Chapter 19: Physiology of the Cardiovascular System

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Chapter 19 Physiology of the Cardiovascular
System
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INTRODUCTION

• MAJOR FUNCTION OF THE CV SYSTEM DEPENDS ON
  CONTINOUS AND CONTROLLED FLOW OF BLOOD THROUGH
  CAPILLARIES
• Major Function of the CV System Transportation
• Transportation depends on flow of blood through
  capillaries
• - Blood flow through capillaries must be
• Continuous ? To meet cells needs
• Controlled (changed) ? To meet the changing needs
  of cells
• How is this accomplished?
• Homoeostatic control mechanisms (hemodynamics)
• Hemodynamics collection of mechanisms that
  influence the dynamic (active and changing)
  circulation of blood

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Conduction System of the Heart

• Conduction system of the heart (Figure 19-2)
• System responsible for conducting nerve impulses
  over the heart
• The action potentials (impulses) of the heart
  that trigger contractions must be coordinated
  carefully.
• Composed of four major structures (all modified
  cardiac muscle)
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• AV bundle (bundle of His)
• Subendocardial branches (Purkinje fibers)

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Conduction System of the Heart (cont.)

• SINOATRIAL (SA) NODE
• Location in the right atrium near the opening of
  the superior vena cava
• Pacemaker of the Heart
• Nerve impulses start here
• Discharges a set of nerve impulses per minute
• ATRIOVENTRICULAR (AV) NODE
• Location in the right atrium along the
  interatrial septum
• ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS)
• Location originates in the AV node, spreads down
  the interventricular septum in 2 branches - L R
  
• PURKINGE FIBERS
• Location extensions of the AV Bundle into the
  walls of the ventricles

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Sequence of Cardiac Stimulation

• Specific sequence
• SA node discharges a nerve impulse ? travels to
  LA and to AV node ? atria contract
• Nerve impulse travels from AV node to AV bundle
  (LR branches) to purkinge fibers ? ventricles
  contract
• Result 1 complete cardiac cycle (pumping cycle),
  Assoc. with 1 heartbeat
• Process repeats
• Interatrial bundle of conducting fibers
  facilitates rapid conduction to left atrium
• As signal enters AV node through internodal
  bundles of conducting fibers, conduction slows,
  permitting contraction of both atrial chambers
  before impulse reaches the ventricles

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Electrocardiogram (ECG)

• Measures hearts electrical activity (graphic
  record)
• Provides a record of the electrical events that
  precede the contractions of the heart
• Electrodes of an electrocardiograph are attached
  to the subject
• Changes in voltage are recorded that represent
  changes in the hearts electrical activity
  (Figure 19-4)
• EKG waves
• Normal waves
• P wave depolarization of the atria
• QRS complex depolarization of the ventricles,
  repolarization of the atria
• T wave repolarization of the ventricles
• Measurement of the intervals between P, QRS, and
  T waves can provide information about the rate of
  conduction of an action potential through the
  heart
• Clinical significance
• EKG can show problems related to the spread of
  nerve impulses over the conduction system

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Cardiac Cycle

• Cardiac cycle a complete heartbeat consisting of
  contraction (systole) and relaxation (diastole)
  of both atria and both ventricles (5 steps)
• The cycle is often divided into time intervals
• Step 1 Atrial systole
• Contraction of atria completes emptying of blood
  out of the atria into the ventricles
• AV valves are open semiluminar (SL) valves are
  closed
• Ventricles are relaxed and fill with blood
• This cycle begins with the P wave of the ECG

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Important Events of the Cardiac Cycle

• Step 2 Isovolumetric ventricular contraction
• Occurs between the start of ventricular systole
  and the opening of the SL valves
• Ventricular volume remains constant as the
  pressure increases rapidly
• Onset of ventricular systole coincides with the R
  wave of the ECG and the appearance of the first
  heart sound
• Step 3 Ejection
• SL valves open and blood is ejected from the
  heart when the pressure gradient in the
  ventricles exceeds the pressure in the pulmonary
  artery and aorta
• Rapid ejection initial short phase characterized
  by a marked increase in ventricular and aortic
  pressure and in aortic blood flow
• Reduced ejection characterized by a less-abrupt
  decrease in ventricular volume coincides with
  the T wave of the ECG

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Important Events of the Cardiac Cycle (cont.)

• Step 4 Isovolumetric ventricular relaxation
• Ventricular diastole begins with this phase
• Occurs between closure of the SL valves and
  opening of the AV valves
• A dramatic fall in intraventricular pressure but
  no change in volume
• The second heart sound is heard during this
  period
• Step 5 Passive ventricular filling
• Returning venous blood increases intra-atrial
  pressure until the AV valves are forced open and
  blood rushes into the relaxing ventricles
• Influx lasts approximately 0.1 second and results
  in a dramatic increase in ventricular volume
• Diastasis later, longer period of slow
  ventricular filling at the end of ventricular
  diastole lasting approximately 0.2 second
  characterized by a gradual increase in
  ventricular pressure and volume

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Heart Sounds

• During each cardiac cycle the heart makes sounds.
• Systolic sound (contraction sound)
• First sound, believed to be caused primarily by
  the contraction of the ventricles and vibrations
  of the closing AV valves (step 2 of the cardiac
  cycle)
• Heart sound - lubb-dupp
• Diastolic sound (relaxation sound)
• short, sharp sound thought to be caused by
  vibrations of the closing of SL valves
• Step 4 of cardiac cycle
• Heart sound dupp
• Heart sounds have clinical significance because
  they provide information about the functioning of
  the valves of the heart
• Heart murmur
• Abnormal heart sound - swishing

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PRIMARY PRINCIPLE OF CIRCULATION

• Blood flows because of a pressure gradient
• high pressure (aorta 100 mm Hg) ? low pressure
  (venae cavae 0 mm Hg)
• Pumping action causes fluctuation in aortic blood
  pressure (systolic 120 mm Hg diastolic 80 mm Hg)
• Blood circulates from the left ventricle to the
  right atrium of the heart because of blood
  pressure gradient
• Measurement of blood flow is based on Newtons
  first and second law of motion
• P1-P2 is the symbol used to represent a pressure
  gradient, with P1 representing the higher
  pressure and P2 the lower pressure
• Perfusion pressure pressure gradient needed to
  maintain blood flow through a local tissue

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Arterial Blood Pressure

• High blood pressure must be maintained in the
  arteries to keep blood flowing in the CV system
• Primary determinant of arterial blood pressure is
  the volume of blood in the arteries
• A direct relation exists between arterial blood
  volume and arterial pressure (Figure 19-10)
• Cardiac output volume of blood pumped out of a
  ventricle of the heart per unit of time (ml/min
  or L/min)
• General principles and definitions
• Cardiac output (CO) determined by stroke volume
  and heart rate
• Stroke volume (SV) volume pumped per heartbeat
• CO (volume/min) SV (volume/beat) ? HR
  (beats/min)
• In practice, CO is computed by Ficks formula
• Heart rate and SV determine CO, so anything that
  changes either also tends to change CO, arterial
  blood volume, and blood pressure in the same
  direction

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Arterial Blood Pressure

• Relationship between arterial blood volume and
  blood pressure.

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Factors That Affect Stroke Volume

• Starlings law of the heart (Frank-Starling
  mechanism)
• Mechanical factor that affects stroke volume
• The longer, or more stretched, the heart fibers
  at the beginning of contraction, the stronger the
  contraction (i.e. the more blood returned to the
  heart, the stronger the contraction)
• The amount of blood in the heart at the end of
  diastole determines the amount of stretch placed
  on the heart fibers
• Exceptions Too much stretching of cardiac
  muscle fibers has the opposite effect (i.e. makes
  the contraction less strong)
• The myocardium contracts with enough strength to
  match its pumping load (within certain limits)
  with each stroke, unlike mechanical pumps

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Factors that Affect Stroke Volume

• Contractility (strength of contraction) can also
  be influenced by chemical factors (Figure 19-13)
• Neural factors
• Norepinephrine
• Endocrine factors
• Epinephrine
• Mechanical factors
• Triggered by stress, exercise

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Factors that Affect Heart Rate

• SA node normally initiates each heartbeat BUT the
  rate of the heartbeat can be altered
• HOW?
• 1. Cardiac pressor reflexes (pressoreflexes)
• Receptors sensitive to changes in pressure
  (baroreflexes)
• Ex. aortic baroreceptors and carotid
  baroreceptors
• located in the aorta and carotid sinus
• Send afferent nerve fibers to cardiac control
  centers in medulla oblongata
• Work with integrators in the cardiac control
  centers through negative feedback loop called
  pressoreflexes or baroreflexes to oppose changes
  in pressure by adjusting heart rate

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Factors that Affect Heart Rate (cont.)

• 2. Carotid sinus reflex (negative feedback loop)
• Sensory fibers from carotid sinus baroreceptors
  run through the carotid sinus nerve and the
  glossopharyngeal nerve to the cardiac control
  center
• Parasympathetic impulses leave the cardiac
  control center, travel through the vagus nerve to
  reach the SA node
• Acetylcholine released from vagus fibers
  decreases the rate of SA firing and heart rate
• Vagal inhibition break of the heart
• Aortic reflex (negative feedback loop)
• Sensory fibers extend from baroreceptors located
  in the wall of the arch of the aorta through the
  aortic nerve and through the vagus nerve to
  terminate in the cardiac control center
• END RESULT Decreased heart rate

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Other Reflexes that Influence Heart Rate
? Heart Rate ? Heart Rate
Anxiety, fear, and anger Grief
Exercise Decreased blood temperature
Increased blood temperature Stimulation of skin cold receptors
Stimulation of skin heat receptors Norepinephrine (released from sympathetic response)
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Peripheral Resistance

• Helps determine aterial blood pressure
• Definition - resistance to blood flow imposed by
  the force of friction between blood and the walls
  of its vessels
• Factors that influence peripheral resistance
• 1. Blood viscosity the thickness of blood as a
  fluid
• High plasma protein concentration can slightly
  increase blood viscosity
• High hematocrit can increase blood viscosity
• Anemia, hemorrhage, or other abnormal conditions
  may also affect blood viscosity
• Means blood meets friction as it flows

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Factors that Affect Peripheral Resistance

• 2. Diameter of arterioles (Figure 19-17)
• Vasomotor control mechanism muscles in walls of
  arteriole may constrict (vasoconstriction) or
  dilate (vasodilation), thus changing diameter of
  arteriole
• Controls amount of blood that runs from arteries
  to arterioles
• Small changes in blood vessel diameter cause
  large changes in resistance
• Means blood meets resistance in arteries as it
  flows (ideal control system)

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Parts of the Vasomotor Control Mechanism

• Vasomotor center or vasoconstrictor center
  area in the medulla
• When stimulated initiates an impulse outflow by
  sympathetic fibers that ends in the smooth
  muscle surrounding resistant vessels, arterioles,
  venules, and veins of the blood reserviors
  causing their constriction
• Vasomotor pressoreflexes
• Initiated by change in aterial blood pressure
• The change stimulates aortic and carotid
  baroreceptors
• Results in arterioles and venules of the blood
  reservoirs dilating
• Decrease in arterial blood pressure results in
  stimulation of vasoconstrictor centers, causing
  vascular smooth muscle to constrict

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Parts of the Vasomotor Control Mechanism (cont.)

• 3. Vasomotor chemoreflexes
• - chemoreceptors located in aortic and carotid
  bodies are sensitive to hypercapnia, hypoxia, and
  decreased arterial blood pH
• 4. Medullary ischemic reflex
• - acts during emergency situation when blood
  flow to the medulla is decreased causes marked
  arteriole and venous constriction
• 5. Higher brain centers
• - impulses from centers in cerebral cortex and
  hypothalamus transmitted to vasomotor centers in
  medulla to help control vasoconstriction and
  dilation

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VENOUS RETURN TO THE HEART

• Venous return amount of blood returned to the
  heart by the veins (venous blood deoxygenated
  blood)
• Affected by several factors
• Stress-relaxation effect occurs when a change in
  blood pressure causes a change in vessel diameter
  (because of elasticity) and thus adapts to the
  new pressure to keep blood flowing (works only
  within certain limits)
• Gravity the pull of gravity on venous blood
• While sitting or standing tends to cause a
  decrease in venous return (orthostatic effect)
• Venous pumps help to overcome the influence of
  gravity to maintain the pressure gradient of
  blood
• Pump unoxygenated blood back to the heart
• 2 kinds
• Respirations
• Skeletal muscle contractions

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Mechanisms of Venous Pumps

• Respirations Create pressure changes that act as
  venous pumps
• During inspiration pressure changes cause blood
  to be pumped from abdominal vena cava to thoracic
  vena cava
• During expiration pressure changes cause blood
  to be pumped into the atria
• Skeletal muscle contractions promote venous
  return by squeezing veins through a contracting
  muscle and milking the blood toward the heart
• Contraction squeezes veins within ? pumps blood
  toward heart
• One-way valves in veins prevent backflow

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Total Blood Volume

• Total blood volume changes in total blood volume
  change the amount of blood returned to the heart
• HOW?
• Capillary exchange governed by Starlings law of
  the capillaries (Figure 19-26)
• At arterial end of capillary, outward hydrostatic
  pressure is strongest force moves fluid out of
  plasma and into intracellular fluid
• At venous end of capillary, inward osmotic
  pressure is strongest force moves fluid into
  plasma from intracellular fluid 90 of fluid
  lost by plasma at arterial end is recovered
• Lymphatic system recovers fluid not recovered by
  capillary and returns it to the venous blood
  before it is returned to the heart
• Note If lymphatic system operates normally there
  is no net loss of blood volume resulting from
  capillary exchange

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Mechanisms that Change Total Blood Volume

• Mechanisms that change total blood volume most
  quickly cause water to move into or out of the
  plasma
• Antidiuretic hormone mechanism
• Involves secretion/release of ADH (water
  retention)
• Increases TBV and venous return
• Renin- Angiotension Mechanism
• Involves secretion of aldosterone (sodium
  retention followed by water retention)
• Increases TBV and venous return
• Atrial natriuretic peptide mechanism
• Involves secretion of atrial natriuretic hormone
  (sodium loss, followed by water loss)
• Decreases TBV and venous return

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MEASURING BLOOD PRESSURE

• Arterial blood pressure
• Measured with a sphygmomanometer and stethoscope
  listen for Korotkoff sounds as the pressure in
  the cuff is gradually decreased (Figure 19-29)
• Systolic blood pressure force of the blood
  pushing against the artery walls while ventricles
  are contracting
• Diastolic blood pressure force of the blood
  pushing against the artery walls when ventricles
  are relaxed
• Pulse pressure difference between systolic and
  diastolic blood pressure

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MINUTE VOLUME OF BLOOD

• The volume of blood circulating through the body
  per minute minute blood volume
• Determined by magnitude of the blood pressure
  gradient and peripheral resistance
• Caculated based on mathematical equation
• (Poiseuilles Law) Minute volume Pressure
  gradient (mean arterial BP - central venous BP) ?
  Resistance
• MV Minute Volume (volume of blood circulated
  per minute)
• BP Blood Pressure
• PR Peripheral Resistance
• PR Has 2 effects on circulation
• 1. PR can increase circ (increases artery blood
  volume)
• 2. PR can decrease circ (allows less blood to
  flow)
• Relation to arterial and venous bleeding
• Arterial bleeding blood escapes from artery in
  spurts because of alternating increase and
  decrease of arterial blood pressure
• Venous bleeding blood flows slowly and steadily
  because of low, nearly constant pressure

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FACTORS THAT INFLUENCE THE FLOW OF BLOOD
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VELOCITY OF BLOOD FLOW

• Velocity of blood is governed by the physical
  principle that states when a liquid flows from an
  area of one cross-sectional size to an area of
  larger size, its velocity decreases in the area
  with the larger cross section (Figure 19-31)
• Blood flows fastest in arteries, slowest in
  capillaries
• Venule cross-sectional area is smaller than
  capillary cross-sectional area, causing blood
  velocity to increase in venules and veins

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PULSE

• Pulse alternate expansion and recoil of an
  artery (Figure 19-32)
• Causes LV contraction and relaxation
  elasticity of artery walls
• Clinical significance reveals important
  information regarding the cardiovascular system,
  blood vessels, and circulation
• Physiological significance expansion stores
  energy released during recoil, conserving energy
  generated by the heart and maintaining relatively
  constant blood flow (Figure 19-33)
• Pulse wave
• Spread of pulse through arteries, each LV
  contraction starts a new pulse wave that spreads
  as a wave throughout arteries
• Where is the pulse felt?
• Superficial arteries that lie over a firm surface
• Examples Radial artery, common carotid artery,
  brachial artery
• Venous pulse
• Detectable in large veins that lie near the heart
• Due to contraction/relaxation of the atria