose contraction and
relaxation help regulate blood flow. The distal end of
the vessel, which has no smooth muscle and
resembles a capillary, is called a thoroughfare channel.
Such a channel provides a direct route for blood from
an arteriole to a venule, thus bypassing capillaries.

25. TYPES OF CAPILLARIES
The body contains three different types of capillaries:
continuous capillaries, fenestrated capillaries, and sinusoids.
Most capillaries are continuous capillaries, in which the
plasma membranes of endothelial cells form a continuous
tube that is interrupted only by intercellular clefts, gaps
between neighbouring endothelial cells.
Continuous capillaries are found in the central nervous
system, lungs, skin, skeletal and smooth muscle, and
connective tissues.

26. CONT.
Sinusoids: a small irregular blood vessel found in organs, especially in
liver.
Fenestrated capillaries: These are “leakier” than continuous capillaries. They contain
small pores, in addition to small gaps between cells, in their walls that allow for the
exchange of larger molecules.
• Examples of these areas include:
• The small intestine, where nutrients are absorbed from food
• The kidneys, where waste products are filtered out of the blood

28. VENULES
Venules drain the capillary blood and begin the return flow of
blood back toward the heart. Because they carry blood
toward the heart, veins are referred to as afferent vessels. As
noted earlier, venules that initially receive blood from
capillaries are called postcapillary venules.
They are the smallest venules, measuring 10 m to 50 m in
diameter. Because they are the weakest endothelial contacts
encountered along the entire vascular tree, venules are very
porous.

29. CONT.
They function as significant sites of exchange of nutrients
and wastes and white blood cell emigration, and for this
reason form part of the microcirculatory exchange unit
along with the capillaries.
The thin walls of the postcapillary and muscular venules
are the most distensible elements of the vascular system;
this allows them to expand and serve as excellent
reservoirs for accumulating large volumes of blood.

31. VEINS
While veins do show structural changes as they
increase in size from small to medium to large, the
structural changes are not as distinct as they are in
arteries.
Veins, in general, have very thin walls relative to their
total diameter. They range in size from 0.5 mm in
diameter for small veins to 3 cm in the large superior
and inferior venae cava entering the heart.

32. CONT.
Although veins are composed of essentially the same three
layers as arteries, the relative thicknesses of the layers are
different. The tunica interna of veins is thinner than that of
arteries; the tunica media of veins is much thinner than in
arteries, with relatively little smooth muscle and elastic fibers.
The tunica externa of a vein is its thickest layer and consists
of collagen and elastic fibers.
Veins lack the external or internal elastic laminae found in
arteries. They are distensible enough to adapt to variations in
the volume and pressure of blood passing through them, but
are not designed to withstand high pressure.

33. CONT.
The contraction of skeletal muscles in the free lower limbs
also helps boost venous return to the heart.
The average blood pressure in veins is considerably lower
than in arteries. Because of the difference in pressure, it is
easy to tell whether a cut vessel is an artery or a vein. Blood
leaves a cut vein in an even, slow flow but spurts rapidly from
a cut artery.

34. CONT.
Many veins, especially those in the limbs, also contain
valves, thin folds of tunica interna that form flaplike cusps.
The valve cusps project into the lumen, pointing toward the
heart. The low blood pressure in veins allows the flow of
blood returning to the heart to slow and even back up; the
valves aid in venous return by preventing backflow. Veins are
more numerous than arteries.

35. CONCLUSION
Blood vessels are needed to sustain the life
because all of the body’s tissues rely on
their functionality.