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
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
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
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
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
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
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
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
