Blood Vessels

• Closed pathway that carries blood

from the heart to cells and back
to the heart.

• There are three types of blood

vessels
 Arteries: take blood away from the heart,

 Capillaries allow exchange of substances

 Veins convey blood back to the heart.

• Most vessels of the circulatory system have three coats, or
tunics:
 Tunica intima
 Tunica Media
 Tunica Adventitia

Vessels can be classified as

Macrovascular vessels
 Large artery
 Medium artery
Large Vein
Medium vein
Microvascular
 Arterioles
 Capillary
 Venules
 Histology of arteries
Tunica intima: Inner most layer
• In direct contact with the blood as it flows through the lumen
• Consists of
1. Endothelium: a single layer of simple squamous epithelium continuous
with endocardium
2. Basement membrane: thin extracellular layer composed chiefly of
collagen, proteoglycans, and glycoproteins
Provides a physical support for the endothelium.
Tunica intima…
3. Sub-endothelial layer :
• Consisting of loose connective tissue.
• Occasional smooth muscle cells are found in the loose connective
tissue.

4. Internal elastic membrane (in arteries and arterioles )

• Contain sheet-like layer of fenestrated elastic material

• Fenestrations enable substances to diffuse readily through the layer

and reach cells deep within the wall of the vessel.
 Endothelium
• Its is a specialized epithelium that acts as a semipermeable barrier
between blood plasma and the interstitial tissue fluid

• Endothelial cells are squamous, and elongated with the long axis in the
direction of blood flow.

• Besides their key role in metabolite exchanges between blood and

tissues, endothelial cells have several other functions:
o The endothelium presents a nonthrombogenic surface on which blood
will not clot and actively secretes agents that control local clot formation

o The cells regulate local vascular tone and blood flow by secreting various
factors that stimulate smooth muscle contraction or relaxation

o Endothelium has several roles in inflammation and local immune

responses. 7
 Tunica media: Middle layer
Muscular and connective tissue layer that displays the greatest
variation among the different vessel types
Consisting
 Primarily of smooth muscle.
 External elastic lamina
 Network of elastic fibers that
separate tunica externa and media

 Allow the vessels to stretch and recoil under the applied pressure of the blood
Variable amounts of elastin, reticular fibers, and proteoglycans
are interposed between the smooth muscle cells of the tunica
media.
 Tunica Media…
• The lamellae of elastin are
fenestrated and arranged in circular
concentric layers.

• All of the extracellular components

of the tunica media are produced by
the smooth muscle cells.

Tunica media responsible for regulating blood flow and

blood pressure, contracts when vessels are damaged to help
limit loss of blood through the injured vessel.

• Sympathetic fibers stimulates the smooth muscle to contract, causing

vasoconstriction
 Tunica externa: Outer layer
• Connective tissue layer
• Consists of
Primarily of longitudinally arranged collagen fiber (type I)
A few elastic fibers.

Ranges from relatively thin in most of the arteries to quite

thick in the veins

Tunica adventitia of large arteries and veins contains

• Small vessels and nerves that supply the blood vessel wall called
vasa vasorum and vasa nervosum, respectively.
• The walls of larger arteries and veins are too thick to receive
nourishment by direct diffusion from their lumina.

• As a result, these walls are supplied by their own small blood vessels
called the vasa vasorum (vessels of the vessel).
 Arteries
• Carries oxygen-rich blood away from the
heart. (Except pulmonary and umbilical artery)

• Carry blood under relatively high pressure

from the heart
• Distribute it to the body
• Accompanied by vein and nerves.
• Small Lumen with no valves
• Repeated branching.
• Strongest of the blood vessels.
 ARTERIES
• The different types of arteries are distinguished from each other on
the basis of
Overall size
Relative amounts of elastic tissue or muscle in the tunica media
Thickness of the wall relative to the lumen
Function
• There are three types of arteries:
1. Large elastic arteries (conducting arteries)
2. Medium muscular arteries (distributing arteries)
3. Small arteries and arterioles
 Types of Artery
1. Large elastic arteries/conducting
• Have a thick tunica media that is dominated by elastic
lamellae(up to 50 layers)

• They are called conducting arteries

• Their major role is to carry blood to medium sized arteries.

• They also facilitate the continuous and uniform movement of

blood along the tube.
Elastic arteries
Elastic arteries
Elastic arteries
 Elastic Arteries
• Their elasticity enables them to expand when they receive the
cardiac output from the ventricles during systole

• Examples of large elastic arteries are

 Aorta & Pulmonary trunk and arteries
 Brachiocephalic trunk,
 Subclavian, and carotid arteries, and
 Common iliac arteries
• They have the largest diameter among the arteries but their wall is
relatively thin compared to the overall size of the vessel.

22
Elastic arteries
 Arterial sensory structures
• Carotid sinuses- are slight dilations on elastic artery wall.
oAct as baroreceptors to monitor arterial blood pressure.

• Carotid/aortic bodies
oChemoreceptors to monitor blood levels of CO2, O2 and
pH

24
 2. Medium muscular arteries
• Distributing blood to various parts of the body
• Tunica Media Consist of
Chiefly of circularly disposed smooth muscle fibers
Fewer elastic fibers than elastic arteries.

• Smooth muscle is three-quarters(40 layers) of the total mass

of the wall.

• Have a well-defined internal elastic lamina but a thin external

elastic lamina

• Because of the reduced amount of elastic tissue they do not

have the ability to recoil.
 Muscular Arteries
• Most of the named arteries, including those observed in the
body wall and limbs during dissection.
E.g. brachial or femoral arteries

• They control blood flow and blood pressure through

vasoconstriction or vasodilation of their lumina.

• Therefore, the wall is relatively thicker than the lumen

28
Muscular Arteries…
 3. Small arteries
• Small arteries which can be observed only under
magnification

• Small arteries and arterioles are distinguished from one

another by the number of smooth muscle cell layers in the
tunica media.
Arterioles have only one or two layers

A small artery may have as many as eight layers of

smooth muscle in the tunica media

• Typically, the tunica intima of a small artery has an internal

elastic membrane, whereas this layer may or may not be
present in the arteriole.
 Arterioles
• Abundant microscopic vessels

• Arterioles are usually not named or specifically identified

• Have relatively narrow lumina and thick muscular walls.

• They have a thin tunica interna with a very thin or no internal

elastic lamina.

• Their tunica externa consists areolar CT with sympathetic

nerves.
 Arterioles
• Arterioles play a key role in regulating blood flow from
arteries into capillaries by regulating resistance.

• Arterioles are known as resistance vessels.

• The terminal end of the arteriole, the region called the

metarteriole

• At the metarteriole–capillary junction, the most distal muscle

cell forms the precapillary sphincter

• This sphincter monitors the blood flow into the capillary.

33
 Types of arteries
Type of Size Intima Media Adventitia
artery
Elastic >10 mm Endothelium; Many elastic Connective tissue,
arteries Loose connective lamellae thinner than media,
tissue alternating with vasa vasorum
with smooth
muscle
Muscular 2-10 mm Endothelium; Many smooth Connective tissue,
arteries -Loose connective muscle layers, thinner than media;
tissue with vasa vasorum may
-internal elastic much less elastic be present
lamina prominent material
Small 0.1-2 Endothelium; 8-10 layers of Connective tissue,
arteries mm Loose connective smooth muscle thinner than media;
tissue no vasa vasorum
Arterioles 10-100 Endothelium; 1-2 layers of Very thin connective
μm No loose connective smooth muscle tissue layer
tissue

36
Anastomoses:
• Union of the branches of ≥2 arteries supplying the same body region

• Provide collateral circulation in case the usual pathway is obstructed

• Collateral pathways require time to open adequately

• Usually insufficient to compensate for sudden occlusion or ligation.

• Anastomoses may also occur between veins and between arterioles

and venules.
• Arteries that do not anastomose with adjacent arteries are true
(anatomic) terminal arteries (end arteries).

• Obstruction of an end artery interrupts the blood supply to a whole

segment of an organ, producing necrosis (death) of that segment.
E.g. Occlusion of end arteries supplying retina will result in blindness.
 Capillaries
• The smallest of blood vessels which have diameters of 4–10 µm

• Form the “U-turns” that connect arterioles to venules

• The primary function of capillaries is the exchange of substances

between the blood and interstitial fluid.

• Because of this, these thin-walled vessels are referred to as exchange

vessels.

• The flow of blood from a metarteriole through capillaries and into a

postcapillary venule is called the microcirculation
• Oxygen and nutrients can pass out of a capillary into a body cell.

• Carbon dioxide and other waste products pass out of a body cell into
a capillary
 Capillary…
• Consists of a single layer of endothelial cells and their basal
lamina.
• Capillary structure varies in different tissues and organs
There are 3 types of capillary
A. Continuous capillaries
B. Fenestrated capillaries
C. Sinusoids
 Types of capillaries
1. Continuous capillaries:- most common type

• Have tight, occluding junctions sealing the intercellular clefts between

all the endothelial cells to produce minimal fluid leakage.

• Continuous basement membrane

• All molecules exchanged across the

endothelium must cross the cells by
diffusion or transcytosis.

• Found in muscle, connective tissue, lungs, exocrine glands, and CNS

44
2. Fenestrated capillaries:- also have tight junctions, but
perforations (fenestrations) through the endothelial cells

• Allow greater molecular exchange across the endothelium.

• The basal lamina is continuous and

covers the Fenestrations

• Found in endocrine
organs, intestinal walls, and
choroid plexus. 45
3. Discontinuous capillaries:- they are called sinusoids
• usually have wider diameter

• Have discontinuities between the endothelial cells

• Discontinuous basement membrane.

• Permit maximal exchange of

macromolecules

• Found in organs where

exchange of macromolecules.

E.g. bone marrow, liver, and spleen. 46

 Veins
• Carries oxygen-poor blood toward the heart
• Veins are more abundant than arteries
• Have valves
• Only 20% of the blood occupies arteries, 80% is in the veins
• The tunics of veins are not as distinct or well defined as the tunics of
arteries.

Types of veins
 Superficial
 Deep
 Perforating
 Veins
• Tend to have wider lumens than arteries
• Arteries have thicker walls than veins
• Veins have:
oThinner tunica intima
oThinner tunica media
oThicker tunica externa
Artery

Vein • Veins lack the external or internal

elastic laminae found in arteries
48
There are three sizes of veins
 Venules
 Medium veins
 Large veins
• Large- and medium-sized veins usually travel with large- and
medium-sized arteries, respectively.

•Arterioles
and Venules
also
sometimes
travel
together
 Types of veins
Type of diamet Intima Media Adventitia
veins er

venules 10-100 Endothelium; Scattered smooth None

μm No Valves muscle cells
and pericytes
Small 0.1-1 Endothelium; Thin, 2-3 loose Connective tissue,
veins mm connective tissue layers of smooth thicker than media
muscle cells

Medium 1-10 Endothelium; 3-5 more distinct CT thicker than media;

veins mm connective tissue, layers of smooth longitudinal smooth
with valves muscle muscle may be present

Large >10 Endothelium; >5 layers of smooth CT is thickest layer, with

veins mm connective tissue, muscle, with much bundled longitudinal
smooth muscle cells; collagen smooth muscle
prominent valves

51
 1. Venules and small veins
• Venules are the smallest veins
• They drain capillary beds and join similar vessels to form
small veins
• Magnification is required to observe venules.
• Small veins are the tributaries of larger veins that unite to
form venous plexuses
• Have no valve
• Small veins are unnamed.
 2. Medium veins
• Drain venous plexuses and accompany medium arteries.

• Most of the named veins are medium veins

• In the limbs, and in some other locations where the flow of blood
is opposed by the pull of gravity, the medium veins have venous
valves.

Examples include the

Named superficial veins of UPL and LL
 Cephalic and basilic veins of the upper limbs
 Great and small saphenous veins of the lower limbs
Accompanying veins that are named according to the artery they
accompany (Deep Veins)
 3. Large veins
• Characterized by wide bundles of longitudinal smooth muscle
and a well-developed tunica adventitia.

• An example is the superior vena cava (SVC), Inferior Vena cava (IVC)