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Chapter 19
The
Cardiovascular
system: Blood
Vessels
Blood Vessels
• Delivery system of dynamic structures that
begins and ends at the heart
– Arteries: carry blood away from the heart;
oxygenated except for pulmonary circulation and
umbilical vessels of a fetus
– Capillaries: contact tissue cells and directly serve
cellular needs
– Veins: carry blood toward the heart
Venous system
Large veins
(capacitance
vessels)
Small veins
(capacitance
vessels)
Postcapillary
venule
Thoroughfare
channel
Arterial system
Heart
Large
lymphatic
vessels
Lymph
node
Lymphatic
system
Arteriovenous
anastomosis
Elastic arteries
(conducting
vessels)
Muscular arteries
(distributing
vessels)
Lymphatic
Sinusoid
capillary
Arterioles
(resistance vessels)
Terminal arteriole
Metarteriole
Precapillary sphincter
Capillaries
(exchange vessels)
Figure 19.2
Structure of Blood Vessel Walls
• Arteries and veins
– Tunica intima, tunica media, and tunica externa
• Lumen
– Central blood-containing space
• Capillaries
– Endothelium with sparse basal lamina
Tunica intima
• Endothelium
• Subendothelial layer
Internal elastic lamina
Tunica media
(smooth muscle and
elastic fibers)
External elastic lamina
Tunica externa
(collagen fibers)
Lumen
Artery
(b)
Capillary
network
Valve
Lumen
Vein
Basement membrane
Endothelial cells
Capillary
Figure 19.1b
Capillaries
• Microscopic blood vessels
• Walls of thin tunica intima, one cell thick
• Pericytes help stabilize their walls and control
permeability
• Size allows only a single RBC to pass at a time
• In all tissues except for cartilage, epithelia, cornea
and lens of eye
• Functions: exchange of gases, nutrients, wastes,
hormones, etc.
Capillaries
•
Three structural types
1. Continuous capillaries
2. Fenestrated capillaries
3. Sinusoidal capillaries (sinusoids)
Continuous Capillaries
• Abundant in the skin and muscles
– Tight junctions connect endothelial cells
– Intercellular clefts allow the passage of fluids and
small solutes
• Continuous capillaries of the brain
– Tight junctions are complete, forming the bloodbrain barrier
Pericyte
Red blood
cell in lumen
Intercellular
cleft
Endothelial
cell
Basement
membrane
Tight junction
Pinocytotic
Endothelial
vesicles
nucleus
(a) Continuous capillary. Least permeable, and
most common (e.g., skin, muscle).
Figure 19.3a
Fenestrated Capillaries
• Some endothelial cells contain pores
(fenestrations)
• More permeable than continuous capillaries
• Function in absorption or filtrate formation
(small intestines, endocrine glands, and
kidneys)
Pinocytotic
vesicles
Red blood
cell in lumen
Fenestrations
(pores)
Endothelial
nucleus
Intercellular
cleft
Basement membrane
Tight junction
Endothelial
cell
(b) Fenestrated capillary. Large fenestrations
(pores) increase permeability. Occurs in special
locations (e.g., kidney, small intestine).
Figure 19.3b
Sinusoidal Capillaries
• Fewer tight junctions, larger intercellular
clefts, large lumens
• Usually fenestrated
• Allow large molecules and blood cells to pass
between the blood and surrounding tissues
• Found in the liver, bone marrow, spleen
Endothelial
cell
Red blood
cell in lumen
Large
intercellular
cleft
Tight junction
Nucleus of
Incomplete
endothelial
basement
cell
membrane
(c) Sinusoidal capillary. Most permeable. Occurs in
special locations (e.g., liver, bone marrow, spleen).
Figure 19.3c
Capillary Beds
•
Interwoven networks of capillaries form the
microcirculation between arterioles and venules
• Consist of two types of vessels
1. Vascular shunt (metarteriole—thoroughfare
channel):
•
Directly connects the terminal arteriole and a
postcapillary venule
2. True capillaries
•
•
10 to 100 exchange vessels per capillary bed
Branch off the metarteriole or terminal arteriole
Blood Flow Through Capillary Beds
• Precapillary sphincters regulate blood flow
into true capillaries
• Regulated by local chemical conditions and
vasomotor nerves
Precapillary
sphincters
Vascular shunt
Metarteriole
Thoroughfare channel
True capillaries
Terminal arteriole
Postcapillary venule
(a) Sphincters open—blood flows through true capillaries.
Terminal arteriole
Postcapillary venule
(b) Sphincters closed—blood flows through metarteriole
thoroughfare channel and bypasses true capillaries.
Figure 19.4
Venules
• Formed when capillary beds unite
• Very porous; allow fluids and WBCs into
tissues
• Postcapillary venules consist of endothelium
and a few pericytes
• Larger venules have one or two layers of
smooth muscle cells
Veins
• Formed when venules converge
• Have thinner walls, larger lumens compared
with corresponding arteries
• Blood pressure is lower than in arteries
• Thin tunica media and a thick tunica externa
consisting of collagen fibers and elastic
networks
• Called capacitance vessels (blood reservoirs);
contain up to 65% of the blood supply
Vein
Artery
(a)
Figure 19.1a
Pulmonary blood
vessels 12%
Systemic arteries
and arterioles 15%
Heart 8%
Capillaries 5%
Systemic veins
and venules 60%
Figure 19.5
Veins
•
Adaptations that ensure return of blood to
the heart
1. Large-diameter lumens offer little resistance
2. Valves prevent backflow of blood
•
•
Most abundant in veins of the limbs
Venous sinuses: flattened veins with
extremely thin walls (e.g., coronary sinus of
the heart and dural sinuses of the brain)
Vascular Anastomoses
• Interconnections of blood vessels
• Arterial anastomoses provide alternate
pathways (collateral channels) to a given body
region
– Common at joints, in abdominal organs, brain, and
heart
• Vascular shunts of capillaries are examples of
arteriovenous anastomoses
• Venous anastomoses are common
Physiology of Circulation: Definition of
Terms
• Blood flow
– Volume of blood flowing through a vessel, an organ, or the
entire circulation in a given period
• Measured as ml/min
• Equivalent to cardiac output (CO) for entire vascular
system
• Relatively constant when at rest
• Varies widely through individual organs, based on
needs
Physiology of Circulation: Definition of
Terms
• Blood pressure (BP)
– Force per unit area exerted on the wall of a blood
vessel by the blood
• Expressed in mm Hg
• Measured as systemic arterial BP in large arteries near
the heart
– The pressure gradient provides the driving force
that keeps blood moving from higher to lower
pressure areas
Physiology of Circulation: Definition of
Terms
• Resistance (peripheral resistance)
– Opposition to flow
– Measure of the amount of friction blood encounters
– Generally encountered in the peripheral systemic
circulation
• Three important sources of resistance
– Blood viscosity
– Total blood vessel length
– Blood vessel diameter
Resistance
• Factors that remain relatively constant:
– Blood viscosity
• The “stickiness” of the blood due to formed elements
and plasma proteins
– Blood vessel length
• The longer the vessel, the greater the resistance
encountered
Resistance
• Frequent changes alter peripheral resistance
• Varies inversely to the fourth power of vessel
radius
– E.g., if the radius is doubled, the resistance is 1/16
as much
Resistance
• Small-diameter arterioles are the major
determinants of peripheral resistance
• Abrupt changes in diameter or fatty plaques
from atherosclerosis dramatically increase
resistance
– Disrupt laminar flow and cause turbulence
Relationship Between Blood Flow,
Blood Pressure, and Resistance
• Blood flow (F) is directly proportional to the
blood (hydrostatic) pressure gradient (P)
– If P increases, blood flow speeds up
• Blood flow is inversely proportional to
peripheral resistance (R)
– If R increases, blood flow decreases: F = P/R
• R is more important in influencing local blood
flow because it is easily changed by altering
blood vessel diameter
Systemic Blood Pressure
• The pumping action of the heart generates blood flow
• Pressure results when flow is opposed by resistance
• Systemic pressure
– Is highest in the aorta
– Declines throughout the pathway
– Is 0 mm Hg in the right atrium
• The steepest drop occurs in arterioles
Systolic pressure
Mean pressure
Diastolic
pressure
Figure 19.6
Arterial Blood Pressure
• Reflects two factors of the arteries close to the
heart
– Elasticity (compliance or distensibility)
– Volume of blood forced into them at any time
Arterial Blood Pressure
• Systolic pressure: pressure exerted during
ventricular contraction
• Diastolic pressure: lowest level of arterial
pressure
• Pulse pressure = difference between systolic
and diastolic pressure
Arterial Blood Pressure
• Mean arterial pressure (MAP): pressure that
propels the blood to the tissues
MAP = diastolic pressure + 1/3 pulse pressure
• Pulse pressure and MAP both decline with
increasing distance from the heart
Capillary Blood Pressure
• Ranges from 15 to 35 mm Hg
• Low capillary pressure is desirable
– High BP would rupture fragile, thin-walled
capillaries
– Most are very permeable, so low pressure forces
filtrate into interstitial spaces
Venous Blood Pressure
• Changes little during the cardiac cycle
• Small pressure gradient, about 15 mm Hg
• Low pressure due to cumulative effects of
peripheral resistance
Factors Aiding Venous Return
1. Respiratory “pump”: pressure changes created
during breathing move blood toward the heart by
squeezing abdominal veins as thoracic veins
expand
2. Muscular “pump”: contraction of skeletal
muscles “milk” blood toward the heart and
valves prevent backflow
3. Vasoconstriction of veins under sympathetic
control
Valve (open)
Contracted
skeletal
muscle
Valve (closed)
Vein
Direction of
blood flow
Figure 19.7
Maintaining Blood Pressure
• Requires
– Cooperation of the heart, blood vessels, and
kidneys
– Supervision by the brain
Maintaining Blood Pressure
• The main factors influencing blood pressure:
– Cardiac output (CO)
– Peripheral resistance (PR)
– Blood volume
Maintaining Blood Pressure
• Flow = P/PR and CO = P/PR
• Blood pressure = CO x PR (and CO depends on
blood volume)
• Blood pressure varies directly with CO, PR, and
blood volume
• Changes in one variable are quickly
compensated for by changes in the other
variables
Cardiac Output (CO)
• Determined by venous return and neural and
hormonal controls
• Resting heart rate is maintained by the
cardioinhibitory center via the
parasympathetic vagus nerves
• Stroke volume is controlled by venous return
(EDV)
Cardiac Output (CO)
• During stress, the cardioacceleratory center
increases heart rate and stroke volume via
sympathetic stimulation
– ESV decreases and MAP increases
Exercise
BP activates cardiac centers in medulla
Activity of respiratory pump
(ventral body cavity pressure)
Activity of muscular pump
(skeletal muscles)
Parasympathetic
activity
Sympathetic activity
Epinephrine in blood
Sympathetic venoconstriction
Venous return
Contractility of cardiac muscle
EDV
ESV
Stroke volume (SV)
Heart rate (HR)
Initial stimulus
Physiological response
Result
Cardiac output (CO = SV x HR
Figure 19.8
Control of Blood Pressure
• Short-term neural and hormonal controls
– Counteract fluctuations in blood pressure by
altering peripheral resistance
• Long-term renal regulation
– Counteracts fluctuations in blood pressure by
altering blood volume
Short-Term Mechanisms: Neural
Controls
• Neural controls of peripheral resistance
– Maintain MAP by altering blood vessel diameter
– Alter blood distribution in response to specific
demands
Short-Term Mechanisms: Neural
Controls
• Neural controls operate via reflex arcs that
involve
– Baroreceptors
– Vasomotor centers and vasomotor fibers
– Vascular smooth muscle
The Vasomotor Center
• A cluster of sympathetic neurons in the
medulla that oversee changes in blood vessel
diameter
• Maintains vasomotor tone (moderate
constriction of arterioles)
• Receives inputs from baroreceptors,
chemoreceptors, and higher brain centers
Short-Term Mechanisms:
Baroreceptor-Initiated Reflexes
• Baroreceptors are located in
– Carotid sinuses
– Aortic arch
– Walls of large arteries of the neck and thorax
Short-Term Mechanisms:
Baroreceptor-Initiated Reflexes
• Increased blood pressure stimulates
baroreceptors to increase input to the
vasomotor center
– Inhibits the vasomotor center, causing arteriole
dilation and venodilation
– Stimulates the cardioinhibitory center
3 Impulses from baroreceptors
stimulate cardioinhibitory center
(and inhibit cardioacceleratory
center) and inhibit vasomotor
center.
4a Sympathetic
impulses to heart
cause HR,
contractility, and
CO.
2 Baroreceptors
in carotid sinuses
and aortic arch
are stimulated.
4b Rate of
vasomotor impulses
allows vasodilation,
causing R
1 Stimulus:
Blood pressure
(arterial blood
pressure rises above
normal range).
Homeostasis: Blood pressure in normal range
5
CO and R
return blood
pressure to
homeostatic range.
1 Stimulus:
5
CO and R
return blood pressure
to homeostatic range.
Blood pressure
(arterial blood
pressure falls below
normal range).
4b Vasomotor
fibers stimulate
vasoconstriction,
causing R
2 Baroreceptors
in carotid sinuses
and aortic arch
are inhibited.
4a Sympathetic
impulses to heart
cause HR,
contractility, and
CO.
3 Impulses from baroreceptors stimulate
cardioacceleratory center (and inhibit cardioinhibitory
center) and stimulate vasomotor center.
Figure 19.9
Short-Term Mechanisms:
Baroreceptor-Initiated Reflexes
• Baroreceptors taking part in the carotid sinus
reflex protect the blood supply to the brain
• Baroreceptors taking part in the aortic reflex
help maintain adequate blood pressure in the
systemic circuit
Short-Term Mechanisms:
Chemoreceptor-Initiated Reflexes
• Chemoreceptors are located in the
– Carotid sinus
– Aortic arch
– Large arteries of the neck
Short-Term Mechanisms:
Chemoreceptor-Initiated Reflexes
• Chemoreceptors respond to rise in CO2, drop
in pH or O2
– Increase blood pressure via the vasomotor center
and the cardioacceleratory center
• Are more important in the regulation of
respiratory rate (Chapter 22)
Influence of Higher Brain Centers
• Reflexes that regulate BP are integrated in the
medulla
• Higher brain centers (cortex and
hypothalamus) can modify BP via relays to
medullary centers
Short-Term Mechanisms: Hormonal
Controls
• Adrenal medulla hormones norepinephrine
(NE) and epinephrine cause generalized
vasoconstriction and increase cardiac output
• Angiotensin II, generated by kidney release of
renin, causes vasoconstriction
Short-Term Mechanisms: Hormonal
Controls
• Atrial natriuretic peptide causes blood volume
and blood pressure to decline, causes
generalized vasodilation
• Antidiuretic hormone (ADH)(vasopressin)
causes intense vasoconstriction in cases of
extremely low BP
Long-Term Mechanisms: Renal
Regulation
•
•
•
Baroreceptors quickly adapt to chronic high
or low BP
Long-term mechanisms step in to control BP
by altering blood volume
Kidneys act directly and indirectly to regulate
arterial blood pressure
1. Direct renal mechanism
2. Indirect renal (renin-angiotensin) mechanism
Direct Renal Mechanism
• Alters blood volume independently of
hormones
– Increased BP or blood volume causes the kidneys
to eliminate more urine, thus reducing BP
– Decreased BP or blood volume causes the kidneys
to conserve water, and BP rises
Indirect Mechanism
• The renin-angiotensin mechanism
–  Arterial blood pressure  release of renin
– Renin production of angiotensin II
– Angiotensin II is a potent vasoconstrictor
– Angiotensin II  aldosterone secretion
• Aldosterone  renal reabsorption of Na+ and  urine
formation
– Angiotensin II stimulates ADH release
Arterial pressure
Indirect renal
mechanism (hormonal)
Direct renal
mechanism
Baroreceptors
Sympathetic stimulation
promotes renin release
Kidney
Renin release
catalyzes cascade,
resulting in formation of
Angiotensin II
Filtration
ADH release
by posterior
pituitary
Aldosterone
secretion by
adrenal cortex
Water
reabsorption
by kidneys
Sodium
reabsorption
by kidneys
Blood volume
Vasoconstriction
( diameter of blood vessels)
Initial stimulus
Physiological response
Result
Arterial pressure
Figure 19.10
Activity of
muscular
pump and
respiratory
pump
Release
of ANP
Fluid loss from Crisis stressors:
hemorrhage, exercise, trauma,
excessive
body
sweating
temperature
Conservation
of Na+ and
water by kidney
Blood volume
Blood pressure
Blood pH, O2,
CO2
Blood
volume
Baroreceptors
Chemoreceptors
Venous
return
Stroke
volume
Bloodborne
Dehydration,
chemicals:
high hematocrit
epinephrine,
NE, ADH,
angiotensin II;
ANP release
Body size
Activation of vasomotor and cardiac
acceleration centers in brain stem
Heart
rate
Cardiac output
Diameter of
blood vessels
Blood
viscosity
Blood vessel
length
Peripheral resistance
Initial stimulus
Physiological response
Result
Mean systemic arterial blood pressure
Figure 19.11
Monitoring Circulatory Efficiency
• Vital signs: pulse and blood pressure, along
with respiratory rate and body temperature
• Pulse: pressure wave caused by the expansion
and recoil of arteries
• Radial pulse (taken at the wrist) routinely used
Superficial temporal
artery
Facial artery
Common carotid
artery
Brachial artery
Radial artery
Femoral artery
Popliteal artery
Posterior tibial
artery
Dorsalis pedis
artery
Figure 19.12
Measuring Blood Pressure
• Systemic arterial BP
– Measured indirectly by the auscultatory method
using a sphygmomanometer
– Pressure is increased in the cuff until it exceeds
systolic pressure in the brachial artery
Measuring Blood Pressure
• Pressure is released slowly and the examiner
listens for sounds of Korotkoff with a
stethoscope
• Sounds first occur as blood starts to spurt
through the artery (systolic pressure, normally
110–140 mm Hg)
• Sounds disappear when the artery is no longer
constricted and blood is flowing freely
(diastolic pressure, normally 70–80 mm Hg)
Variations in Blood Pressure
• Blood pressure cycles over a 24-hour period
• BP peaks in the morning due to levels of
hormones
• Age, sex, weight, race, mood, and posture may
vary BP
Alterations in Blood Pressure
• Hypotension: low blood pressure
– Systolic pressure below 100 mm Hg
– Often associated with long life and lack of
cardiovascular illness
Homeostatic Imbalance: Hypotension
• Orthostatic hypotension: temporary low BP
and dizziness when suddenly rising from a
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
Alterations in Blood Pressure
• Hypertension: high blood pressure
– Sustained elevated arterial pressure of 140/90 or
higher
• May be transient adaptations during fever, physical
exertion, and emotional upset
• Often persistent in obese people
Homeostatic Imbalance: Hypertension
• Prolonged hypertension is a major cause of
heart failure, vascular disease, renal failure,
and stroke
• Primary or essential hypertension
– 90% of hypertensive conditions
– Due to several risk factors including heredity, diet,
obesity, age, stress, diabetes mellitus, and
smoking
Homeostatic Imbalance: Hypertension
• Secondary hypertension is less common
– Due to identifiable disorders, including kidney
disease, arteriosclerosis, and endocrine disorders
such as hyperthyroidism and Cushing’s syndrome
Blood Flow Through Body Tissues
• Blood flow (tissue perfusion) is 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 the right amount to
provide for proper function
Brain
Heart
Skeletal
muscles
Skin
Kidney
Abdomen
Other
Total blood
flow at rest
5800 ml/min
Total blood flow during strenuous
exercise 17,500 ml/min
Figure 19.13
Velocity of Blood Flow
• Changes as it travels through the systemic
circulation
• Is inversely related to the total cross-sectional
area
• Is fastest in the aorta, slowest in the
capillaries, increases again in veins
• Slow capillary flow allows adequate time for
exchange between blood and tissues
Relative crosssectional area of
different vessels
of the vascular bed
Total area
(cm2) of the
vascular
bed
Velocity of
blood flow
(cm/s)
Figure 19.14
Autoregulation
• Automatic adjustment of blood flow to each
tissue in proportion to its requirements at any
given point in time
• Is controlled intrinsically by modifying the
diameter of local arterioles feeding the
capillaries
• Is independent of MAP, which is controlled as
needed to maintain constant pressure
Autoregulation
•
Two types of autoregulation
1. Metabolic
2. Myogenic
Metabolic Controls
• Vasodilation of arterioles and relaxation of
precapillary sphincters occur in response to
– Declining tissue O2
– Substances from metabolically active tissues (H+,
K+, adenosine, and prostaglandins) and
inflammatory chemicals
Metabolic Controls
• Effects
– Relaxation of vascular smooth muscle
– Release of NO from vascular endothelial cells
• NO is the major factor causing vasodilation
• Vasoconstriction is due to sympathetic
stimulation and endothelins
Myogenic Controls
• Myogenic responses of vascular smooth
muscle keep tissue perfusion constant despite
most fluctuations in systemic pressure
• Passive stretch (increased intravascular
pressure) promotes increased tone and
vasoconstriction
• Reduced stretch promotes vasodilation and
increases blood flow to the tissue
Intrinsic mechanisms
(autoregulation)
• Distribute blood flow to individual
organs and tissues as needed
Extrinsic mechanisms
• Maintain mean arterial pressure (MAP)
• Redistribute blood during exercise and
thermoregulation
Amounts of:
Sympathetic
pH
O2
Metabolic
a Receptors
b Receptors
controls
Amounts of:
Nerves
Epinephrine,
norepinephrine
CO2
K+
Angiotensin II
Hormones
Prostaglandins
Adenosine
Nitric oxide
Endothelins
Myogenic
controls
Stretch
Antidiuretic
hormone (ADH)
Atrial
natriuretic
peptide (ANP)
Dilates
Constricts
Figure 19.15
Long-Term Autoregulation
• Angiogenesis
– Occurs when short-term autoregulation cannot
meet tissue nutrient requirements
– The number of vessels to a region increases and
existing vessels enlarge
– Common in the heart when a coronary vessel is
occluded, or throughout the body in people in
high-altitude areas
Blood Flow: Skeletal Muscles
• At rest, myogenic and general neural mechanisms
predominate
• During muscle activity
– Blood flow increases in direct proportion to the
metabolic activity (active or exercise hyperemia)
– Local controls override sympathetic vasoconstriction
• Muscle blood flow can increase 10 or more during
physical activity
Blood Flow: Brain
• Blood flow to the brain is constant, as neurons are
intolerant of ischemia
• Metabolic controls
– Declines in pH, and increased carbon dioxide cause
marked vasodilation
• Myogenic controls
– Decreases in MAP cause cerebral vessels to dilate
– Increases in MAP cause cerebral vessels to constrict
Blood Flow: Brain
• The brain is vulnerable under extreme
systemic pressure changes
– MAP below 60 mm Hg can cause syncope
(fainting)
– MAP above 160 can result in cerebral edema
Blood Flow: Skin
• Blood flow through the skin
– Supplies nutrients to cells (autoregulation in
response to O2 need)
– Helps maintain body temperature (neurally
controlled)
– Provides a blood reservoir (neurally controlled)
Blood Flow: Skin
• Blood flow to venous plexuses below the skin
surface
– Varies from 50 ml/min to 2500 ml/min, depending
on body temperature
– Is controlled by sympathetic nervous system
reflexes initiated by temperature receptors and
the central nervous system
Temperature Regulation
• As temperature rises (e.g., heat exposure,
fever, vigorous exercise)
– Hypothalamic signals reduce vasomotor
stimulation of the skin vessels
– Heat radiates from the skin
Temperature Regulation
• Sweat also causes vasodilation via bradykinin
in perspiration
– Bradykinin stimulates the release of NO
• As temperature decreases, blood is shunted to
deeper, more vital organs
Blood Flow: Lungs
• Pulmonary circuit is unusual in that
– The pathway is short
– Arteries/arterioles are more like veins/venules
(thin walled, with large lumens)
– Arterial resistance and pressure are low (24/8 mm
Hg)
Blood Flow: Lungs
• Autoregulatory mechanism is opposite of that
in most tissues
– Low O2 levels cause vasoconstriction; high levels
promote vasodilation
– Allows for proper O2 loading in the lungs
Blood Flow: Heart
• During ventricular systole
– Coronary vessels are compressed
– Myocardial blood flow ceases
– Stored myoglobin supplies sufficient oxygen
• At rest, control is probably myogenic
Blood Flow: Heart
• During strenuous exercise
– Coronary vessels dilate in response to local
accumulation of vasodilators
– Blood flow may increase three to four times
Blood Flow Through Capillaries
• Vasomotion
– Slow and intermittent flow
– Reflects the on/off opening and closing of
precapillary sphincters
Capillary Exchange of Respiratory
Gases and Nutrients
• Diffusion of
– O2 and nutrients from the blood to tissues
– CO2 and metabolic wastes from tissues to the 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
Pinocytotic vesicles
Red blood
cell in lumen
Endothelial cell
Endothelial cell nucleus
Basement membrane
Tight junction
Fenestration
(pore)
Intercellular cleft
Figure 19.16 (1 of 2)
Lumen
Intercellular
cleft
Caveolae
Pinocytotic
vesicles
Endothelial
fenestration
(pore)
4 Transport
via vesicles or
caveolae (large
substances)
3 Movement
Basement through
membrane fenestrations
1 Diffusion
through
membrane
(lipid-soluble
substances)
2 Movement
through intercellular
clefts (water-soluble
substances)
(water-soluble
substances)
Figure 19.16 (2 of 2)
Fluid Movements: Bulk Flow
• Extremely important in determining relative
fluid volumes in the blood and interstitial
space
• Direction and amount of fluid flow depends
on two opposing forces: hydrostatic and
colloid osmotic pressures
Hydrostatic Pressures
• Capillary hydrostatic pressure (HPc) (capillary
blood pressure)
– Tends to force fluids through the capillary walls
– Is greater at the arterial end (35 mm Hg) of a bed
than at the venule end (17 mm Hg)
• Interstitial fluid hydrostatic pressure (HPif)
– Usually assumed to be zero because of lymphatic
vessels
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
Net Filtration Pressure (NFP)
• NFP—comprises all the forces acting on a
capillary bed
• NFP = (HPc—HPif)—(OPc—OPif)
• At the arterial end of a bed, hydrostatic forces
dominate
• At the venous end, osmotic forces dominate
• Excess fluid is returned to the blood via the
lymphatic system
Arteriole
Venule
Interstitial fluid
Net HP—Net OP
(35—0)—(26—1)
Net
HP
35
mm
Capillary
Net
OP
25
mm
NFP (net filtration pressure)
is 10 mm Hg; fluid moves out
Net HP—Net OP
(17—0)—(26—1)
Net
HP
17
mm
Net
OP
25
mm
NFP is ~8 mm Hg;
fluid moves in
HP = hydrostatic pressure
• Due to fluid pressing against a wall
• “Pushes”
• In capillary (HPc)
• Pushes fluid out of capillary
• 35 mm Hg at arterial end and
17 mm Hg at venous end of
capillary in this example
• In interstitial fluid (HPif)
• Pushes fluid into capillary
• 0 mm Hg in this example
OP = osmotic pressure
• Due to presence of nondiffusible
solutes (e.g., plasma proteins)
• “Sucks”
• In capillary (OPc)
• Pulls fluid into capillary
• 26 mm Hg in this example
• In interstitial fluid (OPif)
• Pulls fluid out of capillary
• 1 mm Hg in this example
Figure 19.17
Circulatory Pathways
• Two main circulations
– Pulmonary circulation: short loop that runs from
the heart to the lungs and back to the heart
– Systemic circulation: long loop to all parts of the
body and back to the heart
Pulmonary capillaries
of the R. lung
R. pulmonary L. pulmonary Pulmonary capillaries
of the L. lung
artery
artery
To
systemic
circulation
Pulmonary
trunk
R. pulmonary veins
From
systemic
circulation
RA
LA
RV
LV
L. pulmonary
veins
(a) Schematic flowchart.
Figure 19.19a
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
Inferior
vena
cava
LV
Azygos
system
Thoracic
aorta
Venous
drainage
Arterial
blood
Capillary beds of
mediastinal structures
and thorax walls
Diaphragm
Abdominal
aorta
Inferior
vena
cava
Capillary beds of
digestive viscera,
spleen, pancreas,
kidneys
Capillary beds of gonads,
pelvis, and lower limbs
Figure 19.20
Differences Between Arteries and Veins
Arteries
Veins
Delivery
Blood pumped into single
systemic artery—the aorta
Blood returns via
superior and interior
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
Arteries of the head and trunk
Internal carotid artery
External carotid artery
Common carotid arteries
Vertebral artery
Subclavian artery
Brachiocephalic trunk
Aortic arch
Ascending aorta
Coronary artery
Thoracic aorta (above
diaphragm)
Celiac trunk
Abdominal aorta
Superior mesenteric artery
Renal artery
Gonadal artery
Common iliac artery
Inferior mesenteric artery
Internal iliac artery
(b) Illustration, anterior
view
Arteries that supply
the upper limb
Subclavian artery
Axillary artery
Brachial artery
Radial artery
Ulnar artery
Deep palmar arch
Superficial palmar arch
Digital arteries
Arteries that supply
the lower limb
External iliac artery
Femoral artery
Popliteal artery
Anterior tibial artery
Posterior tibial artery
Arcuate artery
Figure 19.21b
Ophthalmic artery
Basilar artery
Vertebral artery
Internal
carotid artery
External
carotid artery
Common
carotid artery
Thyrocervical
trunk
Costocervical
trunk
Subclavian
artery
Axillary
artery
Branches of
the external
carotid artery
• Superficial
temporal artery
• Maxillary artery
• Occipital artery
• Facial artery
• Lingual artery
• Superior thyroid
artery
Larynx
Thyroid gland
(overlying trachea)
Clavicle (cut)
Brachiocephalic
trunk
Internal thoracic
artery
(b) Arteries of the head and neck, right aspect
Figure 19.22b
Anterior
Frontal lobe
Optic chiasma
Cerebral arterial
circle
(circle of Willis)
• Anterior
communicating
artery
• Anterior
cerebral artery
• Posterior
communicating
artery
• Posterior
cerebral artery
Basilar artery
Middle
cerebral
artery
Internal
carotid
artery
Mammillary
body
Temporal
lobe
Pons
Occipital lobe
Vertebral artery
Cerebellum
Posterior
(d) Major arteries serving the brain (inferior view, right side
of cerebellum and part of right temporal lobe removed)
Figure 19.22d
Vertebral artery
Thyrocervical trunk
Suprascapular artery
Costocervical trunk
Thoracoacromial artery
Axillary artery
Posterior circumflex
humeral artery
Anterior circumflex
humeral artery
Subscapular artery
Brachial artery
Deep artery of arm
Common
interosseous
artery
Common carotid
arteries
Left subclavian artery
Right subclavian artery
Brachiocephalic trunk
Posterior intercostal
arteries
Anterior intercostal
artery
Internal thoracic artery
Descending aorta
Lateral thoracic artery
Radial artery
Ulnar artery
Deep palmar arch
Superficial palmar arch
Digital arteries
(b) Illustration, anterior view
Figure 19.23b
Hiatus (opening)
for inferior
vena cava
Hiatus (opening)
for esophagus
Adrenal
(suprarenal)
gland
Kidney
Celiac trunk
Abdominal aorta
Lumbar arteries
Ureter
Median sacral
artery
Diaphragm
Inferior
phrenic artery
Middle
suprarenal
artery
Renal artery
Superior
mesenteric
artery
Gonadal
(testicular or
ovarian) artery
Inferior
mesenteric
artery
Common
iliac artery
(c) Major branches of the abdominal aorta.
Figure 19.24c
Common iliac artery
Internal iliac artery
Superior gluteal artery
External iliac artery
Deep artery of thigh
Lateral circumflex
femoral artery
Medial circumflex
femoral artery
Obturator artery
Femoral artery
Adductor hiatus
Popliteal artery
Anterior tibial artery
Posterior tibial artery
Fibular artery
Dorsalis pedis artery
Arcuate artery
Dorsal metatarsal
arteries
(b) Anterior view
Figure 19.25b
Popliteal artery
Anterior tibial artery
Posterior tibial
artery
Lateral plantar
artery
Medial plantar
artery
Fibular artery
Dorsalis pedis artery
(from top of foot)
Plantar arch
(c) Posterior view
Figure 19.25c
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
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
Superior mesenteric
vein
Inferior vena cava
Inferior mesenteric vein
External iliac vein
Common iliac vein
Popliteal vein
Internal iliac vein
Posterior tibial vein
Femoral vein
Great saphenous vein
Anterior tibial vein
(b) Illustration, anterior
view. The vessels of the
pulmonary circulation
are not shown.
Small saphenous vein
Dorsal venous arch
Dorsal metatarsal veins
Figure 19.26b
Superficial temporal
vein
Occipital vein
Posterior
auricular vein
External
jugular vein
Ophthalmic vein
Facial vein
Vertebral vein
Internal jugular
vein
Superior and middle
thyroid veins
Brachiocephalic
vein
Subclavian vein
Superior
vena cava
(b) Veins of the head and neck, right superficial aspect
Figure 19.27b
Superior sagittal
sinus
Falx cerebri
Inferior sagittal
sinus
Cavernous
sinus
Straight sinus
Confluence
of sinuses
Transverse
sinuses
Sigmoid sinus
Jugular foramen
Right internal
jugular vein
(c) Dural venous sinuses of the brain
Figure 19.27c
Brachiocephalic veins
Right subclavian vein
Axillary vein
Brachial vein
Cephalic vein
Basilic vein
Median cubital vein
Internal jugular vein
External jugular vein
Left subclavian vein
Superior vena cava
Azygos vein
Accessory hemiazygos
vein
Hemiazygos vein
Posterior intercostals
Inferior vena cava
Ascending lumbar vein
Median antebrachial
vein
Cephalic vein
Basilic vein
Radial vein
Ulnar vein
Deep palmar venous arch
Superficial palmar venous arch
Digital veins
(b) Anterior view
Figure 19.28b
Cystic vein
Hepatic
portal
system
Inferior
vena cava
Inferior phrenic veins
Hepatic veins
Hepatic portal vein
Superior mesenteric vein
Splenic vein
Suprarenal
veins
Renal veins
Inferior
mesenteric
vein
Gonadal veins
Lumbar veins
R. ascending
lumbar vein
L. ascending
lumbar vein
Common iliac veins
External iliac vein
(a) Schematic flowchart.
Internal iliac veins
Figure 19.29a
Hepatic veins
Inferior vena cava
Right suprarenal
vein
Right gonadal
vein
Inferior phrenic
vein
Left suprarenal
vein
Renal veins
Left ascending
lumbar vein
Lumbar veins
Left gonadal vein
Common iliac
vein
Internal iliac vein
External iliac
vein
(b) Tributaries of the inferior vena cava. Venous drainage
of abdominal organs not drained by the hepatic portal vein.
Figure 19.29b
Inferior vena cava
(not part of hepatic
portal system)
Hepatic veins
Liver
Hepatic portal
vein
Small intestine
Gastric veins
Spleen
Inferior vena cava
Splenic vein
Right gastroepiploic
vein
Inferior
mesenteric vein
Superior
mesenteric vein
Large intestine
Rectum
(c) The hepatic portal circulation.
Figure 19.29c
Common iliac vein
Internal iliac vein
External iliac vein
Inguinal ligament
Femoral vein
Great saphenous
vein (superficial)
Popliteal vein
Small saphenous
vein
Fibular vein
Anterior tibial vein
Dorsalis pedis vein
Dorsal venous arch
Dorsal metatarsal
veins
(b) Anterior view
Figure 19.30b
Great saphenous
vein
Popliteal vein
Anterior tibial vein
Fibular vein
Small saphenous
vein (superficial)
Posterior tibial vein
Plantar veins
Deep plantar arch
(c) Posterior view
Digital veins
Figure 19.30c