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The Circulatory System: Blood
Vessel and Circulation-1
Lecture # 5
(Chapter 20)

General Anatomy of Blood Vessels

Blood Pressure, Resistance, and Flow

Capillary Exchange

Venous Return and Circulatory Shock

Special Circulatory Routes

Anatomy of:
◦ pulmonary circuit
◦ systemic vessels of the axial region
◦ systemic vessels of the appendicular
region
Blood Vessels
Pulmonary
arteries
Pulmonary
veins
Aorta
Vena cava
Arteries
They carry blood away from
the heart (they are efferent
vessels).
Veins
They carry blood back to
(toward) the heart (they
are afferent vessels.
Capillaries
They connect the
with the veins.
arteries
Conducting
(large) artery
Large
vein
Distributing
(medium) artery
Medium
vein
Arteriole
Venule
Capillary
Metarteriole
The Vessel Wall
Lumen
Tunica interna
It lines the blood vessel and is exposed to blood.
Endothelium
Tunica media
It is simple squamous epithelium.
Functions:
It consists of smooth muscle,
collagen, and elastic tissue.
1- Acts as a selectively permeable barrier.
2- Secretes chemicals that stimulate dilation
or constriction of the vessel.
3- Normally repels blood cells and platelets
that may adhere to it and form a clot.
4- When tissue around vessel is inflamed, the
endothelial
cells
produce
cell-adhesion
molecules that induce leukocytes to adhere to
the surface and causes leukocytes to
congregate in tissues where their defensive
actions are needed.
Functions:
1- Strengthens vessel and
prevents blood pressure from
rupturing them.
2- Vasomotion – changes in
diameter of the blood vessel
brought about by smooth
muscle.
Tunica externa
It consists of loose connective
tissue that often merges with
that of neighboring blood
vessels, nerves, or other
organs.
Functions:
1- Anchors the vessel and provides passage
for small nerves, lymphatic vessels.
Basement membrane
Areolar connective tissue.
Nerve
Vasa vasorum
They are small vessels that supply blood to at
least the outer half of the larger vessels.
Arteries
1- Conducting (elastic or large) arteries
The biggest arteries: Aorta, common carotid, subclavian, pulmonary trunk,
and common iliac arteries. They have a layer of elastic tissue, internal
elastic lamina, at the border between interna and media. They have also
an external elastic lamina at the border between media and externa.
They expand during systole, recoil during diastole which lessens
fluctuations in blood pressure.
2- Distributing (muscular or medium) arteries
They distribute blood to specific organs: brachial, femoral, renal, and splenic arteries. The smooth
muscle layers constitute three-fourths of wall thickness.
3- Resistance (small) arteries
They control the amount of blood to various organs. They exhibit up to 25 layers of smooth muscle
tissue (a thick tunica media) and relatively little elastic tissue. The smallest resistance arteries are
the arterioles.
Metarteriole
4- Metarterioles
They are short vessels that link arterioles to capillaries. Muscle cells
form a precapillary sphincter about entrance to capillary. Constriction
of these sphincters reduces or shuts off blood flow through their
respective capillaries and diverts blood to other tissues.
Arteriole
Venule
Arterial Sense Organs
External carotid artery
Internal carotid artery
Carotid body
Carotid sinus
It contains chemoreceptors
They monitor blood chemistry and
transmit signals to the brainstem
respiratory centers to adjust respiratory
rate to stabilize pH, CO2, and O2
It contains baroreceptors
They monitor blood pressure and
send signals to the brainstem, which
decreases the heart rate and dilates
blood in response to high blood
pressure.
Common carotid artery
Aortic bodies
It contains chemoreceptors
They monitor blood chemistry and
transmit signals to the brainstem
respiratory centers to adjust respiratory
rate to stabilize pH, CO2, and O2
Aorta
Baroreceptors
They monitor blood pressure and
send signals to the brainstem, which
decreases the heart rate and dilates
blood in response to high blood
pressure.
Capillaries
Tissues: Oxygen, nutrients and hormones pass from the blood to the
tissue fluid and the waste passes from the tissue fluid to the blood
through the capillaries.
Lungs: Oxygen passes from the air to the blood and carbon dioxide
passes from the blood to the air through the capillaries.
Nutrients,
hormones,
Wastes
O2
CO2
Capillary
O2
CO2
Capillary
Tissue
Metarteriole
Lung
Metarteriole
Capillaries are composed of an endothelium and a basal lamina.
They are absent or scarce in tendons, ligaments, epithelia, cornea and
lens of the eye.
Types of Capillaries
Sinusoids (discontinuous
capillaries)
Continuous Capillaries
Fenestrated capillaries
They occur in most tissues.
Endothelial cells have tight
junctions forming a continuous
tube with intercellular clefts
allow passage of solutes
such as glucose.
Pericytes wrap around the
capillaries and contain the
same contractile protein as
muscle.
They occur in organs that
require rapid absorption or
filtration (kidneys, small
intestine).
The sinusoids are discontinuous capillaries that occur
in liver, bone marrow, and
spleen.
The endothelial cells riddled
with holes called filtration
pores
(fenestrations),
which allow passage of
only small molecules.
The endothelial cells are separated by wide gaps with no
basal membrane.
They contract and regulate
blood flow.
They
allow
proteins
(albumin), clotting factors,
and new blood cells to enter
the circulation.
Capillary Beds
Capillaries are organized into networks called capillary
beds, usually supplied by a single metarteriole.
When the sphincters are open, the
capillaries are well perfused and
engaged in exchanges with the
tissue fluid.
When the sphincters are closed,
little to no blood flow occurs
(skeletal muscles at rest).
Veins
They have a greater capacity for blood
containment than arteries.
In large arteries, blood pressure
averages 90 to 100 mm Hg, whereas
in veins it averages about 10 mm Hg.
Veins, therefore, do not require thick,
pressure resistant walls
They have thinner and flaccid walls and contain less muscular and
elastic tissue than the arteries, that is why they can expand easily.
Small veins merge to form larger and
larger ones as they approach to the heart.
Postcapillary
venules
Muscular
venules
They are even more
porous than
capillaries so also
exchange fluid with
surrounding tissues.
They are up to 1 mm
in diameter.
Tunica interna with
a few fibroblasts
around it.
No tunica media.
No tunica externa.
They have a thin
tunica externa.
Most leukocytes
emigrate from the
bloodstream through
venule walls.
They have 1 or 2 layers
of smooth muscle in
tunica media.
Medium
veins
They are up to 10 mm
in diameter (radial vein,
ulnar vein, saphenous
veins).
Large
veins
They are larger than
10 mm (venae cavae,
pulmonary veins,
internal jugular veins,
and renal veins)
They have a thin tunica
media and thick tunica
externa.
They have some
smooth muscle in all
three tunics
The tunica interna forms
venous valves.
The tunica externa is
the thickest layer
contains longitudinal
bundles of smooth
muscle
The varicose veins
result in part from the
failure of these valves.
Skeletal muscle pump
propels venous blood
back toward the heart.
Venous sinuses:
They are veins with especially thin walls, large lumens, and no
smooth muscle (dural venous sinus and coronary sinus of the heart).
Unlike other veins, sinuses are not capable of vasomotion.
Vasomotion:
It is the capacity to adjust the radius of the blood vessels. This includes
vasoconstriction, the narrowing of a vessel, vasodilation, the
widening of a blood vessel.
Vasomotion
has two
purposes
A generalized raising or lowering of blood pressure of
blood pressure.
A selectively modification of the perfusion of particular
organs and rerouting blood from one region of the body
to another
Redirection of Blood Flow in Response to Metabolic Needs
Portal Systems
Simplest and Most Common
Route of Blood Flow
heart  arteries  arterioles 
capillaries  venules  veins
Blood passes through only one
network of capillaries from the
time it leaves the heart until the
time it returns.
The Portal Systems
Blood flows through two consecutive
capillary networks before returning to
heart.
1- Between hypothalamus and anterior
pituitary.
2- In kidneys.
3- Between intestines to liver.
To the
heart
Anastomosis: It is the point where two blood vessels merge.
1- Arteriovenous anastomosis (shunt)
The artery flows directly into the vein bypassing
capillaries.
2- Venous anastomosis
One vein empties directly into another
They are the most common
anastomosis. They provide several
alternative routes of drainage. For
that reason vein blockage less
serious than an arterial blockage.
(a) Simplest pathway
(1 capillary bed)
(b) Portal system
(2 capillary beds)
(c) Arteriovenous
anastomosis
(shunt)
3- Arterial anastomosis
Two arteries merge and provide collateral
(alternative) routes of blood supply to a
tissue.
Coronary circulation and around joints.
Circle of Willis (cerebral circulation)
(d) Venous
anastomoses
(e) Arterial
anastomoses
Blood Pressure
Blood pressure: It is the force that blood exerts against a vessel wall.
Two pressures are recorded:
Systolic pressure: It is the peak arterial BP taken during ventricular
contraction (ventricular systole).
Diastolic pressure: It is the minimum arterial BP taken during ventricular
relaxation (diastole) between heart beats.
Systolic
blood
pressure
Ventricular
systole
Ventricular
diastole
Diastolic
blood
pressure
Blood pressure:
It is the force that blood exerts against a vessel wall.
Two pressures are recorded
Systolic pressure: It is the peak arterial BP taken during
ventricular contraction (ventricular systole).
Diastolic pressure: It is the minimum arterial BP taken
during ventricular relaxation (diastole) between heart beats.
The Blood Pressure is measured at brachial artery of arm
using sphygmomanometer.
Normal value, young adult: 120/75 mm Hg
Hypertension (high blood pressure):
It is a chronic resting blood pressure higher than 140/90.
Hypotension: It is chronic low resting BP
Hypotension is caused by blood loss, dehydration, anemia
Blood pressure is physiologically determined by 3 principal
variables:
1- Cardiac output
2- Blood volume
It is regulated mainly by the kidneys, which have a greater influence
than any other organ on blood pressure.
3- Peripheral resistance to flow
Blood viscosity
Peripheral resistance depends on 3 factors Vessel length
Vessel radius
Blood viscosity:
RBC count and albumin concentration elevate viscosity the most.
Decreased viscosity with anemia and hypoproteinemia speed flow.
Increased viscosity with polycythemia and dehydration slow flow.
Vessel length:
The farther liquid travels through a tube, the more cumulative friction it
encounters.
Pressure and flow decline with distance.
Systemic blood pressure (mm Hg)
120
100
Systolic pressure
80
60
40
Diastolic
pressure
20
0
Increasing distance from left ventricle
Because of arterial elasticity and the effect of friction against the vessel
walls, all measures of blood pressure declines with distance.
Vessel Radius
In a healthy individual, blood viscosity is quite stable, and vessel
lengths do not change in the short term. Therefore, the only significant
way of controlling peripheral resistance from moment to moment is by
vasomotion.
Vasomotion is a change in vessel radius produced by:
1- Vasoconstriction by smooth muscle contraction.
2- Vasodilation by relaxation of the smooth muscle.
The arterioles are the most significant
point of control over peripheral
resistance and blood pressure. They
produce half of the total peripheral
resistance.
A single drop of epinephrine here has
caused the arteriole to constrict about
one-third of its dilated diameter.
Vasomotion is a quick and powerful way of altering blood
pressure and flow. There are three ways of controlling
vasomotion:
1- Local control
2- Neural control
3- Hormonal control
1- Local control
Tissues have the ability to regulate their own blood supply (autoregulation)
a) Metabolic Theory of Autoregulation
Perfusion
It is the blood flow
per given volume
or mass of tissue
(mL/min/g)
Wastes
( CO2, H+, K+, lactic
acid, adenosine)
Wastes
Perfusion
H+,
K+ ,
( CO2,
lactic
acid, adenosine)
Vasodilation
Vasoconstriction
Perfusion
Perfusion
Other mechanisms of local control
b) Vasoactive chemicals:
They are substances secreted by platelets, endothelial cells, and
perivascular tissue that stimulate vasomotion.
Histamine, bradykinin, prostaglandins, prostacyclin and nitric oxide
stimulate vasodilation.
Endothelins stimulate vasoconstrictor.
c) Reactive hyperemia:
If blood supply is cut off and then restored, flow increases above normal.
d) Angiogenesis:
It is the growth of new blood vessels
Angiogenesis occurs in regrowth of uterine lining, around coronary artery
obstructions, in exercised muscle, and malignant tumors
controlled by growth factors
2- Neural control
The blood vessels are under remote control by the
central and autonomic nervous systems
Baroreceptors: They monitor blood
pressure and send signals to the
brainstem.
BP
1- Heart rate decreases
1- Baroreflex
2- Blood vessels dilate
BP
Sympathetic tone
Chemoreceptors: They monitor blood
chemistry, mainly transmit signals to the
brainstem respiratory centers to adjust
respiratory rate to stabilize pH, CO2,
and O2
When O2 , CO2 , and pH ,
2- Chemoreflex
they also induce a widespread
vasoconstriction in the vasomotor
center.
BP
Perfusion of lungs and gas
exchange increases.
3- Medullary ischemic reflex
A drop in the perfusion of the brain produces
an increase in the sympathetic tone with:
- An increase the heart rate
- Widespread vasoconstriction
BP
Sympathetic tone
Perfusion of brain increases.
Sympathetic tone
Vasomotor
center
Vasomotor
center
3- Hormonal control
Hormones influence blood pressure by two mechanisms:
Vasoactive effects
Regulating water balance
1- Angiotensin II
1- Aldosterone
It is a potent vasoconstrictor that
raises the blood pressure.
It is produced by the adrenal cortex. It
promotes retention of sodium and water
in the kidneys, which increase blood
volume and blood pressure.
Angiotensin I
Angiotensin II
ACE
ACE inhibitors are drugs used to
treat hypertension
2- Epinephrine and Norepinephrine
They are adrenal and sympathetic
catecholamines that stimulate smooth
muscle to contract
They produce vasoconstriction and
rise the blood pressure (a adrenergic
receptors), except in the coronary
blood vessels and skeletal muscles
where they produce vasodilation
(b adrenergic receptors).
2- Atrial natriuretic peptide (ANP)
It is secreted by the heart and antagonizes aldosterone.
It promotes excretion of sodium and
water in the kidneys, which lower blood
volume and blood pressure
3- Antidiuretic hormone
It is produced by the hypothalamus and
released by the posterior pituitary gland.
It promotes water retention in the
kidneys, which increases blood volume
and blood pressure.
Capillary Exchange
The most important blood in the body is in the capillaries because
only through capillary walls are exchanges made between the blood
and surrounding tissues.
Capillary exchange:
It is the two way movement of fluid across capillary walls.
Chemicals given off by the capillaries
Chemicals taken up by the capillaries
Water, oxygen, glucose, amino
acids, lipids, minerals, antibodies,
hormones.
ROUTES
Water, carbon dioxide, ammonia and other
wastes.
1- Filtration pores
(fenestrations) of the
fenestrated capillaries
MECHANISMS
1- Diffusion
2- Transcytosis
3- Filtration
2- Endothelial cell
cytoplasm
3- Intercellular clefts
between endothelial
cells
4- Reabsorption
1- Diffusion
It is the most important form of capillary exchange.
Glucose and oxygen being more concentrated in blood
diffuse out of the blood.
Carbon dioxide and other waste being more concentrated
in tissue fluid diffuse into the blood.
Capillary diffusion can only occur if the solute can
permeate the plasma membranes of the endothelial cell,
or find passages large enough to pass through (filtration
pores and intracellular clefts).
Lipid soluble substance (steroid hormones, O2 and CO2)
diffuse easily through plasma membranes.
Water soluble substances (glucose and electrolytes)
must pass through filtration pores and intercellular clefts.
2- Transcytosis
Endothelial cells pick up material on one side of the plasma membrane
by pinocytosis or receptor-mediated endocytosis, transport vesicles
across cell, and discharge material on other side by exocytosis.
It is important for fatty acids, albumin and some hormones (insulin).
3- Filtration and Reabsorption
 Anatomy
of:
◦pulmonary circuit
◦systemic vessels of the axial region
◦systemic vessels of the
appendicular region
Aorta and Major Branches
L. common
carotid a.
R. common
carotid a.
R. subclavian a.
L. subclavian a.
Brachiocephalic trunk
Aortic arch
Superior vena
cava
Ascending
aorta
Descending
aorta, thoracic
(posterior to
heart)
Inferior vena
cava
Diaphragm
Aortic hiatus
Descending
aorta,
abdominal
Anterior view
Anatomy of the Pulmonary Circuit
Right pulmonary
artery
Superior lobar
artery
Superior lobar arteries
Left pulmonary artery
Middle lobar
artery
Inferior lobar
artery
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Inferior lobar artery
Pulmonary trunk
Superficial Veins of the
Head and Neck
Arteries of the Head and Neck
Vertebral v.
External carotid a.
Internal carotid a.
They supply blood
to the brain.
Vertebral a.
External
jugular v .
Carotid sinus
Thyroid gland
Internal
jugular v .
Thyroid gland
Axillary v.
Brachiocephalic v .
Subclavian v .
Common
carotid a.
(c) Superficial veins of the head and neck
Subclavian a.
Axillary a.
Brachiocephalic
trunk
Lateral view
Anterior
communicating
artery
R. and L.
posterior
communicating
arteries
Cerebral
Arterial Circle
or Circle of
Willis
Cerebral Arterial Circle or Circle
of Willis
Anterior
communicating a.
(1)
Anterior cerebral a.
(2)
Internal carotid a.
Posterior
communicating a.
(2)
Middle cerebral a.
Posterior cerebral a.
(2)
Basilar a.
Vertebral a.
Inferior view
Arteries of the
Upper Limb
Right subclavian
artery
Right axillary
artery
Right
brachial
artery
Right radial
artery
Superficial
palmar arch
Right ulnar
artery
Deep palmar
arch
Veins of the
Upper Limb
Left subclavian
vein
Left Brachiocephalic vein
Left axillary
vein
Cephalic
Brachial
Basilic
Basilic
Brachial
Median cubital
Cephalic
Radial
Ulnar
Superficial and
deep palmar
plexuses
Major Branches of Abdominal Aorta
Celiac trunk
Celiac trunk
Liver
Superior mesenteric a.
Spleen
Pancreas
Pancreatic aa.
Renal a.
L. gastric a.
Gonadal a.
Common hepatic a.
Splenic a.
Inferior mesenteric a.
Common iliac a.
Internal iliac a.
External iliac a.
Celiac trunk:
1- Common hepatic artery
2- Splenic artery
3- Left gastric artery
Arteries of the Lower Limb
External
iliac
Internal
iliac
Femoral
artery
Popliteal
artery
Posterior
tibial
artery
Anterior
view
Anterior
tibial
artery
Fibular or
peroneal
artery
Posterior
view
Veins of the Lower Limb
Great
saphenous
Femoral
vein
Popliteal
Posterior
tibial vein
Anterior
tibial vein
Peroneal or fibular vein
Lesser
saphenous
The Hepatic Portal System
Hepatic
veins
Hepatic
portal v.
Superior
mesenteric v.
Gastric vv.
Splenic v.
Inferior
mesenteric v.
The hepatic portal system all the
blood
draining
from
the
abdominal digestive tract, as
well as from the pancreas,
gallbladder and spleen.
The hepatic portal system gives
the liver first claim to the
nutrients before the blood is
distributed to the rest of the
body.
It also allows the blood to be
cleansed of bacteria and toxins
picked up from the intestine.