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Blood Vessels
Part 2
Slides by Barbara Heard and W. Rose.
figures from Marieb & Hoehn 9th eds.
Portions copyright Pearson Education
Monitoring Cardiovascular Performance
Vital signs: HR, blood pressure, respiratory rate,
body temperature
Radial artery routinely used for HR
Other locations also work: where arteries are close
to body surface
– Carotid, femoral, dorsalis pedis, posterior tibial
K.A.A.P.
Figure 19.12 Body sites where the pulse is most easily palpated.
Superficial temporal artery
Facial artery
Common carotid artery
Brachial artery
Radial artery
Femoral artery
Popliteal artery
Posterior tibial
artery
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Dorsalis pedis
artery
Measuring Blood Pressure
• Systemic arterial BP
– Measured indirectly by auscultatory method
using a sphygmomanometer
– Pressure increased in cuff until it exceeds
systolic pressure in brachial artery
– Pressure released slowly and examiner
listens for sounds of Korotkoff with a
stethoscope
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Measuring Blood Pressure
• Systolic pressure, normally less than 120
mm Hg, is pressure when sounds first
occur as blood starts to spurt through
artery
• Diastolic pressure, normally less than 80
mm Hg, is pressure when sounds
disappear because artery no longer
constricted; blood flowing freely
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Alterations in Blood Pressure
• Hypertension: high blood pressure
Systolic pressure >=140 mmHg or
Diastolic pressure >=90 (or both)
• Chronic (sustained) hypertension
Increased risk of MI (heart attack), heart failure,
stroke, peripheral vascular disease…
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Primary or Essential Hypertension
• Most (90% of) hypertension is “Essential
hypertension”
• No obvious other cause (such as
endocrine disorder, etc.)
• Risk factors: family history, diet, obesity,
age, diabetes mellitus, stress, smoking
• No cure but can be controlled
– Restrict salt, fat intake
– Increase exercise, lose weight, stop smoking
– Antihypertensive drugs
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Homeostatic Imbalance: Hypertension
• Secondary hypertension less common
– Due to identifiable disorders including
obstructed renal arteries, kidney disease, and
endocrine disorders such as hyperthyroidism
and Cushing's syndrome
– Treatment focuses on correcting underlying
cause
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Alterations in Blood Pressure
• Hypotension: low blood pressure
– Blood pressure below 90/60 mm Hg
– Usually not a concern
• Only if leads to inadequate blood flow to tissues
– Often associated with long life and lack of
cardiovascular illness
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Homeostatic Imbalance: Hypotension
• Orthostatic hypotension: temporary low
BP and dizziness when suddenly rising
from sitting or reclining position
• Chronic hypotension: hint of poor
nutrition and warning sign for Addison's
disease or hypothyroidism
• Acute hypotension: important sign of
circulatory shock; threat for surgical
patients and those in ICU
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Blood Flow Through Body Tissues
• Tissue perfusion involved in
– Delivery of O2 and nutrients to, and removal of
wastes from, tissue cells
– Gas exchange (lungs)
– Absorption of nutrients (digestive tract)
– Urine formation (kidneys)
• Rate of flow is precisely right amount to
provide proper function
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Figure 19.13 Distribution of blood flow at rest and during strenuous exercise.
750
750
Brain
750
Heart
250
12,500
1200
Skeletal
muscles
500
Skin
Kidneys
1100
Abdomen
1400
1900
Other
600
Total blood
flow at rest
5800 ml/min
600
600
400
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Total blood flow during
strenuous exercise
17,500 ml/min
Velocity of Blood Flow
• Changes as travels through systemic
circulation
• Inversely related to total cross-sectional
area
• Fastest in aorta; slowest in capillaries;
increases in veins
• Slow capillary flow allows adequate time
for exchange between blood and tissues
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Figure 19.14 Blood flow velocity and total cross-sectional area of vessels.
Relative crosssectional area of
different vessels
of the vascular bed
5000
4000
Total area
(cm2) of the 3000
vascular
2000
bed
1000
0
Velocity of
blood flow
(cm/s)
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50
40
30
20
10
0
Autoregulation
Automatic adjustment of blood flow to tissues to
meet metabolic requirements
Controlled intrinsically (i.e. locally) by modifying
arteriolar diameter - does not require autonomic
nervous system activity
Organs regulate their own blood flows by varying
resistance of their own arterioles
Autoregulation
Two types of autoregulation
• Metabolic control
• Myogenic control
Combination of both determines overall response
Autoregulation
Keeping flow constant when pressure gradient changes.
In a rigid pipe:
Resistance (R) does not change. The equation
Flow = ΔP / R
predicts that when pressure changes, flow will change by
the same percent as pressure.
In an elastic tube:
A pressure change causes a diameter change, and hence a
resistance change:
pressure ↑ ⇒ diameter ↑ ⇒ resistance ↓
As a result, a pressure change will cause a larger flow
change than what you’d get in a rigid pipe.
In a vascular bed with autoregulation:
Flow changes little when the pressure gradient changes.
Percent change in flow is smaller than percent change in
pressure.
Autoregulation
Keeping flow constant when pressure gradient changes.
Graph shows pressure-flow
relation (vertical axis=change in
flow, horiz. axis=change in
pressure) for rigid pipes (),
elastic tubes (), “perfect”
autoregulation (), and actual
measurements in dogs whose
neural reflexes are temporarily
blocked (). We expect
“passive “ blood vessels to look
like . The fact that they look
like  shows they are not
passive – they autoregulate.
Coleman, Granger, Guyton 1971
Measurements in
animals with neural
reflexes blocked
Metabolic Autoregulation
Vasodilation of arterioles and relaxation of
precapillary sphincters occurs in response to
– Declining tissue O2
– Substances from metabolically active tissues (H+,
K+, adenosine, and prostaglandins) and
inflammatory chemicals
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Metabolic Autoregulation
Effects
– Relaxation of vascular smooth muscle if flow
is insufficient for demand
– Release of NO (powerful vasodilator) by
endothelial cells
Endothelins released from endothelium are potent
vasoconstrictors
NO and endothelins balanced unless blood flow
inadequate, then NO wins
Inflammatory chemicals also cause vasodilation
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Myogenic Autoregulation
How it seems to work:
Pressure ↑ ⇒ tension in vessel wall ↑
⇒ vascular smooth muscle constricts
⇒ diameter ↓ ⇒ resistance ↑
Therefore, a pressure change causes a resistance
change that prevents flow from changing as
much as it would have, if resistance had just
stayed the same (or worse, if resistance had
changed in the way you’d expect for an elastic
tube)
P.C. Johnson, Circ Res. 1986.
Figure 19.15 Intrinsic and extrinsic control of arteriolar smooth muscle in the systemic circulation.
Vasodilators
Metabolic
O2
CO2
H+
K+
• Prostaglandins
• Adenosine
• Nitric oxide
Neuronal
Sympathetic tone
Hormonal
• Atrial natriuretic
peptide
Extrinsic mechanisms
Intrinsic mechanisms
(autoregulation)
Vasoconstrictors
• Metabolic or myogenic controls
• Distribute blood flow to individual
organs and tissues as needed
Myogenic
• Stretch
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Neuronal
Sympathetic tone
Metabolic
Hormonal
• Endothelins
• Angiotensin II
• Antidiuretic hormone
• Epinephrine
• Norepinephrine
• Neuronal or hormonal controls
• Maintain mean arterial pressure
(MAP)
• Redistribute blood during exercise
and thermoregulation
Long-term Autoregulation
• Occurs when short-term autoregulation
cannot meet tissue nutrient requirements
• Angiogenesis
– Number of vessels to region increases and
existing vessels enlarge
– Common in heart when coronary vessel
occluded, or throughout body in people in
high-altitude areas
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Blood Flow: Skeletal Muscles
• Varies with fiber type and activity
– At rest, myogenic and general neural
mechanisms predominate - maintain ~ 1L
/minute
– During muscle activity
• Active or exercise hyperemia - blood flow
increases in direct proportion to metabolic activity
• Local controls override sympathetic
vasoconstriction
• Muscle blood flow can increase 10
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Blood Flow: Brain
• Blood flow to brain constant as neurons
intolerant of ischemia; averages 750
ml/min
• Metabolic controls
– Decreased pH of increased carbon dioxide
cause marked vasodilation
• Myogenic controls
– Decreased MAP causes cerebral vessels to
dilate
– Increased MAP causes cerebral vessels to
constrict
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Blood Flow: Brain
• Brain vulnerable under extreme systemic
pressure changes
– MAP below 60 mm Hg can cause syncope
(fainting)
– MAP above 160 can result in cerebral edema
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Blood Flow: Skin
• Blood flow through skin
– Supplies nutrients to cells (autoregulation in
response to O2 need)
– Helps regulate body temperature (neurally
controlled) – primary function
– Provides a blood reservoir (neurally
controlled)
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Blood Flow: Skin
• Blood flow to venous plexuses below skin
surface regulates body temperature
– Varies from 50 ml/min to 2500 ml/min,
depending on body temperature
– Controlled by sympathetic nervous system
reflexes initiated by temperature receptors
and central nervous system
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Temperature Regulation
• As temperature rises (e.g., heat exposure,
fever, vigorous exercise)
– Hypothalamic signals reduce vasomotor
stimulation of skin vessels 
– Warm blood flushes into capillary beds 
– Heat radiates from skin
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Temperature Regulation
• Sweat also causes vasodilation via
bradykinin in perspiration
– Bradykinin stimulates NO release
• As temperature decreases, blood is
shunted to deeper, more vital organs
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Blood Flow: Lungs
• Pulmonary circuit unusual
– Pathway short
– Arteries/arterioles more like veins/venules
(thin walled, with large lumens)
– Arterial resistance and pressure are low
(24/10 mm Hg)
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Blood Flow: Lungs
• Autoregulatory mechanism opposite that in
most tissues
– Low O2 levels cause vasoconstriction; high
levels promote vasodilation
• Allows blood flow to O2-rich areas of lung
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Blood Flow: Heart
• During ventricular systole
– Coronary vessels are compressed
• Myocardial blood flow ceases
• Stored myoglobin supplies sufficient oxygen
• During diastole high aortic pressure forces
blood through coronary circulation
• At rest ~ 250 ml/min; control probably
myogenic
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Blood Flow: Heart
• During strenuous exercise
– Coronary vessels dilate in response to local
accumulation of vasodilators
– Blood flow may increase three to four times
• Important–cardiac cells use 65% of O2 delivered
so increased blood flow provides more O2
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Blood Flow Through Capillaries
• Vasomotion
– Slow, intermittent flow
– Reflects on/off opening and closing of
precapillary sphincters
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Capillary Exchange of Respiratory Gases
and Nutrients
• Diffusion down concentration gradients
– O2 and nutrients from blood to tissues
– CO2 and metabolic wastes from tissues to blood
• Lipid-soluble molecules diffuse directly through
endothelial membranes
• Water-soluble solutes pass through clefts and
fenestrations
• Larger molecules, such as proteins, are actively
transported in pinocytotic vesicles or caveolae
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Figure 19.16 Capillary transport mechanisms. (1 of 2)
Pinocytotic
vesicles
Red blood
cell in lumen
Endothelial
cell
Fenestration
(pore)
Endothelial
cell nucleus
Basement
membrane
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Tight
junction
Intercellular
cleft
Figure 19.16 Capillary transport mechanisms. (2 of 2)
Lumen
Caveolae
Pinocytotic
vesicles
Intercellular
cleft
Endothelial
fenestration
(pore)
Basement
membrane
1 Diffusion
through
membrane
(lipid-soluble
substances)
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2 Movement
through
intercellular
clefts (watersoluble
substances)
4 Transport
via vesicles
or caveolae
(large
substances)
3 Movement
through
fenestrations
(water-soluble
substances)
Fluid Movements: Bulk Flow
• Fluid leaves capillaries at arterial end;
most returns to blood at venous end
– Extremely important in determining relative
fluid volumes in blood and interstitial space
• Direction and amount of fluid flow depend
on two opposing forces: hydrostatic and
colloid osmotic pressures
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Hydrostatic Pressures
• Capillary hydrostatic pressure (HPc)
(capillary blood pressure)
– Tends to force fluids through capillary walls
– Greater at arterial end (35 mm Hg) of bed
than at venule end (17 mm Hg)
• Interstitial fluid hydrostatic pressure
(HPif)
– Pressure that would push fluid into vessel
– Usually assumed to be zero because of
lymphatic vessels
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Colloid Osmotic Pressures
• Capillary colloid osmotic pressure
(oncotic pressure) (OPc)
– Created by nondiffusible plasma proteins,
which draw water toward themselves
– ~26 mm Hg
• Interstitial fluid osmotic pressure (OPif)
– Low (~1 mm Hg) due to low protein content
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Hydrostatic-osmotic Pressure Interactions:
Net Filtration Pressure (NFP)
• NFP—comprises all forces acting on
capillary bed
– NFP = (HPc + OPif) – (HPif + OPc)
• Net fluid flow out at arterial end
• Net fluid flow in at venous end
• More leaves than is returned
– Excess fluid returned to blood via lymphatic
system
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Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the
interstitial fluid compartments, and maintains the interstitial environment. (1 of 5)
The big picture
Fluid filters from capillaries at their arteriolar
end and flows through the interstitial space.
Most is reabsorbed at the venous end.
Arteriole
Fluid moves through
the interstitial space.
For all capillary beds,
20 L of fluid is filtered
out per day—almost 7
times the total plasma
volume!
Net filtration pressure (NFP) determines the
direction of fluid movement. Two kinds of
pressure drive fluid flow:
Hydrostatic pressure (HP)
• Due to fluid pressing against a
boundary
• HP “pushes” fluid across the
boundary
• In blood vessels, is due to blood
pressure
Osmotic pressure (OP)
• Due to nondiffusible solutes that
cannot cross the boundary
• OP “pulls” fluid across the
boundary
• In blood vessels, is due to
plasma proteins
Piston
Boundary
“Pushes”
Solute
molecules
(proteins)
17 L of fluid per
day is reabsorbed
into the capillaries
at the venous end.
Boundary
“Pulls”
Venule
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About 3 L per day
of fluid (and any
leaked proteins) are
removed by the
lymphatic system
(see Chapter 20).
Lymphatic
capillary
Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the
interstitial fluid compartments, and maintains the interstitial environment. (3 of 5)
Net filtration pressure (NFP) determines the
direction of fluid movement. Two kinds of
pressure drive fluid flow:
Hydrostatic pressure (HP)
Osmotic pressure (OP)
• Due to fluid pressing against a
boundary
• HP “pushes” fluid across the
boundary
• In blood vessels, is due to blood
pressure
• Due to nondiffusible solutes that
cannot cross the boundary
• OP “pulls” fluid across the
boundary
• In blood vessels, is due to
plasma proteins
Piston
Solute
molecules
(proteins)
Boundary
“Pushes”
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“Pulls”
Boundary
Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the
interstitial fluid compartments, and maintains the interstitial environment. (4 of 5)
How do the pressures drive fluid flow across a capillary?
Net filtration occurs at the arteriolar end of a capillary.
Capillary
Hydrostatic pressure
in capillary “pushes”
fluid out of capillary.
Osmotic pressure in
capillary “pulls” fluid
into capillary.
Boundary
(capillary wall)
HPc = 35 mm Hg
OPc = 26 mm Hg
HPif = 0 mm Hg
OPif = 1 mm Hg
NFP = 10 mm Hg
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Interstitial fluid
Hydrostatic
pressure in
interstitial fluid
“pushes” fluid
into capillary.
Osmotic pressure
in interstitial fluid
“pulls” fluid out
of capillary.
To determine the pressure driving
the fluid out of the capillary at any
given point, we calculate the net
filtration pressure (NFP)––the
outward pressures (HPc and OPif)
minus the inward pressures
(HPif and OPc). So,
NFP = (HPc + OPif) – (HPif + OPc)
= (35 + 1) – (0 + 26)
= 10 mm Hg (net outward
pressure)
As a result, fluid moves from the
capillary into the interstitial space.
Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the
interstitial fluid compartments, and maintains the interstitial environment. (5 of 5)
Net reabsorption occurs at the venous end of a capillary.
Boundary
(capillary wall)
Capillary
Interstitial fluid
Hydrostatic pressure in capillary
HPc = 17 mm Hg
“pushes” fluid out of capillary.
The pressure has dropped
because of resistance encountered
along the capillaries.
Osmotic pressure in capillary
“pulls” fluid into capillary.
OPc = 26 mm Hg
HPif = 0 mm Hg
Hydrostatic pressure in
interstitial fluid “pushes”
fluid into capillary.
OPif = 1 mm Hg
Osmotic pressure in
interstitial fluid “pulls”
fluid out of capillary.
NFP= –8 mm Hg
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Again, we calculate the NFP:
NFP = (HPc + OPif) – (HPif + OPc)
= (17 + 1) – (0 + 26)
= –8 mm Hg (net inward
pressure)
Notice that the NFP at the venous
end is a negative number. This
means that reabsorption, not
filtration, is occurring and so fluid
moves from the interstitial space
into the capillary.
Circulatory Shock
• Any condition in which
– Blood vessels inadequately filled
– Blood cannot circulate normally
• Results in inadequate blood flow to meet
tissue needs
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Circulatory Shock
• Hypovolemic shock: results from largescale blood loss
• Vascular shock: results from extreme
vasodilation and decreased peripheral
resistance
• Cardiogenic shock results when an
inefficient heart cannot sustain adequate
circulation
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Figure 19.18 Events and signs of hypovolemic shock.
Acute bleeding (or other events that reduce
blood volume) leads to:
1. Inadequate tissue perfusion
resulting in O2 and nutrients to cells
Initial stimulus
Physiological response
Signs and symptoms
Result
2. Anaerobic metabolism by cells, so lactic
acid accumulates
3. Movement of interstitial fluid into blood,
so tissues dehydrate
Chemoreceptors activated
(by in blood pH)
Baroreceptor firing reduced
(by blood volume and pressure)
Hypothalamus activated
(by blood pressure)
Brain
Minor effect
Major effect
Respiratory centers
activated
Cardioacceleratory and
vasomotor centers activated
Heart rate
Sympathetic nervous
system activated
ADH
released
Neurons
depressed
by pH
Intense vasoconstriction
(only heart and brain spared)
Central
nervous system
depressed
Kidneys
Renal blood flow
Adrenal
cortex
Renin released
Angiotensin II
produced in blood
Aldosterone
released
Rate and
depth of
breathing
CO2 blown
off; blood
pH rises
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Tachycardia;
weak, thready
pulse
Kidneys retain
salt and water
Skin becomes
cold, clammy,
and cyanotic
Blood pressure maintained;
if fluid volume continues to
decrease, BP ultimately
drops. BP is a late sign.
Water
retention
Urine output
Thirst
Restlessness
(early sign)
Coma
(late sign)
Circulatory Pathways: Blood Vessels of the
Body
• Two main circulations
– Pulmonary circulation: short loop that runs
from heart to lungs and back to heart
– Systemic circulation: long loop to all parts of
body and back to heart
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Figure 19.19a Pulmonary circulation.
Pulmonary
capillaries
of the
R. lung
R. pulmonary
artery
Pulmonary
capillaries
of the
L. lung
L. pulmonary
artery
To
systemic
circulation
Pulmonary
trunk
R. pulmonary veins
From
systemic
circulation
LA
RA
L. pulmonary
veins
RV
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Schematic flowchart.
LV
Figure 19.19 Pulmonary circulation.
Pulmonary
capillaries
of the
R. lung
Pulmonary
capillaries
of the
L. lung
R. pulmonary L. pulmonary
artery
artery
To
systemic
circulation
Pulmonary
trunk
R. pulmonary veins
From
systemic
circulation
RA
LA
RV
LV
L. pulmonary
veins
Schematic flowchart.
Left pulmonary
artery
Air-filled
alveolus
of lung
Aortic arch
Pulmonary trunk
Right pulmonary
artery
Three lobar arteries
to right lung
Pulmonary
capillary
Gas exchange
Two lobar arteries
to left lung
Pulmonary
veins
Pulmonary
veins
Right
atrium
Left atrium
Right
ventricle
Left
ventricle
Illustration. The pulmonary arterial system is shown in blue to indicate that the blood it carries is oxygen-poor.
The pulmonary venous drainage is shown in red to indicate that the blood it transports is oxygen-rich.
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Figure 19.20 Schematic flowchart showing an overview of the systemic circulation.
Common
carotid arteries
to head and
subclavian
arteries to
upper limbs
Capillary beds of
head and
upper limbs
Superior
vena cava
Aortic
arch
Aorta
RA
LA
RV LV
Azygos
system
Venous
drainage
Inferior
vena
cava
Thoracic
aorta
Arterial
blood
Capillary beds of
mediastinal structures
and thorax walls
Diaphragm
Abdominal
aorta
Inferior
vena
cava
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Capillary beds of
digestive viscera,
spleen, pancreas,
kidneys
Capillary beds of
gonads, pelvis, and
lower limbs
Differences Between Arteries and Veins
Arteries
Veins
Delivery
Blood pumped into single
systemic artery—the aorta
Blood returns via
superior and inferior
venae cavae and the
coronary sinus
Location
Deep, and protected by tissues
Both deep and superficial
Pathways
Fairly distinct
Numerous
interconnections
Supply/drainage
Predictable supply
Usually similar to
arteries, except dural
sinuses and hepatic
portal circulation
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