The document summarizes the structure and function of the cardiovascular system. It describes the three main types of blood vessels - arteries, capillaries, and veins - and their roles in circulating blood throughout the body. Arteries carry oxygenated blood away from the heart, branching into smaller vessels. Capillaries allow for exchange of oxygen, nutrients, waste at the cellular level. Veins then collect deoxygenated blood and return it to the heart. The document provides detailed information on the layers, tissue composition, and regulatory mechanisms of different sections of the cardiovascular system.

2. The blood vessels of the body form a closed
delivery system that begins and ends at the heart
 Often compared to a plumbing system, it is a far
more dynamic system of structures that pulse,
constrict and relax and even proliferate to meet
changing body needs


3.  The
major types of blood vessels are:
◦ Arteries
 The large distributing vessels that bring blood to the body
◦ Capillaries
 The tiny vessels that distribute blood to the cells
◦ Veins
 The large collecting vessels that bring blood back to the
heart

Intermediate vessels connect
◦ Arterioles bring blood to the capillaries
◦ Venules drain blood from the capillaries

4. The pattern of distribution starts with arteries to
arterioles to capillaries to venules to veins
 The blood vessels in the adult human body
carry blood in a distribution network that is
approximately 60,000 miles in length
 Only capillaries come into intimate contact with
tissue cells and serve cellular needs


7. • The walls of blood vessels are composed of
three distinct layers or tunics
1.Tunica Intima
2.Tunica Media
3.Tunica Adventitia
The tunics surround a
central opening called a lumen

8. The innermost tunic is the tunica intima
 This tunic contains the endothelium, the simple
squamous endothelium that lines all vessels
 Its flat cells fit closely together, forming a slick
surface that minimizes friction as blood moves
through the vessel lumen


10. 
In blood vessels
larger than 1 mm in
diameter, a subendothelial layer of
loose connective
tissue,
subendothelial
layer, (basement
membrane)
supports the
endothelium

11. The middle tunic, the tunica media, is mostly
circularly arranged smooth muscle cells and
sheets of elastin
 The activity of the smooth muscle is regulated by
vasomotor nerve fibers of the sympathetic
division of the autonomic nervous system


12. Tunica
Media

13. Depending on the needs of the body, the
vasomotor fibers can cause vaso-constriction or
vasodilation
 The activities of the tunica media are critical in
regulating circulatory dynamics
 Generally, the tunica media is the bulkiest layer
in arteries, which bear the chief responsibility for
maintaining blood pressure and continuous
blood circulation


14. The outermost layer of a blood vessel is the tunica
externa or tunica adventitia
 This tunic is composed largely of loosely woven
collagen fibers that protect blood vessels and
anchor it to surrounding structures


15. Tunica
externa

16. The tunica externa is infiltrated with nerve fibers
and lymphatic vessels and, in larger vessels, a
system of tiny blood vessels
 These vessels, the vasa vasorum nourish the
external tissues of the blood vessel wall


18. Arteries are vessels that carry blood away from
the heart
 All arteries carry oxygen rich blood with the
exception of those in the pulmonary circuit
 Blood proceeds to the tissues through these

arteries, which are classified into three types:
◦ Elastic arteries
◦ Muscular arteries
◦ Arterioles

19. Elastic arteries are thick walled arteries near the
heart - the aorta and its major branches
 These arteries are the largest in diameter and the
most elastic
 A large lumen allows them to serve as low
resistance pathways that conduct blood from the
heart to medium-sized arteries and thus are called
conducting arteries


20. The elastic arteries contain more elastin than
any other type of vessel
 While present in all three layers, the tunica
media contains the most
 The abundant elastin enables these arteries to
withstand and smooth out large pressure
fluctuations by expanding when the heart forces
blood into them and then recoiling to propel
blood onward into the circulation when the heart
relaxes


21. Elastic arteries also contain substantial amounts
of smooth muscle, but they are relatively inactive
in vasoconstriction
 Because elastic arteries expand and recoil
passively to accommodate changes in blood
volume, the blood is kept under pressure
 Thus, blood flows continuously rather than starting
and stopping with each heart beat


22. The muscular distributing arteries deliver blood
to specific body organs and account for most of
the named arteries
 Proportionately, they have the thickest media
of all vessels
 Their tunica media contains relatively more
smooth muscle and less elastic tissue than that
of elastic arteries
 They are more active in vasoconstriction and
are less distensible


23. 
As in all vessels, concentric sheets of elastin
occur within the tunica media of muscular arteries
although these sheets are not as thick or
abundant as those of elastic arteries

24. 

A feature unique
to muscular
arteries,
especially thick
sheets of elastin
lie on each side
of the tunica
media
An external
elastic lamina lies
between the
tunica media and
tunica externa

25. 
The elastin in muscular arteries, like that in elastic
arteries, helps dampen the pulsatile pressure
produced by the heartbeat

26. 
Arterioles have a lumen diameter from 0.3 mm to
10 µm, and are the smallest of the arteries
Larger arterioles exhibit all three tunics, but their
tunica media is chiefly smooth muscle with a few
scattered muscle fibers
 The smaller arterioles that lead into capillary beds,
are little more than a single layer of smooth
muscle cells spiraling around the endothelial lining


27. 
The diameter of each arteriole is regulated in two
ways:
◦ Local factors in the tissues signal the smooth
musculature to contract or relax, thus regulating the
amount of blood sent downstream to each capillary bed
◦ Sympathetic nervous system adjusts the diameter of
arterioles throughout the body to regulate systemic
blood pressure

29. 


The microscopic
capillaries are the
smallest blood
vessels
In some cases, one
endothelial cell forms
the entire
circumference of the
capillary wall
The average length of
a capillary is 1 mm
and the average
diameter is 8-10 µm

30. 
Capillaries have a
lumen just large
enough for blood
cells to slip through
in single file

31. Capillaries are the body’s most important blood
vessels because they renew and refresh the
surrounding tissue fluid (interstitial fluid) with
which all cells in the body are in contract
 Capillaries deliver to interstitial fluid the oxygen
and nutrients that cells need while removing
carbon dioxide and nitrogenous wastes that cells
deposit in the fluid


32. Given their location and the thinness of their
walls capillaries are ideally suited for their role
of providing access to nearly every cell
 Along with the universal functions just
described some capillaries also perform sitespecific functions

◦
◦
◦
◦
Lungs: gas exchanges
Endocrine glands: pick up hormones
Small intestine: nutrients
Kidneys: removal of nitrogenous wastes

33. A capillary bed is a network of the body’s smallest
vessels that run throughout almost all tissues,
especially the loose connective tissue
 This flow is also called a “microcirculation”


34.  The
capillary bed consist of two
types of vessels: Shunt vessels(met-
arterioles) and true capillaries
 The terminal arteriole leads into a
metarteriole which is directly continuous with
the thorough- fare channel

35. The thoroughfare channel joins the post- capillary
venule that drains the capillary bed
 The true capillaries number 10 to 100 per
capillary bed, depending on the organ served
 Branch from metarteriole to thoroughfare channel


36. A cuff of smooth muscle fibers, called a precapillary sphincter surrounds the root of each
capillary at the metarteriole and acts as a valve to
regulate the flow of blood into the capillary
 When the pre-capillary sphincters are relaxed,
blood flows through the true capillaries and takes
part in exchanges with tissue cells
 When the pre-capillary sphincters are contracted,
blood flows through the shunts and bypasses the
tissue cells


37. 
Most tissues have a rich supply, but there are a
few exceptions
◦
◦
◦
◦
Tendons and ligaments / poorly vascularized
Cartilage / from adjacent connective tissue
Epithelia / from adjacent connective tissue
Cornea / nourished by aqueous humor

38. The relative amount of blood entering a capillary
bed is regulated by vasomotor nerve fibers and
local chemical conditions
 A capillary bed may be flooded with blood or
almost completely bypassed, depending on
conditions in the body or in that specific organ
 Example of shunting blood from digestive organs
to skeletal muscles


39. The structure of capillaries is well suited for their
function in the exchange of nutrients and wastes
between the blood and the tissues through the
tissue fluid
 A capillary is a tube consisting of thin endothelial
cells surrounded by a basal lamina
 The endothelial cells are held together by tight
junctions and occasional desmosomes


40. Tight junctions block the passage of small
molecules, but such junctions do not surround the
whole perimeter of the endothelial cells
 Instead, gaps of unjoined membrane called
intercellular clefts occur through which small
molecules exit and enter the capillary


41. 
External to the
endothelial cells,
the delicate
capillary is
strengthened and
stabilized by
scattered
pericytes

42. 
The pericytes are
spider shaped
cells whose thin
processes form a
network that is
widely spaced so
as to not to
interfere with
capillary
permeability

43. 
Structurally there are three types of capillaries
◦ Continuous
◦ Fenestrated
◦ Sinusoidal

44. Continuous
capillaries are
abundant in the
CNS, skin and
muscles and are
the most common
 They are
continuous in the
sense that their
endothelial cells
provide an
uninterrupted
lining


45. Adjacent cells are joined laterally by tight
junctions
 However, these are usually incomplete and leave
gaps of unjoined membrane called intracellular
clefts that are just large enough to allow limited
passage of fluids


46. Fenestrated capillaries have fenestrations (pores)
spanning the endothelial cells
 Fenestrated capillaries occur only where there are
exceptionally high rates of exchange of small
molecules between blood and the surrounding
tissue


48. The fenestrations are usually covered by a thin
diaphragm but this variety has much greater
permeability to fluids and small solutes
 Fenestrated capillaries are found where active
capillary absorption or filtrate formation occurs


49. 
Fenestrated
capillaries are
found in the small
intestine to receive
digested nutrients
Intercellular
clefts
 These capillaries are also found in the synovial
membranes of joints to allow water molecules to
exit the blood to form synovial fluid

50. 
Molecules pass into and out of capillaries via four
routes
◦
◦
◦
◦
Direct diffusion through endothelial cell membranes
Through the intercellular clefts
Through cytoplasmic vesicles or caveolae
Through fenestrations in fenestrated capillaries

51. Most exchange of small molecules is thought to
occur through intercellular clefts
 Caveolae apparently transport a few larger
molecules, such as small proteins
 Carbon dioxide and oxygen seem to be the only
important molecules that diffuse directly through
endothelial cells because these uncharged
molecules easily diffuse through lipid containing
membranes of cells


52. The blood-brain barrier prevents all but the most
vital molecules(even leukocytes) from leaving the
blood and entering brain tissue
 The blood-brain barrier derives its structure from
the capillaries of the brain
 Brain capillaries have complete tight junctions, so
intercellular clefts are absent


53. Brain capillaries are continuous, not fenestrated
and they also lack caveolae
 Vital capillaries that must cross brain capillaries
are “ushered through” by highly selective transport
mechanisms in the plasma membranes of the
endothelial cells


54. Some organs
contain wide,
leaky capillaries
called sinusoids
 Each sinusoid
follows a twisted
path and has both
expanded and
narrowed regions


55. 
Sinusoids are
usually
fenestrated and
their endothelial
cells have fewer
cell junctions than
do ordinary
capillaries

56. In some sinusoids
the intercellular
cleft is wide open
 Sinusoids occur
wherever there is
an extensive
exchange of large
materials, such as
proteins or cells,
between the blood
and surrounding
tissue


57. Sinusoids are found in only in bone marrow and
spleen, where many blood cells move through
their walls
 The large diameter and twisted course of
sinusoids ensure that blood slows when flowing
through these vessels, allowing time for the many
exchanges that occur across their walls


59. Veins are the blood vessels that conduct blood
from the capillaries back to the heart
 Because blood pressure declines substantially
while passing through the high-resistance
arterioles and capillary beds, blood pressure in
the venous part of the circulation is much lower
than in the arterial part


60. Because they need not withstand as much
pressure, the walls of veins are thinner than those
of comparable arteries
 The venous vessels increase in diameter, and
their walls gradually thicken as they progress from
venules to the larger and larger veins leading to
the heart


61. Venules, ranging from 8 to 100 µm in diameter are
formed when capillaries unite
 The smallest venules, the postcapillary venules,
consist of endothelium on which lie pericytes


62. Venules join to form veins
 With their large lumens and thin walls, veins can
accommodate a fairly large blood volume
 Up to 65%of the body’s total blood supply is found
in the veins at any one time although the veins are
normally only partially filled with blood


63. Veins have three distinct tunics, but their walls are
always thinner and their lumens larger than those of
corresponding arteries
 There is little smooth muscle even in the largest veins


64. The tunica externa is the heaviest wall layer
and is often several times thicker than the
tunica media
 In the venae cavae, the largest veins, which
return blood directly to the heart the tunica
externa is further thickened by longitudinal
bands of smooth muscle


65. Tunica externa

66. Veins have less elastin in their walls than do
arteries, because veins do not dampen any
pulsations (these have been smoothed out by
the arteries)
 Because blood pressure within veins is low, they
can be much thinner walled than arterioles
without danger of bursting


67. Low-pressure conditions demand some special
adaptations to help return blood to the heart at
the same rate as it was pumped into circulation
 One structural feature that prevents the
backflow of blood away from the heart is the
presence of valves within veins


68. Venous valves are formed
from folds of the tunica
intima and they resemble
the semilunar valves of the
heart in structure and
function
 Venous valves are most
abundant in the veins of
the limbs, where the
upward flow of blood is
opposed by gravity


69. A few valves occur in the veins of the head and
neck, but none are located in veins of the thoracic
and abdominal cavities
 A functional mechanism that aids the return of
venous blood to the heart is the normal movement
of our body and limbs


70. Another mechanism of
venous return is called
the skeletal muscular
pump
 Here contracting
muscles press against
the thin-walled veins
forcing valves proximal
to the contraction to
open and propelling the
blood toward the heart


72. Where vessels unite or interconnect, they form
vascular anastomoses
 Most organ receive blood from more than one
arterial branch and arteries supplying the same
area often merge, forming arterial anastomoses
 Arterial anastomoses provide alternative pathways
called collateral channels for blood to reach a
given body region


73. If one arterial branch is blocked arterial
anastomoses provide the region with an adequate
blood supply
 Arterial anastomoses are abundant in abdominal
organs and around joints, where active movement
may hinder blood flow through one channel


74. Anastomoses are also prevalent in the abdominal
organs, brain, and heart
 Because of the many anastomoses among the
smaller branches of the coronary artery in the
heart wall, a coronary artery can be 90% occluded
by atherosclerosis (plaque) before a myocardial
infarction (heart attack) occurs


75. Arteries that do not anastomose, or which have
a poorly developed collateral circulation (retina,
kidneys, spleen) may be vulnerable if their
blood flow is interrupted
 Veins anastomoses much more freely than
arteries and because of abundant collateral
circulation occlusion of a vein rarely blocks
blood flow leading to tissue death


76. The wall of the blood vessels contain living cells and
therefore require a blood supply of their own
 For this reason the larger arteries and veins have
tiny arteries, capillaries and veins in their tunica
externa
 These tiny vessels the vasa vasorum nourish the
outer half of the wall of a large vessel with the inner
half being nourished by the blood in the lumen
