Chapter 20–
Blood Vessels &
Circulation
20-1
Ch. 20 (Blood vessels) Study Guide
1. Critically read Chapter 20 pp. 756-774 right
before 20.4 “Venous return and circulatory
shock” section. Also read Table 20.3 (p.782)
and Insight 20.5 (p.808) in the textbook.
2. Comprehend Terminology (those in bold)
3. Study-- Figure questions, Think About It
questions, and Before You Go On (sectionending) questions
4. Do Testing Your Recall— 1-8, 10-12, 14-16, 18
5. Do True or False– 1-2, 4-5, 8-9
6. Do Testing Your Comprehension-- #5
2
§ 20.1—General
Anatomy of Blood
Vessels
20-3
§ Introduction of Blood Vessels
1A. Closed circulatory system– Def. Blood flows
in a continuous circuit through the body under
pressure generated by the heart.
– 1B. Open circulatory system-- In what
animals?
2. Three principal categories of blood vessels:
– Arteries: efferent vessels
– Capillaries:
– Veins: afferent vessels
Fig. 20.x
20-4
20-5
§ Vessel wall of arteries/veins-1
1. Innermost layer (tunica interna/intima)
A. Structures: lines the inside of the vessel and is
exposed to the blood; consists of-• Endothelial cells– histology?
• Basement membrane
• Connective tissue (sparse)
B. Functions of the endothelial cells—
• Selectively permeable barrier
• Secrets chemicals--?
• Repels blood cells and platelets
Fig. x
20-6
The vessel wall
1
2
Next
slide
3
20-7
§ Vessel wall of arteries/veins-2
2. Middle layer (tunica media)—
thickest layer
A.Structures:
– Smooth muscle cells-– Collagen fibers
– Elastic fibers (in arteries)
B. Functions of this layer:
– Strengthen the vessel
– Provide vasomotion--?
20-8
§ Vessel wall of arteries/veins-3
3. Outermost layer (tunica externa or
advertitia)—
A. Structures:
– Largely loose connective tissue (collagen
fibers)
B. Functions:
– Protection & anchoring
– Provide passage for-• Vasa vasorum— vessels of the vessels
Fig. 20.2
20-9
Conducting (large) artery
Lumen
Tunica interna:
Endothelium
Basement
membrane
Tunica media
Tunica externa
Vasa
vasorum
Nerve
20-10
§ Arteries
1. More muscular
2. Able to resist high blood pressure
– Thus called resistance vessels
3. Retain their round shape even when
empty
4. Divided into three categories by size
(next slide)
20-11
§ Categories of Arteries-1
1. Conducting (elastic/large) arteries - largest
– Ex. aorta, common carotid, subclavian,
common iliac, and pulmonary trunk (Fig. 20.23)
– Structure– (Slide #10)
• tunica media-- 40-70 layers of smooth
muscle alternating with elastic tissue
• Internal/external elastic lamina— not obvious
• tunica externa– vasa vasorum
– Function-• Able to expand/recoil-• But not so in atherosclerosis– aneurysms
and rupture (Slides #15-16)
20-12
ID—A--The aorta and
its major branches
(Table 20.3)
20-13
R. common
carotid a.
L. common
carotid a.
R. subclavian a.
L. subclavian a.
Brachiocephalic trunk
Aortic arch
Ascending
aorta
Descending
aorta, thoracic
(posterior to
heart)
Diaphragm
Aortic hiatus
Descending
aorta,
abdominal
20-14
Aneurysm (read p. 758 box)
1. Def.– a balloon-like outpocketing of an
artery wall (Fig. Y)
2. Risk– for rupture, most often reflects
gradual weakening of the artery
3. Causes– OFTEN chronic hypertension or
atherosclerosis
4. Common sites– abdominal aorta, renal
arteries, and the arterial circle at base of
brain
Fig. Y
20-15
20-16
§ Categories of Arteries-2
2. Distributing (muscular, medium) arteries
– Distribute blood to specific organs
– Ex. brachial, femoral, renal, and splenic
arteries etc.
– Structure-• tunica media– up to 40 layers of smooth
muscle
• Internal/external elastic lamina—
conspicuous/not conspicuous (circle
one)
Fig. 20.34, 29, 30, 36
20-17
§ Categories of Arteries-3
3. Resistance (small) arteries
– Up to 25 layers of smooth muscle
– Elastic tissue little
– ARTERIOLES (smallest of these); 1-3 smooth
m. layers
• Empty blood into capillaries through
____________________
• Here individual muscle cells form a
precapillary sphincter encircling the
entrance to capillary; function?
Fig. 20.3
20-18
1
2
2a
3
4
20-19
§ Arterial Sense Organs (3 kinds)
• Where– structures in major arteries above heart
• Function– to monitor blood pressure/chemistry
Three kinds (2 categories): Fig. 20.4
1. Carotid sinuses (Baroreceptors)—Details next
– Location-- in walls of ascending aorta etc.
– monitors BP – a rise in BP signals brainstem . . .
2. Carotid bodies (Chemoreceptors)
– Location-- oval bodies near carotids
– monitor blood chemistry
• adjust respiratory rate to stabilize pH, CO2, and O2
3. Aortic bodies (Chemoreceptors)
– Location-- in walls of aortic arch
– same function as carotid bodies
20-20
2

Figure 20.4
 To brain
1C
To the face
3
1A+B
20-21
§ Capillaries
1. Material exchanges– between blood and
tissue fluids
– Locations-- _____________ and smallest of
the venules
2. Structure– endothelium + ____________
– Fig. X next
3. Close vicinity to all cells— Exceptions
– Scarce in: tendons, ligaments, & cartilage
– Absent from (3 locations): __________________________(Epi. & Eyes)
20-22
20-23
§ 3 Types of Capillaries
1. Continuous capillaries- occur in most
tissues, ex. Skeletal muscle
– endothelial cells have tight junctions
with intercellular clefts (allow passage
of solutes)
– What molecules can pass– ex. glucose
– What molecules can not– protein,
formed elements of the blood
– Fig. 20.5
20-24
Continuous capillary
20-25
§ 3 Types of Capillaries
2. Fenestrated capillaries
– Structure – have _____________ on
endothelial cells
– filtration pores – spanned by very thin
glycoprotein layer - allows passage of
molecules such as _____________
– Locations-- organs that require rapid
absorption or filtration - kidneys, small
intestine etc.
– Fig. 20.6 a and b
20-26
Fenestrated Capillary
20-27
20.6b--Surface view of a
fenestrated endothelial cell
20-28
§ 3 Types of Capillaries
3. Sinusoids (discontinuous) capillaries– Structure– endothelial cells separated
by wide gaps; no basal lamina
– Conform to the shape of the
surrounding tissue
– Molecules can pass– proteins and
blood cells
– Locations-- liver, bone marrow, spleen,
lymphatic organs
– Fig. 20.7
20-29
Sinusoid
in Liver
20-30
§ Veins (capacitance vessels; why?)
1. b/c Greater capacity for blood
containment than arteries do (Fig.
20.8)
2. thinner walls—due to less muscular
and elastic tissue; why?
3. lower blood pressure: 10 mm Hg
with little fluctuation
4. ____________ aid skeletal muscles
in upward blood flow
20-31
20-32
§ Types of veins-- Smallest to
largest vessels (A)
1. Postcapillary venules-- only tunica intima
– Receive blood from capillaries
– more porous than capillaries
2. Muscular venules-- receive blood from #1
– have tunica media (1-2 layers of smooth
muscle) + thin tunica externa
3. Medium veins–
– Most have individual names, Examples-radius or ulna veins
– Many have venous valves
20-33
§ Types of veins-- Smallest to
largest vessels (B)
4. Venous sinuses-– veins with thin walls, large lumens, no smooth
muscle; vasomotion– yes/no? (Circle one)
– Examples– coronary sinus of the heart and the
dural sinuses of the brain
5. Large veins-– Greater than 10 mm (diameters)
– Venae cavae, pulmonary veins, internal jugular
veins
20-34
§ Circulatory
Routes
1. Most common route
– heart  arteries 
arterioles  capillaries
 venules  veins
2. Portal system
– blood flows through
two consecutive
capillary networks
before returning to
heart
– 3 places in human
body–
20-35
B. Hepatic portal sys. (p.797)
Figure 20.38b
4
3
5
2
1
20-36
§ 3 Anastomoses
Def. Point where 2 blood
vessels merge
3. Arteriovenous shunt
– artery flows directly into
vein; fingers etc.
4. Venous anastomosis
– most common type
– alternate drainage of
organs; Fig. 20.33
5. Arterial anastomosis
– Two arteries merge
– collateral circulation
20-37
(coronary); Fig. 20.31
What type of circulatory route does
inferior/superior mesenteric vein belong?
§ 20.2— Blood
Pressure, Resistance,
and Flow
20-39
§ Blood pressure, resistance, and flow
• Importance– deliver oxygen and nutrients and
to remove wastes at a rate keeps pace with
tissue metabolism
• Blood flow (F)– is the amount of blood flowing
through an organ, tissue, or blood vessel in a
given time
• Hemodynamics: Blood Flow (F) = ΔP/R
– Where ΔP is the pressure difference and R is the
resistance
20-40
§ Blood Pressure
1. Blood pressure (BP)– Def. the force
per unit area exerted by the blood
against a vessel wall
2. In what vessels can you find BP?
Figure 20.10 has the answer
20-41
Why ?
Why?
20-42
§ Blood Pressure
3.BP is understood to mean the
pressure in the _________________
4.BP rises and falls in a pulsatile
fashion in the arteries and arterioles
Figure Z (what BP do we measure?)
43
Four different kinds of arterial BP-A--?
C.
D. Mean
arterial
pressure
B--?
20-44
§ Blood Pressure
5.Systolic P.– the maximum p.
exerted in the arteries when blood
is ejected into them during ventricular
ejection, averages 120 mm Hg (Mercury)
• Physiology– during ventricular systole,
a volume of blood enters the arteries
from the ventricle. How much actually
moves to the arterioles?
• Status of the semilunar valves in this
particular cardiac cycle? (open or close)
45
§ Blood Pressure
6.Diastolic P.– the arterial p. when blood
is draining off into the arterioles
during diastole, averages ________
Hg. Lowest during cardiac cycle.
• Physiology– during ventricular diastole,
the semilunar valves close, no blood
enters the arteries but the arteries
moves the blood forward. Why?
46
§ Blood Pressure
7.Pulse P.– is the difference between
systolic and diastolic pressure
8.The Mean Arterial P. (MAP)— is the
average blood pressure throughout the
cardiac cycle
• is monitored and regulated by BP
reflexes
• MAP = diastolic p. + 1/3 pulse p.
Figure Z
47
Fig.-- Aortic pressure throughout the cardiac cycle
Systolic pressure
Mean arterial
pressure
A.
Diastolic pressure
20-48
Q.—Peter’s systolic pressure
is 140 mm Hg and his diastolic
pressure is 95 mm Hg (written
140/95).
A) What is his systolic p. and
diastolic p., respectively?
B) His pulse pressure?
C) His mean arterial pressure?
49
§ Hypertension/hypotension
Def.– high blood pressure; a chronic resting
blood pressure higher than 140/90-(hypertension)
Results– aneurysms, atherosclerosis, heart
failure, stroke, etc.
Hypotension– a chronic low resting BP
(90/50 or lower);
Causes– blood loss, dehydration, anemia,
in people approaching death
50
§ Peripheral Resistance
• Resistance depends on three variables
below: (Note: Blood Flow = ΔP/Resistance)
1. Blood viscosity inversely relates to blood flow—
– Anemia & hypoproteinemia -- ___ blood flow
– Polycythemia & dehydration -- ___ blood flow
2. Vessel length– pressure and flow decline with
distance (farther end of the vessel)
The above two variables usually quite stable
3. Vessel radius on blood flow— proportional to
the fourth power of radius
– Blood Flow α radius4
20-51
Table 20.2
Table 20.2
Blood Flow
20-52
§ Blood pressure regulation (A)
1. Neural control–
– Baroreflex autonomic regulation-•
•
BP increases –baroreceptors firing rate
increases
Figure 20.13
– Chemoreflex– response to changes in
blood chemistry
– Medullary ischemic reflex– an automatic
response to a drop in perfusion of the brain
20-53
Figure 20.13
Brain stem
20-54
§ Blood pressure regulation (B)
2. Hormonal control-a) Angiotensin II-- ↑ BP
b) Aldosterone– ↑ BP
c) Atrial natriuretic peptide-- ↓ BP
d) Antidiruetic hormone-- ↑ BP
e) Epinephrine and Norepinephrine-- ↑ BP
20-55
Understanding blood pressure
1. Blood flow= Δ Blood Pressure/Resistance
–  BP = Blood Flow x Resistance (R)
–  BP = Blood Flow x 1/(Radius)4
2. Why vasodilation causes resistance to decrease?
3. BP = Blood Flow x R = Cardiac output x R
–  BP = Heart rate (beats/min) x stroke volume
(ml/beat) x R
– Thus, heart rate and stroke volume impact BP
Fig. 20.13 again
20-56
Figure 20.13


Brain stem
20-57
§ 20.3 Capillary exchange routes
1. Diffusion: (1a, 1b, and 1c of Fig. 20.16)
– From high to low conc. areas
– Via endothelial cells, intercellular clefts, and
filtration pores
2. Transcytosis: (2 of Fig. 20.16)
– Pinocytosis/endocytosis then exocytosis
– Fatty acids, albumin, insulin etc.
Fig. 20.16
20-58
1c
Albumin
Fatty acids,
albumin,
insulin
2
O2, CO2,
steroids
1a
Glucose
ions
1b
20-59
§ Filtration (arterial end) and Reabsorption
(venous end) of the capillary
1. Hydrostatic pressure– due to liquid
– Mainly caused by the blood pressure
– 30 mm Hg at arterial end and 10 mm Hg at the
venous end
2. Colloid osmotic pressure– due to
protein
– Mostly by albumin etc.
Difference of 1-2 above is Net Filtration or
Reabsorption Pressure
Fig. 20.17
20-60
Net
filtration
p.
Net
reabsorption
p.
20-61
§ Edema (pulmonary, cerebral, etc.)
1. Def.– accumulation of fluid in a tissue.
2. Three causes:
– Increased capillary filtration: hypertension
etc.
– Reduced capillary reabsorption: due to
albumin-- hypoproteinemia
– Obstructed lymphatic drainage
3. Edema’s consequences:
– Oxygen delivery/waste removal are impaired
– Tissue death (necrosis)
20-62
When available and time allows
• Watch a video— Baroreceptor reflex control
of blood pressure
• Watch a video— Fluid exchange across the
capillary
20-63