Peripheral Circulation and
Regulation
21-1
Peripheral Circulatory System
• Systemic vessels
– Transport blood through most all body parts
from left ventricle and back to right atrium
• Pulmonary vessels
– Transport blood from right ventricle through
lungs and back to left atrium
• Blood vessels and heart regulated to ensure
blood pressure is high enough for blood
flow to meet metabolic needs of tissues
21-2
Blood Vessel Structure
• Arteries
– Elastic, muscular, arterioles
• Capillaries
– Blood flows from arterioles to capillaries
– Most of exchange between blood and interstitial
spaces occurs across the walls
– Blood flows from capillaries to venous system
• Veins
– Venules, small veins, medium or large veins
21-3
Capillaries
• Capillary wall consists
mostly of endothelial
cells
• Types classified by
diameter/permeability
– Continuous
• Do not have fenestrae
– Fenestrated
• Have pores
– Sinusoidal
• Large diameter with
large fenestrae
21-4
Capillary Network
• Blood flows from
arterioles through
metarterioles, then
through capillary
network
• Venules drain network
• Smooth muscle in
arterioles, metarterioles,
precapillary sphincters
regulates blood flow
21-5
Structure of Arteries and Veins
• Three layers except for
capillaries and venules
• Tunica intima
– Endothelium
• Tunica media
– Vasoconstriction
– Vasodilation
• Tunica adventitia
– Merges with connective
tissue surrounding blood
vessels
21-6
Structure of Arteries
• Elastic or conducting arteries
– Largest diameters, pressure high and fluctuates
• Muscular or medium arteries
– Smooth muscle allows vessels to regulate blood
supply by constricting or dilating
• Arterioles
– Transport blood from small arteries to capillaries
21-7
Structure of Veins
• Venules and small veins
– Tubes of endothelium on delicate basement
membrane
• Medium and large veins
• Valves
– Allow blood to flow toward heart but not in
opposite direction
• Atriovenous anastomoses
– Allow blood to flow from arterioles to small
veins without passing through capillaries
21-8
Blood Vessel Comparison
21-9
Aging of the Arteries
• Arteriosclerosis
– General term for
degeneration changes
in arteries making them
less elastic
• Atherosclerosis
– Deposition of plaque
on walls
21-10
Pulmonary Circulation
• Moves blood to and from the lungs
• Pulmonary trunk
– Arises from right ventricle
• Pulmonary arteries
– Branches of pulmonary trunk which project to
lungs
• Pulmonary veins
– Exit each lung and enter left atrium
21-11
Systemic Circulation: Arteries
• Aorta
– From which all arteries are derived either
directly or indirectly
– Parts
• Ascending, descending, thoracic, abdominal
• Coronary arteries
– Supply the heart
21-12
Branches of the Aorta
21-13
Systemic Circulation: Veins
• Return blood from body to right atrium
• Major veins
– Coronary sinus (heart)
– Superior vena cava (head, neck, thorax, upper
limbs)
– Inferior vena cava (abdomen, pelvis, lower
limbs)
• Types of veins
– Superficial, deep, sinuses
21-14
Major Veins
21-15
Veins of Thorax
21-16
Hepatic Portal System
21-17
Dynamics of Blood Circulation
• Interrelationships between
–
–
–
–
Pressure
Flow
Resistance
Control mechanisms that regulate blood
pressure
– Blood flow through vessels
21-18
Laminar and Turbulent Flow
• Laminar flow
– Streamlined
– Outermost layer
moving slowest and
center moving fastest
• Turbulent flow
– Interrupted
– Rate of flow exceeds
critical velocity
– Fluid passes a
constriction, sharp
turn, rough surface
21-19
Blood Pressure
• Measure of force exerted by blood against
the wall
• Blood moves through vessels because of
blood pressure
• Measured by listening for Korotkoff sounds
produced by turbulent flow in arteries as
pressure released from blood pressure cuff
21-20
Blood Pressure Measurement
21-21
Blood Flow, Poiseuille’s Law
and Viscosity
• Poiseuille’s Law
• Blood flow
– Amount of blood
moving through a
vessel in a given time
period
– Directly proportional
to pressure differences,
inversely proportional
to resistance
– Flow decreases when
resistance increases
– Flow resistance
decreases when vessel
diameter increases
• Viscosity
– Measure of resistance
of liquid to flow
– As viscosity increases,
pressure required to
flow increases
21-22
Critical Closing Pressure,
Laplace’s Law and Compliance
Critical closing pressure
– Pressure at which a blood
vessel collapses and blood
flow stops
Laplace’s Law
– Force acting on blood
vessel wall is proportional
to diameter of the vessel
times blood pressure
Vascular compliance
– Tendency for blood
vessel volume to
increase as blood
pressure increases
– More easily the vessel
wall stretches, the
greater its compliance
– Venous system has a
large compliance and
acts as a blood
reservoir
21-23
Physiology of Systemic Circulation
• Determined by
– Anatomy of circulatory system
– Dynamics of blood flow
– Regulatory mechanisms that control heart and
blood vessels
• Blood volume
– Most in the veins
– Smaller volumes in arteries and capillaries
21-24
Cross-Sectional Area
• As diameter of vessels
decreases, the total
cross-sectional area
increases and velocity
of blood flow
decreases
• Much like a stream
that flows rapidly
through a narrow
gorge but flows slowly
through a broad plane
21-25
Pressure and Resistance
• Blood pressure averages
100 mm Hg in aorta and
drops to 0 mm Hg in the
right atrium
• Greatest drop in
pressure occurs in
arterioles which regulate
blood flow through
tissues
• No large fluctuations in
capillaries and veins
21-26
Pulse Pressure
• Difference between
systolic and diastolic
pressures
• Increases when stroke
volume increases or
vascular compliance
decreases
• Pulse pressure can be
used to take a pulse to
determine heart rate
and rhythmicity
21-27
Capillary Exchange and
Interstitial Fluid Volume Regulation
• Blood pressure, capillary permeability, and
osmosis affect movement of fluid from
capillaries
• A net movement of fluid occurs from blood
into tissues. Fluid gained by tissues is
removed by lymphatic system.
21-28
Fluid Exchange Across
Capillary Walls
21-29
Vein Characteristics and
Effect of Gravity on Blood Pressure
Vein Characteristics
• Venous return to heart
increases due to
increase in blood
volume, venous tone,
and arteriole dilation
Effect of Gravity
• In a standing position,
hydrostatic pressure
caused by gravity
increases blood
pressure below the
heart and decreases
pressure above the
heart
21-30
Control of Blood Flow by Tissues
• Local control
– In most tissues, blood flow is proportional to
metabolic needs of tissues
• Nervous System
– Responsible for routing blood flow and
maintaining blood pressure
• Hormonal Control
– Sympathetic action potentials stimulate
epinephrine and norepinephrine
21-31
Local Control of Blood Flow
by Tissues
• Blood flow can increase 7-8 times as a result of vasodilation of
metarterioles and precapillary sphincters in response to
increased rate of metabolism
– Vasodilator substances produced as metabolism increases
– Vasomotion is periodic contraction and relaxation of precapillary
sphincters
21-32
Nervous Regulation of
Blood Vessels
21-33
Short-Term Regulation of
Blood Pressure
• Baroreceptor reflexes
– Change peripheral resistance, heart rate, and stroke
volume in response to changes in blood pressure
• Chemoreceptor reflexes
– Sensory receptors sensitive to oxygen, carbon dioxide,
and pH levels of blood
• Central nervous system ischemic response
– Results from high carbon dioxide or low pH levels in
medulla and increases peripheral resistance
21-34
Baroreceptor Reflex Control
21-35
Baroreceptor Effects
21-36
Chemoreceptor Reflex Control
21-37
Effects of pH and Gases
21-38
Long-Term Regulation
of Blood Pressure
•
•
•
•
•
Renin-angiotensin-aldosterone mechanism
Vasopressin (ADH) mechanism
Atrial natriuretic mechanism
Fluid shift mechanism
Stress-relaxation response
21-39
Renin-Angiotensin-Aldosterone
Mechanism
21-40
Vasopressin (ADH) Mechanism
21-41
Long Term Mechanisms
• Fluid shift
• Atrial natriuretic
– Hormone released
from cardiac muscle
cells when atrial blood
pressure increases,
simulating an increase
in urinary production,
causing a decrease in
blood volume and
blood pressure
– Movement of fluid
from interstitial spaces
into capillaries in
response to decrease in
blood pressure to
maintain blood volume
• Stress-relaxation
– Adjustment of blood
vessel smooth muscle
to respond to change in
blood volume
21-42
Shock
• Inadequate blood flow throughout body
• Three stages
– Compensated: Blood pressure decreases only a moderate
amount and mechanisms able to reestablish normal blood
pressure and flow
– Progressive: Compensatory mechanisms inadequate and
positive feedback cycle develops; cycle proceeds to next
stage or medical treatment reestablishes adequate blood
flow to tissues
– Irreversible: Leads to death, regardless of medical
treatment
21-43