Biomedical Engineering 403

Vascular Anatomy and Vascular Resistance

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Today's topic:

Many of these topics are incomplete.
They should be completed when the extra credit projects are turned in.

6
Anatomy of the Vascular System
 Basics of Blood Pressure
(Chapter 6, and pp. 1-4)

Function of vascular system:
        Distribute blood to capillary beds to sustain tissues (and for
thermoregulation)

Basic Anatomy
        Aorta
        Arteries
        Arterioles--most muscular
        Capillaries--one blood cell at a time, very slow
        Venules
        Veins--contain the highest proportion of circulating blood
        Vena Cava

Aorta and large arteries--contain unusually large amts of elastin, which
makes them particularly pliable.
Small arteries and arterioles--less elastin, but more circular smooth
muscle fibers.  Therefore, have enhanced ability to       constrict
increasing pressure in the direction of the heart but reducing pressure in
the capillaries.
Veins--Very thin-walled vessels with little elastin or smooth muscle.
Stores most blood.

Percent of total blood volume
        in capillaries          5%
        arterial vessels                11%     (includes aorta, arteries,
arterioles)
        venous vessels          67%     (include venae cavae, veins,
venules)
        heart                   5%
        Pulmonary vessels       12%
cross-sectional area of capillaries is far greater than either veins or
arteries

Systole occupies approx 1/3 of the cardiac cycle, yet blood flows
throughout systole and diastole.
Because blood flow is restricted at the arterioles, only about 40% of
stroke volume gets through the arteries during   each systole; the other
60% stays squeezed into the arteries, which can expand somewhat to hold
it. This        creates pressure.  The amt of pressure is inversely
related to the elasticity of the vessels.
        P=V/C (pressure = volume of blood divided by the capacity of the
vessel to expand).
During the rapid ejection phase of systole, the volume of blood introduced
into the arterial system exceeds the volume  that exits through the
arterioles, increasing arterial pressure; maximum arterial volume is
reached at the end  of the rapid ejection phase--this is the peak (or
systolic) pressure

Pulse Wave--dicrotic notch--reflux of small amt of blood back to aortic
valve and coronary vessels.  Then arterial      walls recoil, sending
stored blood to periphery.  Lowest point is diastolic pressure.

Systolic, diastolic, and mean arterial pressure
Mean arterial pressure is not simply the average of systolic and
diastolic, because blood spends more time near the diastolic than the
systolic level.  Mean arterial pressure is therefore closer to the
diastolic value.  You can get a very good approximation of mean arterial
pressure by adding one third of the pulse pressure (systolic - diastolic)
to the diastolic pressure.

Pulse pressure (difference between systolic and diastolic pressures) is a
function of stroke volume and arterial        compliance--it reflects the
volume of blood discharged by the left ventricle during the rapid ejection
phase    minus the volume that has run off to the periphery during this
same phase of the cardiac cycle


Pressure wave moves at 4-6 m/sec--about 20 faster than mean velocity of
blood flow (~20-40 cm/sec).  The pressure       pulse starts at the aorta
and moves relatively slowly to the periphery (reaching the radial artery,
for instance,       .1 second later).  Consequently, there comes a period
when distal pressure is transiently higher than proximal  pressure.  The
reversed pressure gradient does not immediately reverse blood flow because
of the forward        momentum, but it does decellerate the flow.  Thus
arterial flow accellerates, then decellerates over the first  third of the
cardiac cycle.  During the next two thirds of the cycle, flow is virtually
zero.  The period of    near-zero flow shortens as blood enters small
arteries, and in the smallest arteries flow is continuous, albiet
still pulsatile.

Determinants of BP
                blood volume
                elastic characteristics (capacitance)
                cardiac output, which depends on HR X stroke volume
                peripheral resistance
If arterial inflow exceeds outflow BP rises, if outflow exceeds inflow BP
falls; when inflow equals outflow, arterial   pressure remains constant

Blood Pressure Measurement--sphygmomanometry
The inflatable cuff is placed on the upper arm at about heart level
The cuff is inflated to a pressure that obliterates the pulse
stethoscope applied to skin of the antecubital space over the brachial
artery
Listening to the brachial artery with a stethoscope reveals no sound
Lower cuff pressure
When pressure is just below systolic pressure, the artery opens briefly
during each systole--the transient squirt of    blood vibrates the artery
wall downstream producing a Korotkoff sound (dull tapping).
Korotkoff sounds grow louder, as the cuff pressure is lowered and more
blood spurts through
When cuff pressure approaches diastolic, the artery remains patent for
most of the cardiac cycle and the vibration of   the vessel wall
diminishes.

Factors affecting BP:
Age:  Approx    100/65 mmHg at age 6
                125/80 at 30
                180/90 at 70
The striking increase in pulse pressure with age is due to reduced
arterial compliance as a result of progressive changes in the collagen and
elastin contents of arterial walls.  Hardening of the arteries, or
arteriosclerosis, is almost universal in older people, so systolic
pressure is almost always very high.  Atherosclerosis, buildup of fatty
deposits in the vessels also contributes to reduced distensibility.

Sleep and exercise:  BP often falls to 80/50 or  lower during sleep
In exercise, it may rise or fall depending on the balance between
increased cardiac output and reduced peripheral       vascular resistance
In dynamic exercise BP may go down or increase slightly
In static exercise (i.e., weight lifting, hand grip), BP goes up

Gravity:  BP increases in arteries below the heart due to the weight of
the column of blood between the heart and the   artery--so BP higher in
feet than in head

Stress:  raises BP

Valsalva Maeuver: BP rises

Pregnancy:  BP falls

Many other situations and conditions

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