Blood Flow and Pressure
Exchange
Outline
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Overview of circulation
Components of the Vascular system
Medical physics of blood flow
Vascular distensibility and compliance
Arterial damping of pressure pulses
Veins as reservoirs of blood
Capillary exchange
Learning Objectives
• Know each component of the vascular system.
• Understand blood flow using Ohm’s and
Poiseuille’s laws.
• Know how vascular distensibility allows
arteries to dampen pressure pulses and veins
to act as reservoirs.
• Know how hydrostatic and colloid osmotic
forces determine the flow of fluid in the
capillaries.
Components of the Vascular System Blood Vessels
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Closed circulatory system
Arteries
Arterioles
Capillaries
Venules
Veins
3 tunics
Lumen
• Tunica interna
– Endothelium
– Connective tissue
• Tunica media
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Smooth muscle
Elastin
Vasoconstriction
Vasodilation
• Tunica externa
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Collagen fibers
Nerve fibers
Lymphatic vessels
Elastin fibers
Comparison of Veins and Arteries
Arteries:
Veins:
Histological Structure of Blood Vessels
Arteries
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Away from the heart
Thick, muscular walls
Very elastic
Arterioles
Diameter varies in
response to neural
stimuli and local
chemical influences.
• Capillaries
• Consist of a single tunica
interna
• Gas, nutrient, and waste
exchange
• Brain capillaries
• Blood-brain barrier
• Capillary beds
• Precapillary sphincter
• Shunting of blood
• Digestion
Venous System
• Toward the heart
• Venules—porous—free
movement of fluids and
white blood cells.
• Veins
• 3 tunics—but thin
• Venous valves
• Varicose veins
• Incompetent valves
• hemorrhoids
• Maintenance of Blood
Pressure
– Neural control
• Shunting and vasoconstriction.
– Vasomotor center
– Baroreceptors
• Carotid and aorta
– Chemoreceptors
– Higher brain centers
– Hormones
• Catecholoamines
• Atrial natrietic peptide
• ADH
– Alcohol
– Histamine—other vasodilators
Hypertension
• 30% of people over 50
• Damages arteries
• Causes heart failure,
vascular disease, renal
failure, stroke, and
blindness.
• Enlargement falled by
hypertrophy of the
myocardium
• Contributing factors:
– Diet (sodium, saturated
fat, cholesterol)
– Obesity
– Age
– Race
– Heredity
– Stress
– Smoking—nicotine is a
vasoconstrictor.
Atherosclerosis
• Damage to the tunica
interna
– Viral
– Bacterial
– Hypertension
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Reinjury
Inflammation
LDLs—”bad cholesterol”
Foam cells
Fatty streak stage
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Arteriosclerosis
Hypertension
Stroke
Heart attack
Coronary bypass
Angioplasty
tPA—tissue plasminogen
activator
• Clot buster
• HDL—removes cholesterol
from vessel walls.
Arteries
• Aorta—largest artery
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Ascending
Descending
Right and left coronary arteries
Common carotid arteries—branch to form internal and
external carotids
• External—supply tissues of the head except the brain and orbits.
• Internal—supply the orbits and most of the cerebrum.
– Vertebral arteries—branch to the cervical spinal cord,
neck, cerebellum, pons, and inner ear.
Arteries to Know
• Know the arteries on the proceeding chart
plus:
– Arteries of the arm—brachial, radial, ulnar
– Arteries of the leg—femoral, popliteal, anterior
tibial, posterior tibial
– Be able to identify these arteries on a diagram.
Also know the locations served by these arteries.
Veins
• Dural sinuses—veins of the brain drain into these
enlarged chambers and drain to the internal jugular
veins.
• External jugular veins—superficial head structures.
• Vertebral veins—cervical vertebrae and neck
muscles.
• Brachiocephalic—mammary glands and first 2 or 3
intercostal spaces.
Veins to Know
• Know the veins on the preceding chart plus:
– The veins of the arms—cephalic, axillary, brachial, radial,
ulnar.
– The veins of the legs—external iliac, femoral, popliteal,
anterior tibial, posterior tibial, great saphenous vein,
hepatic portal vein.
– The great saphenous vein is a superficial vein. Connect
with many of the deep veins of the legs and thighs.
– Be able to identify these veins on a diagram. Also know
the locations served by these arteries.
Overview of
Circulatory
System: Arteries +
Veins and
Everything in
Between
Function of Circulatory
System: To carry nutrients
and hormones to tissues
and wastes products away
from tissues.
Basic Circulatory Function
• Rate of blood flow to tissues changes based on
need.
- e.g., during exercise, blood flow to skeletal
muscle increases.
- In most tissues, blood flow increases in
proportion to the metabolism of that tissue.
• Cardiac output is mainly controlled by venous
return.
• Generally, arterial pressure is controlled
independently of local blood flow or cardiac
output control.
Parts of the
Vasculature
•Aorta receives blood from
left ventricle.
•Arteries transport under
high pressure, strong
vascular walls.
• Arterioles control
conduits, last branch of
arterial system, strong
muscular walls that can
strongly constrict or dilate.
• Capillaries exchange
substances through pores.
• Venules collect blood
from capillaries.
• Veins low pressure,
transport blood back to
the heart, controllable
reservoir of extra blood.
Blood Volume and
Vasculature Cross-Sectional
Area
Cross-sectional area (cm2)
Aorta
2.5
Small Arteries
20
Arterioles
40
Capillaries
2500
Venules
250
Small Veins
80
Venae Cavae
8
Normal Blood Pressures in Vasculature
Ohm’s Law Applied to Blood Flow
Blood Pressure
• BP is the force exerted by the blood against the
vessel wall.
- Typically measured as mm Hg.
- E.g., 100 mm Hg is the force needed to push a
column of Hg to a level of 100 mm.
Resistance
• Resistance is the impediment to blood flow.
• Not measured directly, but determined from
pressure and flow measurements.
- If ΔP = 1 mm Hg and F = 1 ml/sec, then R = 1 PRU
(peripheral resistance unit).
- In the adult systemic circulatory system, ΔP = 100
mm Hg, and F = 100 ml/sec; so R = 1 PRU.
- In the pulmonary system, ΔP = 14 mm Hg and F =
100 ml/sec; so R = 0.14 PRU.
Conductance
• Conductance is the opposite of resistance:
Conductance = 1/resistance
• Conductance may be easier to conceptualize
than resistance and is sometimes easier to use
in calculating the total resistance of parallel
vessels.
Vessel Diameter and Blood Flow – Changes in
Resistance
Laminar Flow
Poiseuille’s Law
Turbulant Flow
Adding Resistance in Series and
Parallel
Effect of Viscosity on Resistance and
Blood Flow
Summary of Blood Flow Physics
Vascular Distensibility
• Vascular distensibility is the ability of the
vascular system to expand with increased
pressure, which
Increases blood blow as pressure increases.
- In arteries, averages out pulses.
- Allows veins to act as reservoirs
Calculate Distensibility
• Fractional increase in volume per rise in
pressure:
Vascular
= Increase in Volume
Distensibility
Incr in P x orig Vol
If 1mm Hg increases a vessel from 10mm to
11mm, the distensibility would be 0.1 per
mm Hg or 10% per mm Hg.
Distensibility of Arteries and Veins
• Artery walls are much stronger than those of
veins and thus, much less distensible.
• The larger distensibility of veins allows them
to act as blood reservoirs.
Vascular Compliance
• The quantity of blood that can be stored in a
particular portion of the vasculature for a rise
in pressure:
Vascular compliance = Increase in volume
Increase in pressure
• Compliance = distensibility x vol
Arterial and Venous Volume-Pressure
Curves
Damping of Pulse Pressure in Arterial
System
Athersclerosis – Arteries become less
Compliant
Veins
•Can distend to hold
large amounts of
blood.
•Contraction of
skeletal muscles can
constrict the veins and
propel blood to the
heart and increase
cardiac output.
• The contractioninduced constriction
and the valves prevent
the venous pressure
from building up on
the feet of standing
adults.
Veins as Blood Reservoirs
• > 60% of blood in
the circulatory system is in the veins.
• When blood
is lost, sympathetic stimulation causes veins
to constrict and make up for the lost blood.
• Conversely, veins can distend to hold excess
blood if too much is
given during a transfusion.
The Distribution of Blood
Blood Volume
• Distribution of H2O within the
body:
• Intracellular compartment:
– 2/3 of total body H2O within
the cells.
• Extracellular compartment:
– 1/3 total body H2O.
• 80% interstitial fluid.
• 20% blood plasma.
• Maintained by constant
balance between H2O loss and
gain.
Capillaries
• Exchange nutrients and waste with tissues.
• ~ 10 billion capillaries with 500 – 700 m2
total surface area in whole body.
Capillaries are Porous
• The exchange of watersoluble nutrients and waste between bloo
d plasma and interstitial fluid occurs by di
ffusion through pores in the capillary walls
.
• Lipidsoluble substances pass directly through th
e capillary wall (e.g., O2 and CO2).
Capillary Structure
Capillary Exchange
Diffusion:
Filtration:
Reabsorption:
Capillary Exchange
Molecular Weight and Capillary
Porosity
Colloid Osmotic Pressure
• Starling force=(Pc + Pif) - (Pif + Pp)
• Pc
– Hydrostatic pressure in the capillary.
• Pif
– Colloid osmotic pressure of the interstitial fluid.
• Pif
– Hydrostatic pressure in the interstitial fluid.
• Pp
– Colloid osmotic pressure of the blood plasma.
Cardiac Output (CO)
Recall the Frank-Starling Mechanism?
• Volume of blood
pumped/min. by each
ventricle.
– Pumping ability of the
heart is a function of
the beats/ min. and the
volume of blood
ejected per beat.
• CO = SV x HR
– Total blood volume =
about 5.5 liters.
• Each ventricle pumps
the equivalent of the
total blood volume/ min.
Forces Determining the Flow of Substances
in the Capillary
Forces at Arterial End of Capillary
Forces at Venous End of Capillary
Mean Capillary Forces