Chapter 21:
The Cardiovascular System:
Blood Vessels
Vessel Structure - General
 All vessels same basic
structure
 3 wall layers (or tunics)


Tunica adventitia (externa)
- elastic and laminar fibers
Tunica media
thickest layer
 elastic fibers and smooth
muscle fibers


Tunica interna (intima)

endothelium – non-stick
layer

basement membrane

internal elastic lamina
 Lumen - opening
 Structure/function
relationship changes
as move through
cardiovascular tree
 Tunic thickness and
composition variable
throughout
cardiovascular tree
Vessel Structure – Elastic Arteries
 Elastic
(conducting)
arteries
 Near heart
 Thick walls
 More elastic
fiber, less smooth
muscle
 Lose elasticity
with aging
Vessel Structure - Elastic Arteries
 Aorta and elastic
arteries
 Can vasoconstrict or
vasodilate
 Large arteries expand,
absorb pressure wave
then release it with
elastic recoil -
Windkessel effect
 Help to push blood along
during diastole
 With aging have less
expansion and recoil
Vessel Structure – Muscular Arteries
 Muscular
(distributing)
arteries
 Deliver blood to
organs
 More smooth
muscle
 Less elastic
fibers
Vessel Structure - Arterioles
 Arterioles




Distribution of
blood in organs
Composition varies
depending on
position - more
muscle, less
elasticity nearer
heart
Regulate flow from
arteries to
capillaries

Flow = ΔP/R

vary resistance
by changing
vessels size
Site of blood
pressure regulation
Vessel Structure - Capillaries
 Microcirculation
connects arteries and
veins
 Found in nearly every
tissue in body
 Higher the metabolic
rate, more capillaries in
tissue
 Muscle many caps
(>600/mm2)
 Cartilage none
Vessel Structure - Capillaries
 Allow exchange of nutrients and wastes between blood
and tissue
 Capillary structure - simple
 Basal lamina - connective tissue
 Endothelial cells
 Structure/function
Flow Regulation
 Regulation by vessels with
smooth muscle
 Metarterioles
connect arterioles to
venules through capillary
bed
 allows flow through
capillary bed w/out flow
through caps

Flow Regulation
 True capillaries
 Pre-capillary sphincter
ring of smooth muscle
 open/close to control
flow


regulated by chemicals
 Intermittent vasomotion
– caps open for flow 510X min
Types of Capillaries
 3 types of capillaries
1.
Continuous capillaries
 continuous endothelial
cells except for cleft
between cells
 tight junctions between
endothelial cells
prevent most things
from leaving caps
 most capillaries in body
Types of Capillaries
2. Fenestrated capillaries
fenestrations (slits)
allow for filtration of
small substances
 glomerular capillaries in
kidney

Types of Capillaries
3. Sinusoid capillaries
wider gaps between
endothelial cells
allowing RBC’s to exit
the caps
 found in liver

Vessel Structure - Veins
 Venules
 Collect blood from
caps carry it to
veins
 Structure changes
with position
 Become more
vessel-like (walls)
as move from
capillaries
Vessel Structure – Veins
 Veins
 Interna thicker
than arteries
 Media thinner, less
muscle
 Externa thick
 Valves
 Pressure low
 High compliance -
change volume
easily with small
change in pressure
 Varicose veins
Vessel Structure - Histology
 Very different morphology
under light microscopy
 Tunica media thickness
differentiates artery from vein
Vein
Artery
Artery
Vein
Vessel Structure/Function

At rest




60% of blood located in
veins and venules
Serve as reservoirs for
blood, “storing” it until
needed
Particularly veins of
abdominal organs, skin
ANS regulates volume
distribution



Vasoconstrict
Vasodilate
Open areas of circulation
to be supplied with blood



veins at rest
caps during exercise
Can “shift” volumes to
other areas as needed
Vessel Structure/Function
 0.75 L/min
Rest
CO = 5 L/min
Vessel Structure/Function
CO = 25 L/min
Heavy
 20 L/min
Exercise
 0.75 L/min
Rest
CO = 5 L/min
Physiology of Circulation
 Flow = ΔP/R
 or CO = MAP/R
 MAP - mean arterial pressure
 Higher pressure to lower pressure with resistance
(R) factor
 Blood pressure
 Pressure of blood on vessel wall
 Measurement of pressure of a volume in a space
 Systole/diastole - 120/80
 BP falls progressively from aorta to O mm Hg at RA
Regulation of Blood Flow
 Resistance - opposition to blood flow from blood and
vessel wall friction
 Factors that affect resistance (R)
 Viscosity - V R
thickness of blood
 dehydration, polycythemia
 R proportional to vessel length
 garden hose vs. straw
 obesity
 Vessel diameter
 changes in diameter affect flow




vessel wall drag – blood cells dragging against the wall
laminar flow – layers of flow
R inverse proportional to radius4


decrease in r by 1/2 R 16X
only important in vessels that can change their size actively
Regulation of Pressure, Resistance
 Systemic vascular resistance
(Total Peripheral Resistance TPR)




All vascular resistance
offered by systemic vessels
Which vessels change size?
Resistance highest in
arterioles
Largest pressure drop occurs
in arterioles
 Relationship of radius to
resistance in arterioles
important due to smooth
muscle in walls
Systemic Blood Pressure
 Arterial Blood Pressure
 Pulsatile in arteries due to
pumping of heart
 Systolic/diastolic
 Pulse pressure = systolic
(minus) diastolic
Q - Windkessel effect on
pulse pressure?
A
Q
- Decreases pulse
pressure
- What is the effect of
hardening of the arteries
on pulse pressure?
A
- Increases pulse
pressure
Systemic Blood Pressure
 Capillary Blood
Pressure
 Relatively low blood
pressure
 Low pressure good for
caps because:
 caps are fragile - hi
pressure tears them
up
 caps are very
permeable - hi
pressure forces a lot
of fluid out
Systemic Blood Pressure
 Venous return
 Volume of blood flowing
back to heart from
systemic veins
 Depends on pressure
difference (ΔP) from
beginning of venules (16
mmHg) to heart (0
mmHg)
 Any change in RA
pressure changes
venous return
 Help for venous return
 Skeletal muscle pump
muscles squeeze veins
 force blood back to heart


valves prevent back flow
 Respiratory pump

inhaling pulls air into lungs

helps to pull blood back into
thorax
Velocity of Blood Flow
 Velocity of blood flow -
inversely related to total
cross sectional area (CSA)
of vessels
 Aorta


Total CSA 3-5 cm2
Velocity 40 cm/sec
 Capillaries


Total CSA 4500-6000 cm2
Velocity 0.1 cm/sec
 Vena Cava


Total CSA 14 cm2 in vena
cava
Velocity 5-20 cm/sec
Vessel Structure - Function
 Capillary Function
 Site of exchange between blood and tissues
 Delivery of nutrients and removal of wastes
 Slow flow allows time for exchange
 Mechanisms of nutrient exchange
 Diffusion - O2, CO2, glucose, AA's, hormones
diffuse down [ ] gradients
 If lipid soluble, can travel through cell
 If water soluble, between cells
Capillary Fluid Exchange
 Fluid movement
 Fluid filtered and
reabsorbed across
capillary wall
 Starling’s law of the
capillaries
 Forces driving the movement of fluid
 Hydrostatic pressure capillary (HPc)
 Hydrostatic pressure interstitial fluid (HPif)
 Osmotic pressure capillary(OPc)
 Osmotic pressure interstitial fluid (OPif)
 Net filtration pressure (NFP) is a sum of all
Capillary Fluid Exchange
 On average 85% of fluid filtered at arteriole end is
reabsorbed at venular end
Maintaining Blood Pressure Short Term Mechanisms - CNS
 Neural Control - Cardiac centers in medulla
 Vasomotor center
 medullary area dedicated to control of blood vessels
 sends sympathetic output to blood vessels




Vasoconstricts or vasodilates as needed
tone - normal amount of vasoconstriction or vasodilation
can vary tone which varies delivery of blood
receives input from different sources


baroreceptors
chemoreceptors
Maintaining Blood
Pressure – Short term
mechanisms – CNS reflexes
 Baroreceptor initiated
reflex
 Located at carotid sinus and
aortic arch
 Monitor changes in blood
pressure
 Regulate activity of
Sympathetic Nervous
System (vascular tone)
Maintaining Blood Pressure –
mechanisms – CNS reflexes
Short term
 Chemoreceptor
initiated reflexes
 Carotid bodies,
aortic bodies
 Monitors changes
in chemicals (O2,
CO2, [H+])
  CO2,  H+,  O2
(stresses) result in
 sympathetic
activity and  BP
Maintaining Blood Pressure – Short
term mechanisms – CNS reflexes
 Influence of Higher Brain Centers (areas above
medulla) - Cortex and Hypothalamus
 Not involved in minute to minute regulation
 Influence vasomotor center depending on conditions


public speaking
temperature regulation
Maintaining Blood Pressure -
Short Term Mechanisms - Hormones
 Renin - Angiotensin -
Aldosterone
 Renin
enzyme from kidney
 results in formation of
Angiotensin II (AII)

 AII
vasoconstrictor
 stimulates ADH, thirst


stimulates aldosterone
- Na+ reabsorption
 Why/how would these
things affect blood
pressure?
Maintaining Blood Pressure -
Short Term Mechanisms - Hormones
 Adrenal medulla - Epi and Norepi
  CO (HR,  SV)
 Constrict abdominal, cutaneous
arterioles/venules
 Dilate cardiac, skeletal muscle beds
 Why/how would this affect blood
pressure?
Maintaining Blood Pressure -
Short Term Mechanisms - Hormones
 Antidiuretic Hormone
(ADH)
 Osmoreceptors in
hypothalamus
 Retains fluid (inhibited by
alcohol)
 Vasoconstriction at high
levels
 Why/how would this affect
blood pressure?
Maintaining Blood Pressure -
Short Term Mechanisms - Hormones
 ANP (atrial natriuretic peptide)
 Released from atrial cells in response to BP
 Vasodilator, Na+ and water loss, opposes Aldosterone
 Why/how would this affect blood pressure?
Maintaining Blood Pressure Long Term Regulation
 Renal mechanism
 Volume in a space
 Regulate space in the
short term – we just
talked about it!
 nervous control
 hormones
 Regulate volume in the
long term
 The kidneys!
  BP,  urine flow to 
BP
  BP,  urine flow to 
BP
Control of Blood Flow
 Autoregulation (local control) - local automatic
adjustment of blood flow to match tissue
needs
 Physical changes


Warming -  vasodilation
Cooling -  vasoconstriction
 Chemical changes - metabolic products


Vasodilators
Vasoconstrictors
 Myogenic control
 smooth muscle controls resistance
  stretch  contraction,  stretch  relaxation
Blood Flow in Special Areas
 Skeletal Muscle
 Wide variability in amount of flow
 Sympathetic regulation from brain in response to level of
activity
 α receptors - vasoconstrict
 β receptors - vasodilate
 Metabolic regulation in tissue
 low O2  vasodilate to increase flow
 hi O2  vasoconstrict to decrease flow
 Brain
 Very little variability in flow
 Stores few nutrients so flow must be maintained!
 Metabolic regulation
Blood Flow in Special Areas
 Skin
 Supplies nutrients, aids in temperature regulation, provides
a blood reservoir
 Metabolic and sympathetic regulation
 Lungs
 Low pressure (25/10), low resistance
 Flow regulated by O2 availability in the lungs
hi O2  vasodilate to increase flow – opposite to muscle
 low O2  vasoconstrict to decrease flow – opposite to muscle

 Heart
 Variable flow depending on activity
 Metabolic and sympathetic regulation
Regulation of Blood Pressure
CO = =MAP/R
MAP
CO x R