The document summarizes the structure and function of blood vessels. It describes the three main types of blood vessels - arteries, capillaries, and veins. It explains the layers (tunics) that make up blood vessel walls, and how they differ in each vessel type. It discusses the roles of elastic and muscular arteries in circulation. It also outlines neural, chemical, and renal mechanisms that regulate blood pressure through controlling vessel diameter and cardiac output.

1. Blood Vessels

2. • The three major blood vessels arteries,
capillaries and Veins.
• As the heart alternately contracts and relaxes,
blood is forced into the large arteries leaving
its chambers
• Arteries and Veins simply act as conduits for
blood

3. The Structure of Blood VesselWall
• The walls of all blood vessels, except capillaries, are
composed of three distinct layers or tunics,
surrounding a central tubular opening
• The innermost layer that lines the vessel lumen is
called the tunica intima ( tunica interna)
• The tunica intima consists of two layers, a thin layer of
endothelium which is a continuous with the
endocardial lining of the heart
• It is the only tunic present in all blood vessels
• The endothelial cells fit closely together, forming a slick
surface that helps decrease friction as blood moves
through the vessels lumen

4. Tunica media
• The middle layer, the tunica media consists
mostly of circularly arranged smooth muscle
cells and elastic connective tissue fibers
• The activity of the smooth muscle is regulated
by vasomotor fibers of the sympathetic
division of the autonomic nervous system

5. • Depending on the precise needs of the body at
any moment, the vasomotor fibers can cause
vasoconstriction ( reduction in lumen diameter
due to smooth muscle contraction) or
vasodilation ( widening of the lumen due to
smooth muscle relaxation)
• Since small changes in blood vessel diameter
greatly infuence blood flow and blood pressure,
the activity of the tunica media are critical in
regulating the circulatory dynamics

6. • The tunica media is the bulkiest layer in
ateries, which bear the greatest responsibility
for maintaining blood pressure and continuos
blood circulation

7. Tunica adventitia( tuniva externa)
• The outermost layer of a blood vessel wall, the
tunica adventia(tunica externa)
• Is composed largely of loosely collagen fibers that
protect the blood vessel and anchour it to
surrounding structures
• The tunica adnetitia is inflitrated with nerve
fibers and lymphatic vessels
• The three vessel types vary in length, diameter
and relative thickness and tissue makeup of their
wall

8. Arteries
• Are vessels that transport blood away from
the heart

9. Elastic Arteries
• Are the large, thick-walled arteries close to the
heart, such as aorta and major branches
• These arteries are both the largest in diameter
and the most elastic
• Their large diameter lumen allows them to
serve as low-resistance conduits to convey
blood from the heart to the medium sized
arteries ( conducting arteries)

10. • Because of generous amount of elastic tissue
in the tunica media, these arteries have a
stretchy sheet of elastic fibers called the
elastic lamian which enables the arteries to
withstand large pressure fluctuations by
expanding when the heart contratcs, forcing
blood into them and recoiling as blood flows
forward into the circulation during heart
relaxiation

11. Muscular Arteries
• Are medium and smaller sized arteries farther
along inn the circulatory pathway that carry
blood to the specific body organs
• Their tunica media contains more smooth muscle
and less elastic tissue than that of elastic arteries
• They more in vasoconstriction and are less
distensable
• Because they distribute blood throughout the
body ( distributing arteries)

12. arterioles
• Arterioles have a lumen diameter smaller than
0.5mm and are the smallest of the arterial vessels
• The larger arterioles exhibit all three tunics and
their tunica media is chiefly smooth muscle with
few scattered elastic fibers
• Vasoconstriction and vasodilation of arterioles in
response to changing stimuli from vasomotor
nerve fibers and local chemical influences are the
most important factors determining blood flow
into the capillaryy beds

13. Arterial Pulse
• Alternating expansion and recoil of elastic
arteries during each cardiac cycle creates a
pressure wave (pulse) that is transmitted
through the arterial tree with each heartbeat
• You can feel a pulse in any artery lying close to
the body surface by compressing the artery
against firm tissue and this provide an easy
way of counting heart rate

14. Capillaries
• Form dense networks that branch throughout
nearly all body tissues.
• Thin walls consist of just tunica intima
• Suited for the exchange of materials between
the blood and interstitial fluid
• Structually are classified as continuous or
fenestrated

15. • Continuous capillaries are abundant in the
skin and muscle
• Their endotheilal cells provide an
uninterrupted lining, adjacent cells are joined
laterally by tight junctions are incomplete,
leaving intercellular clefts

16. • Fenestrated capillaries, the endothelial cells
are joined by gap junctions and are riddled
with oval pores of fenestration which are
usually covered by a very thin membrane
• Are found where active capillary absorption
occurs, such as in the mucosa of the small
intestine and in endocrine organs

17. Veins
• Venules , which represent the initial part of the
venous return to the heart, are formed when
capillaries unite
• The smallest venules consist entirely of the tunica
intima and tunica adventitia
• In large venules there is tunica media
• The venules join to form to form veins which
usually have three distinct tunics, but their walls
are always much thinner and their lumens much
larger than those of corresponding artreries

18. • With their large lumens and thin walls, veins
can accommodate a fairly large blood volume,
since up to 65% of the body’s total blood
supply is found in the veins
• Are called capacitance vessels or blood
reservoirs
• Blood pressure within the veins is low,
• Presence of valves that prevent back flow of
blood within the veins

19. Vascular anastomoses
• When vascular channels unite or interconnect vascular
anastomoses are formed
• Most organs receive blood from more than one arterial
branch, and the arterial supplying the same territory
often merge with one another, forming arterial
anastomoses
• Arterial anastomoses permit free communication
between the vessels involved and provide alterative
pathways for blood to reach a given organ
• If one branch becomes blocked by a clot or atheroma,
the alternative or collateral-channels helps ensure that
organ receives adequate nutrition

20. • Arterial anastomoses are abundant around
ifjoints, where active movement may hinder
blood flow through one channel, and in the
abdominal organs
• Arteries that do not anastomose, such as those
supplying the toes and fingers, are called end
arteries. If their blood flow is interrupted, the
cells supplied by that branch die and decompose,
eventually causing gangrene

21. • The thoroughfare channels of capillary beds
that connect arterioles and venules are
examples of arteriovenous anastomoses
• Venous anastomoses are common ( many
veins interconnect freely with others along
their entire length, as a result occlusion of
venous channels is rarely a cause of tissue
death and necrosis

22. Blood Pressure
• is the force per unit area exerted on the wall
of a blood vessel by its contained blood
• This pressure keeps blood moving through the
body

23. Resistance
• Is opposition to flow and is a measure of the
amount of friction the blood encounters as it
passes through the vessels
• Since most of friction is encountered in the
peripheral circulation, away from the heart
and is referred as peripheral resistance

24. Systemic blood pressure
• As the left ventricle contracts and expels blood into the
aorta, it imparts kinetic energy to the blood, which in
turn stretches the elastic walls of the aorta and causes
aortic pressure to reach its peak ( systolic arterial blood
pressure) average about 120mmHg
• During diastole, closure of the aortic semilumar valve
prevents blood from flowing back into the heart, and
the walls of the aorta recoil, maintaining continuous
pressure on the reducing blood volume
• During this time, aortic pressure drops to its lowest
leve (diastole pressure) approximate 70-80mmHg

25. Factors affecting Blood Pressure
• The main factors that influencing blood
pressure are cardiac output, peripheral
resistance and blood volume
• Blood Pressure = Cardiac output * P eripheral
resistance

26. Regulation of blood pressure
• Maintaining a steady flow of blood from the
head to the toes is vital for proper organ
fucntion
• Blood pressure is regulated by neural,
chemical and renal control that act
continuosly to modify and adjust cardiac
output, peripheral resistance and blood
volume

27. Nervous System controls
• Neural controls of blood vessels are directed primary at
maintaining adequate systemic blood pressure and
altering blood distribution to achieve specific function.
• For example, during exercise, blood is shunted
temporarily from the skin and digestive organs to the
skeletal muscles
• Heat loss from the body is enhanced when blood
vessels in the skin are dilated and under conditions of
low blood volume, all vessels except those supplying
the heart and brain are constricted to allow as much
blood as possible to flow to those vital organs

28. • Nervous system controls blood pressure and
blood distribution by altering the diameter of
arterioles
• Most neural controls operate via reflex
involving the following components.
• Pressoreceptors and associated afferent
fibers, vasomotor center of medulla,
vasomotor (efferent) fibers and vascular
smooth muscle

29. Vasomotor fibers
• Are sympathetic nervous system efferents that
innervate the smooth muscle layer of blood
vessels, most the arterioles
• Most vasomotor fibers release norepiniphrine (
vasoconstrictor)
• Some of the vasomotor fibers release
acetylcholine, causing vasodilation
• These vasodilator fibers, though important to
local controls of muscle blood flow during
exercise, are not important to overall regulation
of systemic blood pressure

30. Vasomotor center
• A cluster of sympathetic neurons in the medulla, is the
intergrating center for blood pressure control.
• This center transmits impulses in a fairly steady stream
along the vasomotor fibers, as a result, arterioles are
nearly always in a state of partial constriction
(vasomotor tone)
• Any increase in the rate of vasomotor impulse delivery
intensifies vasoconstriction and leads to a rise in
systemic blood pressure, a decreased rate allows the
vascular muscle to relax somewhat, causing
vasodilation and a drop in blood pressure

31. • The activity of the vasomotor center is
modified by inputs coming from
pressoreceptors, chemoreceptors and higher
brain centers

32. Pressoreceptors
• Pressoreceptors that detect changes in arterial
pressure are located not only in the carotid and
aortic sinuses, but in nearly every large artery of
the neck and thorax.
• When arterial blood pressure rises and stretches
these receptors, they send off a faster stream of
impulses to the vasomotor center. This inhibit the
vasomotor center, reducing impulse transmission
along the vasomotor fibers and results in
vasodilation and a decline in blood pressure

33. • Afferent impulses from the pressoreceptor
also reach the cardaic inhibitory center in the
medulla, leading to a reduction in heart rate
and contractile force.
• Decline in mean arterial pressure initiates
reflex vasoconstriction and increased cardiac
output

34. • The central function of the pressoreceptors is
to protect the circulation against short-term
changes in blood pressure, such as those
occuring with changes in posture
• Pressoreceptors are relatively ineffective in
providing the same protection against
sustained pressure changes, as evidenced by
the fact that some people do have chronic
hypertension.

35. Chemoreceptors
• When the oxygen content of the blood drops
sharply, or when hydrogen ion levels rise,
chemoreceptors in the aortic arch and large
arteries of the neck region transmit impulses
to the vasomotor center, and reflex
vasoconstriction occurs.
• The rise in blood pressure that follows helps
to speed blood return to the heart and lungs

36. High brain centers
• Reflexes that regulate blood pressure are integrated at
the brain stem (medulla) level. Although the cerebral
cortex and hypothalamus are not required for routine
blood pressure maintenance, these higher brain
centers can modify arterial blood pressure via relays to
the medullary center
• For example, flight-or-fight response
• The hypothalamus also mediates the redistribution of
blood flow and other CV responses that e.g exercise
and changes in body temperature

37. Chemical controls
• Numerous other blood-borne chemicals influence
blood pressure by acting directly on vascular
smooth muscle or on the vasomotor center
• Adrenal medulla hormones- During periods of
stress, the adrenal gland releases
norepinnephrine(NE) and epinephrine to the
blood, which enhance the sympathetic fight-or-
flight response
• NE has a vasoconstrictive action, epiniphrine
increases Cardiac output

38. • Antiduretic hormone id produced by
hypothalamus and stimulate the kidneys to
conserve water
• ADH is released in greater amounts when
blood pressure falls and helps to restore
arterial pressure by causing intense
vasoconstriction