Laboratory Exercise 16: Blood Pressure
Heart contractions maintain blood pressure within the blood vessels. This hydrostatic
pressure enables the blood to circulate and forces water, dissolved molecules and ions out
of the capillaries and into the tissue fluid compartment by bulk flow.
The value of normal blood pressure depends on:
1. Pumping action of the heart (cardiac output, CO). The CO is dependent on
stroke volume and heart rate. CO is the principal cause of the blood pressure.
2. Structural characteristics of the blood vessels – amount of muscle and elastic
connective tissue in the wall.
3. Functional characteristics of the blood vessels – if the blood vessels are
constricted or dilated – peripheral resistance.
A. Arterial Blood Pressure
Since the heart is a one cycle pump, blood enters the arteries intermittently during
ventricular systole, causing pressure waves or pulses. The heart is then described as a
pulsatile pump. The systolic pressure from left ventricular systole, causes blood to surge
through the arteries at 120-130 mm of Hg. This is the pressure on the side wall of an
artery when the ventricle is contracting. During left ventricular diastole, the pressure
drops to approximately 80 mm Hg. The diastolic pressure is residual pressure on an
arterial wall when the ventricle is relaxed. It is due to recoil of the elastic membranes in
the walls of the arteries. The difference between the systolic and the diastolic pressure is
the pulse pressure. Normal pulse pressure is 40 mm Hg. Pulse pressure provides
information about the condition of the CV system. For example, in atherosclerosis, pulse
pressure is greatly increased as diastolic pressure increases.
Normal ratio of SP: DP: PP is:
3: 2 : 1
120 : 80 : 40.
Recording an arterial pulse wave with a phototransducer, demonstrates a sharp rise in
arterial pressure during ventricular systole, followed an initial steep drop during the
diastolic phase. A notch appears in the diastolic phase of the pulse wave followed by a
less steep drop of the pressure during diastole. A drop in the ventricular pressure during
diastole causes aortic pressure to decrease. This allows a backflow of aortic blood
toward the aortic semilunar valve, closing the valve. The back flowing blood hitting
against the closed valve and wall of the aorta results in a forward surge of blood. This is
recorded as a small increase in pressure followed by a gradual decline and accounts for
the notch (dicrotic notch) in the pulse record.
Relationships among Pulse Pressure, Stroke Volume, Elasticity of the Blood Vessels and
Blood Pressure
In a normal person the pulse pressure is:
1. Directly related to stroke volume and thus systolic pressure.
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2. Inversely related to elasticity of the blood vessels, i.e., the more elastic the
arteries less the diastolic pressure drops as the elastic recoil keeps pressure on
the flowing blood. This recoil is necessary to force blood out of the arteries,
so that at the end of ventricular diastole there is only a very little volume of
blood left in the large and medium sized arteries. Then these arteries can
easily accommodate the next ejected volume of blood on the next ventricular
systole.
In atherosclerosis (hardening of the arteries) there is a decreased elasticity of the vessels.
As a result, the arterial vessels cannot stretch during systole, this increases systolic
pressure in the artery as the energy of the force of the blood is not taken up in the elastic
stretching of the wall. Since the vessels are less elastic, they cannot recoil during diastole
and apply sufficient pressure to push the blood from the arteries, blood remains in the
vessels so that the end diastolic pressure (EDP) and EDV are higher than normal.
Under conditions of the hardening of the arteries, the pulses of blood entering the arteries cause
greater pressure changes than normal. Systolic pressure rises very high and diastolic pressure
falls relatively low, although the EDV and EDP are higher than normal, because blood is still in
the arteries from the previous cardiac cycle. Pulse pressures under these conditions can be 100
mm Hg. or more.
Transmission of Pressure Wave or Pulse to the Smaller Vessels and Damping of the
Pressure Pulse
When blood is pumped into the aorta at ventricular systole, the pressure distends the
aorta, at diastole the aorta recoils and relatively great pressure changes are observed
during systole and diastole. As blood pushes forward along the arteries pressure builds
up further peripherally. The movement of the pressure along the length of the artery is
the transmission of the pressure wave or the pulse.
As the blood moves further distally the pressure drops and the pressure difference
decreases. This is the damping of the pulses. By the time the blood is in the arterioles
there is practically no systolic and diastolic pressure.
The reasons for damping of the pressure wave are:
1. resistance of the blood flow in small vessels is great enough to impede blood
flow and the transmission of pressure;
2. distensibility of the small vessels is great enough to produce less pressure rise
and fall, as a result pressure smoothes out in the small arteries and the
arterioles.
B. Sphygmomanometer
The sphygmomanometer measures blood pressure indirectly by determining how
forcefully the blood presses against the vessel’s wall. The sounds of Korotkoff are heard
through the stethoscope caused by the turbulence of blood spurting through a partially
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compressed vessel. Systolic pressure – pressure at which the turbulence is first heard.
Diastolic pressure – pressure at which the turbulence is no longer heard.
C. The Cold Pressor Test
Increase in BP due vasoconstriction causes a rise in the peripheral resistance. The cold
pressor test observes the change in blood pressure caused by the ANS response to a cold
stress. Cold will lead to arterial constriction and a rise in the peripheral resistance and
BP.
D. Effect of Rest, Exercise and Smoking on Blood Pressure and Heart Rate
A photoelectric transducer measures indirectly heart rate as the pulse rate by variations in
fingertip blood volume converted into electrical signals which are recorded as pulse
waves. The sphygmomanometer, and the stethoscope pick up the Korotkoff sounds
determine systolic and diastolic blood pressures.
Smoking (nicotine) increases heart (pulse) rate and raises BP due to vasoconstriction.
The vasoconstriction causes increased peripheral resistance and an increase in the
amplitude of the pulse wave. Nicotine in cigarettes stimulates the SNS neurons to release
NE and E to the blood vessels causing the vasoconstriction. Nicotine also inhibits PNS
neurons from releasing ACH.
E. Microcirculation in the Frog Skin
F. Liquid Crystal Thermography
Liquid crystals are molecules which reflect light of different wavelengths. When
exposed to changes in temperature, the molecules alter their shape and reflect different
wavelengths, producing different colors.
Liquid crystals can be used to examine regional variations in peripheral blood flow,
indicating increased circulation (vasodilation) or decreased circulation (vasoconstriction).
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