Blood vessels carry blood throughout the body via different types of vessels - arteries, arterioles, capillaries, venules and veins. The main types of blood vessels are described including their structure, layers, role in circulation and factors that influence blood flow. Arteries carry blood away from the heart, veins carry blood back to the heart, and capillaries allow for exchange of oxygen, nutrients and waste between the blood and tissues. Proper circulation is maintained through vessel structure, autonomic control, and various pumping mechanisms.

1. Blood Vessels:Blood Vessels:
Structure and Function.Structure and Function.
Prepared By;
Mr. Johny Kutty Joseph

2. Blood Vessels
 There are nearly 100000 km of vessels carrying
blood through the body.
 The blood vessels are developed through the
process of angiogenesis during embryonic stage
through the life span.
 There are mainly 5 types of vessels such as;
 Arteries: carry blood AWAY from the heart
 Arterioles: smallest arteries
 Capillaries: site of exchange in tissues
 Venules: smallest veins
 Veins: carry blood TO the heart

3. Arteries
The arteries are vessels that carry blood away
from the heart.
Elastic arteries: Largest
arteries and largest
diameter but walls
relatively thin. Function
as pressure reservoir.
Walls can stretch and
recoil to propel blood
while ventricles relaxing.
Also known as
conducting arteries –
conduct blood to
medium-sized arteries.

4. Arteries
2. Muscular arteries: Tunica media contains more
smooth muscle and fewer elastic fibers than elastic
arteries. Walls relatively thick. Capable of great
vasoconstriction/ vasodilatation to adjust rate of
blood flow. Also called distributing arteries.
3. Arterioles: Also called resistance vessels. They are
smallest arteries. They create resistance against
blood flow to maintain blood pressure. Met-arteriole
is the term used for the connecting vessel of
arterioles and capillaries. Sympathetic innervations
and local chemical mediators can alter diameter
and thus blood flow, resistance, vasoconstriction
and pressure is maintained.

5. Anastamosis and End Arteries
• Some Arteries that form links within main
arteries is called as anastamosis. It is seen in
Palms, Soles, Brain etc. Anastamotic arteries
provide Collateral Circulation.
• Some arteries do not form anastamosis hence
called as End Arteries. No Anastamosis beyond
distal anastamosis. Occlusion of the end artery
causes death of distal tissues as there is no
alternate supply.

6. Capillaries
• They are the microscopic vessels that carry blood
from arterioles to venules; and this is called as
microcirculation.
• The diameter is just large enough for a red blood
cell.
• The walls consist of tunica intima only (i.e. layer of
endothelium).
• Substances pass through just one layer of
endothelial cells and basement membrane.
• Although they are tiny around a billion are present
in body.
• It forms the capillary bed for exchange of materials
across the plasma membrane.

7. Capillary Beds

8. Capillaries
True Capillaries receive blood from arterioles.
Capillaries are classified into three types based on
ease of passage of substances and structural
differences.
1.Continuous capillaries: have a continuous
endothelial cells with small opening in between
called as intercellular clefts. These are found in
skeletal muscles and lungs.

9. Capillaries1. Fenestrated capillaries: they
also have intercellular clefts but
in addition they have small
holes or fenestrations through
the plasma membrane of the
endothelial cells. They are
found in kidneys and intestines.
2. Sinusoid capillaries: they have
wider lumen and incomplete
endothelial cells. They have
large fenestrations too. They
are found in bone marrow and
liver which helps in the transfer
of blood cells from
intravascular space to
surrounding tissues.

10. Veins & Venules
 It is a vessel carrying blood towards the heart.
 The first venous structure are called venules similar
to capillaries.
 The movement of WBC and Platelets to the site of
injury takes place from venules.
 Venules empty the blood to large veins.
 Veins possess the great capacity to stretch to
accommodate all blood emptied to heart.
 Venous sinus is the term used to indicate large
veins such as coronary sinuses and thy have thin
endothelium.
 It has valves which aids in venous return by
preventing backflow

11. Venous Return
Factors aiding venous
return:
• Large lumen which
offers little resistance.
• Valves to prevent
backflow
• Muscular “pump” –
skeletal muscles “push”
blood toward the heart.
• Respiratory “pump” –
pressure gradient sucks
blood into thorax.

12. The Vessels
Functions:
• Distribution of blood
• Exchange of materials with tissues
• Return of blood to the heart
Structure: All the blood vessels are made of four
types of tissues such as endothelial tissue,
collagen fibers, elastic fibers, and smooth muscle
tissue.
Endothelial tissue: It is a line of simple squamous
epithelial cells. They provide smooth luminal
surface and promotes blood flow preventing
coagulation.

13. The Vessels
 Collagen fibers: They are the fibrous tissues
which are flexible but not elastic. They help in
maintenance of shape and size of vessels.
 Elastic Fibers: Composed of elastin protein
fibers and forms a rubber like structure which is
capable of stretching under certain conditions.
They are arranged in circular pattern and they
recoil after distension.
 Smooth Muscle Tissue: Found in the walls and
are involuntary muscles which helps in
maintaining the tension during stretching.

14. General Structure of Blood Vessels
Arteries and veins are
composed of three tunics
or coatings such as
 tunica interna
 tunica media
 tunica externa
They are arranged in
sequence from outside to
inside with a lumen inside.
The general structure is
different in thickness and
layer from vessel to vessel.

15. General Structure

16. Outer Layer: Tunica Externa
 The walls of the large arteries and veins have
three layers.
 It is also called tunica adventitia.
 It is made of strong flexible elastic and collagen
fibers.
 The collagen fibers extend to external structures
to hold it open.
 This is the thickest layer of all three.

17. Middle Layer: Tunica Media
Layer of smooth muscle - circular arrangement –
contains elastin.
Depending on body’s needs – lumen is
narrowed (vasoconstriction) or widened
(vasodilatation)
Smooth muscle regulates diameter of lumen
The autonomic nerve supply called as nervi
vasorum and the blood supply to vessels is
called vasa vasorum: it connects to the midle
layer

18. Inner Layer:Tunica Intima
innermost smooth layer
simple squamous epithelium
continuous with the endocardial layer.
present in all vessels, Inner lining in direct,
contact with blood
In arteries it provides complete smooth surface
whereas in veins it forms valvular structures to
maintain on way of blood flow.

19. Control of Blood Vessels
Responsibility of the vaso-motor centre in the
medulla oblongata.
Change the diameter of the lumen of the blood
vessel.
Medium and small arteries have more muscle
fibres
Large vessels are mostly elastic.
Small arteries and Arterioles respond to nerve
stimuli
Large vessels respond to the amount of blood in
them.

20. Various Circulations?
 Systemic Circulation
 Pulmonary Circulation
 Portal Circulation
 Lymphatic Circulation
 Coronary Circulation

21. CIRCULATORY ROUTES
•The term circulation suggests the flow of blood
through vessels arranged in body.
•The Systemic circulation conducts blood floe
from the heart (left ventricle) to all parts of the
body and back to the heart (right atrium)
•The pulmonary circulation starts from right
ventricle to the pulmonary trunk and then to
lungs for gas exchange and delivers oxygenated
blood into left atrium.
•Portal circulation is the blood flow of liver which
is different from other organs since two capillary
beds (liver & intestine=portal vein) are passed
before reaching vein where as only one capillary
bed is present for other organs.

22. CIRCULATORY ROUTES
•Vascular anastamosis is the term used to
describe a second type of circulation where
there is direct connection between arteries to
arteries and veins to veins without passing
through capillary beds.
•Coronary arteries is the blood supply of the
heart. It promotes collateral circulations.
Absence of this can create coronary block
where bypass surgery may be required.
•Venous anastamosis promotes multiple
drainage facility especially in deep veins and
lack of this can create DVT(deep vein
thrombosis)

23. Factors Affecting Venous Return
1.Pumping action of the heart: Heart beat
creates pressure in arteries to 100mmHg and
it reaches veins around 10mmHg.
2.Suction of venous blood by heart: intra
arterial negative pressure.
3.Thoracic Pump.
4.Intra abdominal pressure.
5.Gravity: seen in head and neck.
6.Muscle Pump
7.Venous valves