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Lecture 5
Blood Vessels
Blood Vessels
• Dynamic structures that pulsate, constrict,
relax and proliferate
• form a closed delivery system that begin and
end at the heart
Blood Vessels
• Arteries: carry blood away from heart
• Capillaries: contact tissue cells; directly serve
cellular needs
• Veins: carry blood toward heart
Figure 19.1b Generalized structure of arteries, veins, and capillaries.
Tunica intima
• Endothelium
• Subendothelial layer
• Internal elastic membrane
Tunica media
(smooth muscle and
elastic fibers)
• External elastic membrane
Valve
Tunica externa
(collagen fibers)
• Vasa vasorum
Lumen
Lumen
Artery
Capillary network
Vein
Basement membrane
Endothelial cells
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Capillary
Tunica Intima
• Endothelium lines lumen of all vessels
• May also contain an internal elastic connective
tissues in certain arteries
Tunica media
• Smooth muscle and sheets of elastin
– Sympathetic vasomotor nerve fibers control
vasoconstriction and vasodilation of vessels
– Influence blood flow and blood pressure
Tunica Externa
• Collagen fibers protect and reinforce; anchor
to surrounding structures
– Contains nerve fibers, lymphatic vessels
– Vasa vasorum of larger vessels nourishes external
layer
Arteries
• Elastic arteries
• Muscular arteries
• Arterioles
• Arteries carry blood from the heart to the
capillary beads
Elastic Arteries
• Thick-walled arteries near the heart
– aorta and its major branches
• Elastin is contained in all three tunics
– swiss cheese appearance
• Inactive to vasocontriction
– expand and recoil as blood flows
Muscular Arteries
• Deliver blood to specific body organs
• Have thickest tunica media of all arteries
– more smooth muscle, less elastin
• More active in vasoconstriction, less
expandable
Arterioles
• the smallest of the arteries
– smooth muscle and elastic fibers
• lead into the capillary beds, blood flow
determined by arteriolar diameter
– vasoconstriction shuts down blood flow
– vasodilation allows blood flow
• Influenced by neural, hormonal and local
chemicals
Capillaries
• smallest blood vessels
– thin tunica intima layer
• Single file blood cell flow
– nourish almost every cell in the body
• Exchange of gases, nutrients, hormones and
waste
Three Types of Capillaries
• Continuous Capillaries
– limited passage of fluids
• Fenestrated Capillaries
– pores allow greater permeability
– active capillary absorption or filtrate formation
• Sinusoid Capillaries (Sinusoids)
– highly leaky found in liver, bone marrow, spleen and
adrenal medulla
– allow passage of large molecules and blood cells
Figure 19.3a Capillary structure.
Pericyte
Red blood
cell in lumen
Intercellular
cleft
Endothelial
cell
Basement
membrane
Tight junction
Endothelial
nucleus
Pinocytotic
vesicles
Continuous capillary. Least permeable, and most
common (e.g., skin, muscle).
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Figure 19.3b Capillary structure.
Pinocytotic
vesicles
Red blood
cell in lumen
Fenestrations
(pores)
Endothelial
nucleus
Basement membrane
Tight junction
Intercellular
cleft
Endothelial
cell
Fenestrated capillary. Large fenestrations (pores)
increase permeability. Occurs in areas of active
absorption or filtration (e.g., kidney, small intestine).
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Figure 19.3c Capillary structure.
Endothelial
cell
Red blood
cell in lumen
Large
intercellular
cleft
Tight junction
Incomplete
basement
membrane
Nucleus of
endothelial
cell
Sinusoid capillary. Most permeable. Occurs in special
locations (e.g., liver, bone marrow, spleen).
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Capillary Beads
• Capillaries do not function independently
• Form an interweaving network of capillaries
– microcirculation
• Two types
– Vascular shunt
– true capillaries (exhange vessels)
Figure 19.4 Anatomy of a capillary bed.
Precapillary sphincters
Vascular shunt
Metarteriole Thoroughfare
channel
True
capillaries
Terminal arteriole
Postcapillary venule
Sphincters open—blood flows through true capillaries.
Terminal arteriole
Postcapillary venule
Sphincters closed—blood flows through metarteriole – thoroughfare
channel and bypasses true capillaries.
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Precapillary Spinchter
• Smooth muscle fibers surround root of each
true capillary at the metarteriole, acting as a
value to regulate blood flow
• Regulated by local conditions, ateriolar
vasomoter nerve fibers
– controls when blood is needed at specific tissue
sites
Venous System
• Veins carry blood from the capillary bead
toward the heart
• Diameter increases the closer you get to the
heart
Venules and Veins
• Capillaries unite to form venules
• Venules join to form veins
– typically thinner than artery with larger lumen
• Designed to carry a large blood supply
– capacitance vessels and blood reservoirs
Figure 19.1a Generalized structure of arteries, veins, and capillaries.
Artery
Vein
Arteries are thick = higher blood pressure conditions, less blood
Viens are thin = lower blood pressure conditions, more blood
© 2013 Pearson Education, Inc.
Venous Valves
Vascular Anastomoses
• meaning “coming together”
• Specialized interconnections of blood vessels
– the merging of blood vessels that supply blood to
the same area, bypassing capillary beds
– collateral channels
– vascular shunts are examples
Figure 19.2 The relationship of blood vessels to each other and to lymphatic vessels.
Venous system
Large veins
(capacitance
vessels)
Arterial system
Heart
Elastic
arteries
(conducting
arteries)
Large
lymphatic
vessels
Lymph
node
Lymphatic
system
Small veins
(capacitance
vessels)
Muscular
arteries
(distributing
arteries)
Arteriovenous
anastomosis
Lymphatic
capillaries
Sinusoid
Arterioles
(resistance
vessels)
Terminal
arteriole
Postcapillary
venule
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Thoroughfare
channel
Capillaries
(exchange
vessels)
Precapillary
sphincter
Metarteriole
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
© 2013 Pearson Education, Inc.
Total blood flow during
strenuous exercise
17,500 ml/min
Organs and Blood Flow
• Autoregulation – automatic adjustments of
blood flow for tissue requirements
– Skeletal Muscles
– Brain
– Skin
– Lungs
– Heart
Physiology of Circulation
• Blood flow – the volume of blood flowing
through a vessel, an organ, or the entire
circulation in a given period of time (ml/min)
– Cardiac Output (CO)
• Blood pressure – the force per unit area
exerted on a vessel wall by the blood (mm Hg)
Blood Pressure
• Usually refers to systemic arterial blood
pressure in the largest arteries near the heart
• Differences in BP within vasculature is the
driving force for blood flow
Resistance
• the opposition of blood flow, a measure of the
amount of friction passing through vessels
– peripheral resistance
• Factors for resistance
– Blood viscosity
– Total blood vessel length
– Blood vessel diameter
Blood Viscosity
• the thickness of a fluid
– greater the viscosity, harder to flow
• Major determinate is number of RBC’s
• Usually unchanging under normal condition
Total Blood Vessel Length
• Longer the vessel – greater the resistance
• Vessel length generally considered a constant
in health adults
Blood Vessel Diameter
• Greatest influence
on resistance
PR = 1/r4
r=1, PR=1
r=2, PR=1/16
Blood vessel diameter and resistance
• Large arteries close to the heart contribute
little to peripheral resistance
• Small-diameter arterioles are major
determinants of peripheral resistance
– these can enlarge or restrict in repsonse to neural
and chemical controls
Blood Pressure
• Blood flow along a pressure gradient from
higher to lower pressure areas
• Pumping action of the heart generates blood
flow
• Pressure is flow opposed by resistance
Figure 19.6 Blood pressure in various blood vessels of the systemic circulation.
120
Systolic pressure
100
Mean pressure
80
60
40
Diastolic
pressure
20
0
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Blood Pressure
• Systolic pressure – pressure generated by
ventricular contraction
• Diastolic pressure – the remaining pressure
after aortic valve closure
• Pulse Pressure = SP - DP
Mean Arterial Pressure
• Flucuation of blood pressure due to heart
beating, an weighted average is measured
• MAP – the pressure that drives the blood to
the tissues
MAP = diastolic pressure + 1/3 pulse pressure
Figure 19.6 Blood pressure in various blood vessels of the systemic circulation.
120
Systolic pressure
100
Mean pressure
80
60
40
Diastolic
pressure
20
0
© 2013 Pearson Education, Inc.
Capillaries Blood Pressure
• Not too high, not too low!
• Capillaries are porous, too much pressure
would burst them open.
• Pressure high enough that force will propel
solutes out, and into interstitial space
Veins and low BP
• Requires help
• The muscular pump
• The respiratory pump
• Sympathetic venoconstriction
The Muscular Pump
• contraction of skeletal muscles "milks" blood
toward heart; valves prevent backflow
Figure 19.7 The muscular pump.
Venous valve (open)
Contracted skeletal
muscle
Venous valve
(closed)
Vein
Direction of blood flow
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The Respiratory Pump
• pressure changes during breathing move
blood toward heart by squeezing abdominal
veins as thoracic veins expand
Sympathetic Venoconstriction
• under sympathetic control, constriction
pushes blood toward heart
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)
© 2013 Pearson Education, Inc.
50
40
30
20
10
0
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)
Diffusion and Flow Direction
• Diffusion down concentration gradients
– O2 and nutrients from blood to tissues
– CO2 and metabolic wastes from tissues to blood
• Fluid leaves capillaries at arterial end; most
returns to blood at venous end
Hydrostatic and Colloid Osmotic
Pressure
• Direction and amount of fluid flow depend on
two opposing forces:
• Hydrostatic
• Colloid osmotic pressures
• Movement out of capillaries is called filtration
Capillary Hydrostatic Pressure
• The force exerted by a fluid pressing against a
wall
• 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 constantly withdraw it
Colloid Osmotic Pressure
• the force opposing hydrostatic pressure
• Created by non-diffusable molecules
– draw water towards themselves, encouraging
osmosis
– occurs in both sides
Hydrostatic-Osmotic Pressure
Interaction
• Hydrostatic pressure pushes, Osmotic
pressure pulls
• Net filtration pressure (NFP) considers all the
forces acting on a capillary bed
• NPF > 0 at arteriolar end
• NPF < 0 at venous end
Net Filtration Pressure
• NFP = (HPc + OPif) – (HPif + OPc)
• NPF > 0 at arteriolar end
• NPF < 0 at venous end
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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.
Role of Lymphatic Vessels
• Net loss of fluid from circulation system
– more fluid enters body than returns
• Lymphatic vessels absorb this fluid and return
it to circulation system (Thursday’s lecture)
– without it we’d empty our blood vessels in 24 hrs!
Maintaining Blood Pressure
• Cooperation of heart, blood vessels, kidneys,
and supervision by brain
• Main factors influencing blood pressure
– Cardiac output (CO)
– Peripheral resistance (PR)
– Blood volume
Let’s do some math!
Blood flow (F) = P/R
Blood flow directly correlates with CO
CO = P/R
P = CO x R
P = (HRxSV) x R
• Homeostasis dictates that changes in one variable
quickly compensated for by changes in other
variables
• Resting heart rate maintained by cardioinhibitory
center via parasympathetic vagus nerves
• Stroke volume controlled by venous return (EDV)
Figure 19.8 Major factors enhancing cardiac output.
Exercise
BP activates cardiac centers in medulla
Activity of respiratory pump
(ventral body cavity pressure)
Sympathetic activity
Parasympathetic activity
Activity of muscular pump
(skeletal muscles)
Epinephrine in blood
Sympathetic venoconstriction
Venous return
Contractility of cardiac muscle
ESV
EDV
Stroke volume (SV)
Heart rate (HR)
Initial stimulus
Physiological response
Result
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Cardiac output (CO = SV x HR)
Neural Control
• Neural controls of peripheral resistance
– Maintain MAP by altering blood vessel diameter
• If low blood volume, all vessels constricted
except those to heart and brain
• Alter blood distribution to organs in response
to specific demands
– capillary sphinchter muscles
Cardiovascular Center
• medulla oblongata control blood pressure by
altering CO and blood vessel diameter
– cardio-acceleratory center
– cardio-inhibitory center
• Arterioles are in a state of constant, moderate
state of constriction
– vasomotor tone
– varies from organ to organ
Regulation of Cardiovascular Center
Activity
• Baroreceptors – pressure-sensitive
mechanoreceptors that respond to changes in
arterial pressure and stretch
– temporary changes but can adapt to higher BP
• Chemoreceptors – respond to changes in CO2,
H+, O2
Baroreceptors
• When active, downregulates vasomotor and
cardiac center of brain:
– Arteriolar vasodilation
– Venodilation
• Result:
– Decrease in blood pressure
– Decreased CO
Chemoreceptors
• Increases in CO2
• Decrease in pH (Increase in H+)
• Decrease in O2
• Activate cardiocenters and vasomotor
– Increase blood pressure via vasoconstriction
– Increase CO
Allows for Local Regulation of Blood
Flow
• Metabolic Control
– NO acts as a vasodialator
– endothelins acts as vasoconstrictors
• Myogenic Control
– vascular smooth muscle responds to changes in
vascular pressure
Hormonal Controls
• Short term regulation via changes in
peripheral resistance
• Long term regulation via changes in blood
volume
– renal mechanisms
Figure 19.10 Direct and indirect (hormonal) mechanisms for renal control of blood pressure.
Direct renal mechanism
Arterial pressure
Indirect renal mechanism (renin-angiotensin-aldosterone)
Initial stimulus
Arterial pressure
Physiological response
Result
Inhibits baroreceptors
Sympathetic nervous
system activity
Filtration by kidneys
Angiotensinogen
Renin release
from kidneys
Angiotensin I
Angiotensin converting
enzyme (ACE)
Angiotensin II
Urine formation
Adrenal cortex
ADH release by
posterior pituitary
Thirst via
hypothalamus
Secretes
Aldosterone
Blood volume
Sodium reabsorption
by kidneys
Water reabsorption
by kidneys
Water intake
Blood volume
Mean arterial pressure
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Mean arterial pressure
Vasoconstriction;
peripheral resistance
Figure 19.11 Factors that increase MAP.
Activity of
muscular
pump and
respiratory
pump
Release
of ANP
Fluid loss from
hemorrhage,
excessive
sweating
Crisis stressors:
exercise, trauma,
body
temperature
Conservation
of Na+ and
water by kidneys
Blood volume
Blood pressure
Blood pH
O2
CO2
Blood
volume
Baroreceptors
Chemoreceptors
Venous
return
Activation of vasomotor and cardioacceleratory centers in brain stem
Stroke
volume
Heart
rate
Vasomotor tone;
bloodborne
chemicals
(epinephrine,
NE, ADH,
angiotensin II)
Diameter of
blood vessels
Cardiac output
Physiological response
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Blood
viscosity
Body size
Blood vessel
length
Peripheral resistance
Initial stimulus
Result
Dehydration,
high hematocrit
Mean arterial pressure (MAP)
Problems with Blood Pressure
• Hypertension
– Primary (90%): Genetic and Lifestyle “choices”
– Secondary (10%): renal, kidney, hormone
syndromes
• Hypotension
– Acute: due to circulatory shock
– Orthostatis: temporary due to quick movement
– Chronic: sign of serious underlying conditions
Circulatory Shock
• Any condition where blood vessels are
inadequately filled and blood cannot circulate
• Hypovolemic
– blood or fluid losss
• Vascular
– drop in blood pressure
• Cardiogenic
– pump failure
Figure 19.21b Major arteries of the systemic circulation.
Arteries of the head and trunk
Internal carotid artery
External carotid artery
Common carotid arteries
Vertebral artery
Subclavian artery
Brachiocephalic trunk
Arteries that supply the upper limb
Subclavian artery
Axillary artery
Brachial artery
Aortic arch
Ascending aorta
Coronary artery
Celiac trunk
Abdominal aorta
Superior mesenteric
artery
Renal artery
Gonadal artery
Radial artery
Ulnar artery
Deep palmar arch
Superficial palmar arch
Digital arteries
Arteries that supply the lower
limb
External iliac artery
Inferior mesenteric artery
Femoral artery
Common iliac artery
Popliteal artery
Internal iliac artery
Anterior tibial artery
Posterior tibial artery
Illustration, anterior view
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Arcuate artery
Figure 19.26b Major veins of the systemic circulation.
Veins of the head and trunk
Dural venous sinuses
External jugular vein
Vertebral vein
Internal jugular vein
Right and left
brachiocephalic veins
Superior vena cava
Great cardiac vein
Hepatic veins
Splenic vein
Hepatic portal vein
Renal vein
Superior mesenteric vein
Inferior mesenteric vein
Inferior vena cava
Common iliac vein
Internal iliac vein
Veins that drain
the upper limb
Subclavian vein
Axillary vein
Cephalic vein
Brachial vein
Basilic vein
Median cubital vein
Ulnar vein
Radial vein
Digital veins
Veins that drain
the lower limb
External iliac vein
Femoral vein
Great saphenous vein
Popliteal vein
Posterior tibial vein
Anterior tibial vein
Small saphenous vein
Dorsal venous arch
Dorsal metatarsal veins
Illustration,
anterior view. The vessels of the pulmonary circulation are not shown.
© 2013 Pearson
Education, Inc.
Lab Exercises 32/33
• Exercise 32 – Anatomy of blood vessels
– use as practice to learn the major arteries and
veins
• Exercise 33 – Cardiovascular Physiology
– Activites 1 and 5,
– 6 and 7 (optional)

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