Heart Rate and Blood Pressure

1
Heart Rate and Blood Pressure

• Learning Objectives
• To understand how the heart rate is affected by
  nervous inputs and the route taken by nervous
  impulses through the heart
• To understand how blood pressure changes with
  activity

2
The Heart Rate

• Contraction of the heart muscle begins at the
  sino-atrial node (SAN) within the right atrium.
  (Stimulation is myogenic because it comes from
  the muscle.)
• The SAN produces an action potential which starts
  the muscle contracting about 70 times a minute
  (normal rates range from 60-80 beats per minute).
• As soon as it is generated the action potential
  spreads through both atria, causing them to
  contract.
• When the action potential reaches the
  atrio-ventricular node (AVN) it sweeps down the
  Purkyne fibres within the septum and spreads
  upwards into the walls of the ventricles causing
  contraction.
• N.B. Conduction is slow through the Purkyne
  fibres and causes a delay so that the ventricles
  contract after the atria (by approx. 0.12 - 0.20
  s). The ventricles also contract from the bottom
  upwards ensuring the blood is squeezed up into
  the arteries.

3
Altering Heart Rate and Stroke Volume

• The heart rate is the number of beats per minute
  and the stroke volume is the volume of blood
  pumped.
• Cardiac output heart rate x stroke volume
• During exercise the cardiac output is greater as
  the delivery of oxygen and removal of carbon
  dioxide must increase.
• Cardiac output increases as the flow of blood
  increases with exercise, stretching the muscles
  of the SAN which responds by increasing the rate
  at which action potentials are fired, increasing
  the heart rate slightly.
• At the same time the increased volume stretches
  the muscles more which respond by contracting
  harder and increasing the stroke volume.
• With even greater exercise levels the heart is
  further stimulated by the cardiovascular centre
  in the brain which sends impulses along
  sympathetic nerves, speeding up the heart rate
  and increasing stroke volume. Impulses sent along
  the vagus (parasympathetic) nerve have the
  opposite effect.

4
Investigating Heart/Pulse Rate

• Measure your heart/pulse rate while at rest and
  after gentle and vigorous exercise.
• How do your readings compare to typical pulse
  rates?
• Explain how these rates are controlled by the
  body in each case.

5
Blood Pressure

• This is the pressure which the blood exerts
  against the walls of the vessel in which it
  flows.
• Blood pressure goes up and down in the arteries
  but gradually decreases once it leaves the heart,
  smoothing out as it moves further away from the
  heart.
• This gradual decrease is due to the frictional
  resistance in the blood vessels which is greatest
  in the very small arterioles.
• Although the capillaries have an even smaller
  diameter than the arterioles as there are so many
  of them the pressure drop is not so great.

6
Measuring Blood Pressure

• Blood pressure is measured using a
  sphygmomanometer.
• This measures the pressure in the brachial artery
  just below your elbow.
• A cuff is wrapped around the upper arm and
  inflated with air until blood flow is stopped.
• Whilst listening to the artery at the elbow air
  is slowly released until blood can be heard
  passing into the artery.
• At this point the pressure exerted by the cuff is
  the same as the maximum pressure in the artery.
• This value is the systolic blood pressure, i.e.
  that produced during contraction of the heart.
• The cuff is then deflated further and when sound
  of the blood flow disappears totally it signals
  that blood can get through even at its minimum
  pressure.
• This is the diastolic pressure, i.e. the pressure
  in the artery while the heart is relaxing.

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Healthy Readings

• In a healthy person the systolic blood pressure
  at rest is about 120 mm Hg.
• The diastolic pressure is about 80 mm Hg.
• This is written as 120/80.

8
Exercise and Blood Pressure

• The sympathetic and parasympathetic nerves can be
  affected by blood pressure.
• Inside the aorta are nerve endings which are
  sensitive to stretching, These are called
  baroreceptors or stretch receptors.
• If the blood pressure increases then the artery
  walls are stretched and stimulate these nerve
  endings.
• Impulses are sent to the brain and the
  parasympathetic (vagus) nerve is stimulated which
  sends signals to the heart slowing heart rate and
  reducing stroke volume.
• Low blood pressure has the opposite effect. No
  stretching occurs so no signals are sent to the
  cardiovascular centre in the brain. Impulses are
  sent down the sympathetic nerve increasing
  cardiac output. Messages are also sent to the
  muscles in the arteriole walls which contact and
  narrow, increasing the blood pressure.

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What Factors Affect Blood Pressure?

• Standing up after sitting for a while causes the
  blood pressure to drop.
• This is because when you relax your heart works
  at a fairly low rate.
• When you stand the blood pressure in the vessels
  in your head and upper body drops causing you to
  feel faint.
• However baroreceptors detect this drop and set
  off the processes which will increase your blood
  pressure, including an increase in cardiac output.

10
Investigating Blood Pressure

• Measure your blood pressure whilst lying down,
  sitting and standing.
• Try to explain your results using the information
  given during this lesson.

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