Cardiac Muscle

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Cardiac Muscle

• Found only in heart
• Striated
• Each cell usually has one nucleus
• Has intercalated disks and gap junctions
• Autorhythmic cells
• Action potentials of longer duration and longer
  refractory period
• Ca2 regulates contraction

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Cardiac Muscle

• Elongated, branching cells containing 1-2
  centrally located nuclei
• Contains actin and myosin myofilaments
• Intercalated disks Specialized cell-cell
  contacts
• Desmosomes hold cells together and gap junctions
  allow action potentials
• Electrically, cardiac muscle behaves as single
  unit

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Cardiac myocyte action potential
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Refractory Period

• Absolute Cardiac muscle cell completely
  insensitive to further stimulation
• Relative Cell exhibits reduced sensitivity to
  additional stimulation
• Long refractory period prevents tetanic
  contractions

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AP-contraction relationship

• AP in skeletal muscle is very short-lived
• AP is basically over before an increase in muscle
  tension can be measured.
• AP in cardiac muscle is very long-lived
• AP has an extra component, which extends the
  duration.
• The contraction is almost over before the action
  potential has finished.

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Functions of the Heart

• Generating blood pressure
• Routing blood
• Heart separates pulmonary and systemic
  circulations
• Ensuring one-way blood flow
• Heart valves ensure one-way flow
• Regulating blood supply
• Changes in contraction rate and force match blood
  delivery to changing metabolic needs

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Orientation of cardiac muscle fibres

• Unlike skeletal muscles, cardiac muscles have to
  contract in more than one direction.
• Cardiac muscle cells are striated, meaning they
  will only contract along their long axis.
• In order to get contraction in two axis, the
  fibres wrap around.

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Circulation circuits
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Heart Wall

• Three layers of tissue
• Epicardium This serous membrane of smooth outer
  surface of heart
• Myocardium Middle layer composed of cardiac
  muscle cell and responsibility for heart
  contracting
• Endocardium Smooth inner surface of heart
  chambers

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Valve function
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Coronary circulation
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Cardiac conducting system
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Pacemaker potential
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EKG
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Heart sounds
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Heart Sounds

• First heart sound or lubb
• Atrioventricular valves and surrounding fluid
  vibrations as valves close at beginning of
  ventricular systole
• Second heart sound or dupp
• Results from closure of aortic and pulmonary
  semilunar valves at beginning of ventricular
  diastole, lasts longer
• Third heart sound (occasional)
• Caused by turbulent blood flow into ventricles
  and detected near end of first one-third of
  diastole

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Cardiac Arrhythmias

• Tachycardia Heart rate in excess of 100bpm
• Bradycardia Heart rate less than 60 bpm
• Sinus arrhythmia Heart rate varies 5 during
  respiratory cycle and up to 30 during deep
  respiration
• Premature atrial contractions Occasional
  shortened intervals between one contraction and
  succeeding, frequently occurs in healthy people

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Cardiac Cycle

• Heart is two pumps that work together, right and
  left half
• Repetitive contraction (systole) and relaxation
  (diastole) of heart chambers
• Blood moves through circulatory system from areas
  of higher to lower pressure.
• Contraction of heart produces the pressure

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Pressure relationships
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Mean Arterial Pressure (MAP)

• Average blood pressure in aorta
• MAPCO x PR
• CO is amount of blood pumped by heart per minute
• COSV x HR
• SV Stroke volume of blood pumped during each
  heart beat
• HR Heart rate or number of times heart beats per
  minute
• Cardiac reserve Difference between CO at rest
  and maximum CO
• PR is total resistance against which blood must
  be pumped

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Factors Affecting MAP
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Regulation of the Heart

• Intrinsic regulation Results from normal
  functional characteristics, not on neural or
  hormonal regulation
• Starlings law of the heart
• Extrinsic regulation Involves neural and
  hormonal control
• Parasympathetic stimulation
• Supplied by vagus nerve, decreases heart rate,
  acetylcholine secreted
• Sympathetic stimulation
• Supplied by cardiac nerves, increases heart rate
  and force of contraction, epinephrine and
  norepinephrine released

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Heart Homeostasis

• Effect of blood pressure
• Baroreceptors monitor blood pressure
• Effect of pH, carbon dioxide, oxygen
• Chemoreceptors monitor
• Effect of extracellular ion concentration
• Increase or decrease in extracellular K
  decreases heart rate
• Effect of body temperature
• Heart rate increases when body temperature
  increases, heart rate decreases when body
  temperature decreases

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Baroreceptor and ChemoreceptorReflexes
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Cardian innervation
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Pacemaker regulation

• Once the pacemaker cells reach threshold, the
  magnitude and duration of the AP is always the
  same.
• In order to change the frequency, the time
  between APs must vary.
• The interval can only be changed in two ways.
• The rate of depolarization can be changed
• The amount of depolarization required to reach
  threshold can be changed.

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Vascular physiology
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Peripheral Circulatory System

• Systemic vessels
• Transport blood through most all body parts from
  left ventricle and back to right atrium
• Pulmonary vessels
• Transport blood from right ventricle through
  lungs and back to left atrium
• Blood vessels and heart regulated to ensure blood
  pressure is high enough for blood flow to meet
  metabolic needs of tissues

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Blood Vessel Structure

• Arteries
• Elastic, muscular, arterioles
• Capillaries
• Blood flows from arterioles to capillaries
• Most of exchange between blood and interstitial
  spaces occurs across the walls
• Blood flows from capillaries to venous system
• Veins
• Venules, small veins, medium or large veins

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Structure of Arteries and Veins

• Three layers except for capillaries and venules
• Tunica intima (interna)
• Endothelium
• Tunica media
• Vasoconstriction
• Vasodilation
• Tunica adventitia (externa)
• Merges with connective tissue surrounding blood
  vessels
• Note mistake on figure

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Structure of Arteries

• Elastic or conducting arteries
• Largest diameters, pressure high and fluctuates
• Muscular or medium arteries
• Smooth muscle allows vessels to regulate blood
  supply by constricting or dilating
• Arterioles
• Transport blood from small arteries to capillaries

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Structure of Veins

• Venules and small veins
• Tubes of endothelium on delicate basement
  membrane
• Medium and large veins
• Valves
• Allow blood to flow toward heart but not in
  opposite direction
• Atriovenous anastomoses
• Allow blood to flow from arterioles to small
  veins without passing through capillaries

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Blood Vessel Comparison
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Capillaries

• Capillary wall consists mostly of endothelial
  cells
• Types classified by diameter/permeability
• Continuous
• Do not have fenestrae
• Fenestrated
• Have pores
• Sinusoidal
• Large diameter with large fenestrae

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Capillary Network

• Blood flows from arterioles through
  metarterioles, then through capillary network
• Venules drain network
• Smooth muscle in arterioles, metarterioles,
  precapillary sphincters regulates blood flow

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Muscular contractions aid venous return
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Pulmonary Circulation

• Moves blood to and from the lungs
• Pulmonary trunk
• Arises from right ventricle
• Pulmonary arteries
• Branches of pulmonary trunk which project to
  lungs
• Pulmonary veins
• Exit each lung and enter left atrium

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Systemic Circulation Arteries

• Aorta
• From which all arteries are derived either
  directly or indirectly
• Parts
• Ascending, descending, thoracic, abdominal
• Coronary arteries
• Supply the heart

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Systemic Circulation Veins

• Return blood from body to right atrium
• Major veins
• Coronary sinus (heart)
• Superior vena cava (head, neck, thorax, upper
  limbs)
• Inferior vena cava (abdomen, pelvis, lower limbs)
• Types of veins
• Superficial, deep, sinuses

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Dynamics of Blood Circulation

• Interrelationships between
• Pressure
• Flow
• Resistance
• Control mechanisms that regulate blood pressure
• Blood flow through vessels

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Blood Pressure

• Measure of force exerted by blood against the
  wall
• Blood moves through vessels because of blood
  pressure
• Measured by listening for Korotkoff sounds
  produced by turbulent flow in arteries as
  pressure released from blood pressure cuff

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Pressure and Resistance

• Blood pressure averages 100 mm Hg in aorta and
  drops to 0 mm Hg in the right atrium
• Greatest drop in pressure occurs in arterioles
  which regulate blood flow through tissues
• No large fluctuations in capillaries and veins

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Blood Pressure Measurement
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Pulse Pressure

• Difference between systolic and diastolic
  pressures
• Increases when stroke volume increases or
  vascular compliance decreases
• Pulse pressure can be used to take a pulse to
  determine heart rate and rhythmicity

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Blood Flow, Poiseuilles Lawand Viscosity

• Blood flow
• Amount of blood moving through a vessel in a
  given time period
• Directly proportional to pressure differences,
  inversely proportional to resistance

• Poiseuilles Law
• Flow decreases when resistance increases
• Flow resistance decreases when vessel diameter
  increases
• Viscosity
• Measure of resistance of liquid to flow
• As viscosity increases, pressure required to flow
  increases

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Critical Closing Pressure, Laplaces Law and
Compliance

• Vascular compliance
• Tendency for blood vessel volume to increase as
  blood pressure increases
• More easily the vessel wall stretches, the
  greater its compliance
• Venous system has a large compliance and acts as
  a blood reservoir

• Critical closing pressure
• Pressure at which a blood vessel collapses and
  blood flow stops
• Laplaces Law
• Force acting on blood vessel wall is proportional
  to diameter of the vessel times blood pressure

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Physiology of Systemic Circulation

• Determined by
• Anatomy of circulatory system
• Dynamics of blood flow
• Regulatory mechanisms that control heart and
  blood vessels
• Blood volume
• Most in the veins
• Smaller volumes in arteries and capillaries

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Cross-Sectional Area

• As diameter of vessels decreases, the total
  cross-sectional area increases and velocity of
  blood flow decreases
• Much like a stream that flows rapidly through a
  narrow gorge but flows slowly through a broad
  plane

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Laminar and Turbulent Flow

• Laminar flow
• Streamlined
• Outermost layer moving slowest and center moving
  fastest
• Turbulent flow
• Interrupted
• Rate of flow exceeds critical velocity
• Fluid passes a constriction, sharp turn, rough
  surface

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Aging of the Arteries

• Arteriosclerosis
• General term for degeneration changes in arteries
  making them less elastic
• Atherosclerosis
• Deposition of plaque on walls

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Capillary Exchange andInterstitial Fluid Volume
Regulation

• Blood pressure, capillary permeability, and
  osmosis affect movement of fluid from capillaries
• A net movement of fluid occurs from blood into
  tissues. Fluid gained by tissues is removed by
  lymphatic system.

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Fluid Exchange Across Capillary Walls
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Vein Characteristics andEffect of Gravity on
Blood Pressure

• Vein Characteristics
• Venous return to heart increases due to increase
  in blood volume, venous tone, and arteriole
  dilation

• Effect of Gravity
• In a standing position, hydrostatic pressure
  caused by gravity increases blood pressure below
  the heart and decreases pressure above the heart

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Control of Blood Flow by Tissues

• Local control
• In most tissues, blood flow is proportional to
  metabolic needs of tissues
• Nervous System
• Responsible for routing blood flow and
  maintaining blood pressure
• Hormonal Control
• Sympathetic action potentials stimulate
  epinephrine and norepinephrine

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Local Control of Blood Flow by Tissues

• Blood flow can increase 7-8 times as a result of
  vasodilation of metarterioles and precapillary
  sphincters in response to increased rate of
  metabolism
• Vasodilator substances produced as metabolism
  increases
• Vasomotion is periodic contraction and relaxation
  of precapillary sphincters

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Nervous Regulation of Blood Vessels
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Short-Term Regulation ofBlood Pressure

• Baroreceptor reflexes
• Change peripheral resistance, heart rate, and
  stroke volume in response to changes in blood
  pressure
• Chemoreceptor reflexes
• Sensory receptors sensitive to oxygen, carbon
  dioxide, and pH levels of blood
• Central nervous system ischemic response
• Results from high carbon dioxide or low pH levels
  in medulla and increases peripheral resistance

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Baroreceptor Reflex Control
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Local mechanisms affect MAP
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Effects of pH and Gases
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Long-Term Regulation of Blood Pressure

• Renin-angiotensin-aldosterone mechanism
• Vasopressin (ADH) mechanism
• Atrial natriuretic mechanism
• Fluid shift mechanism
• Stress-relaxation response

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General control of MAP
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Renin-Angiotensin-AldosteroneMechanism
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Vasopressin (ADH) Mechanism
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Long Term MechanismsWhich Lower Blood Volume

• Fluid shift
• Movement of fluid from interstitial spaces into
  capillaries in response to decrease in blood
  pressure to maintain blood volume
• Stress-relaxation
• Adjustment of blood vessel smooth muscle to
  respond to change in blood volume

• Atrial natriuretic factor
• Hormone released from cardiac muscle cells when
  atrial blood pressure increases, simulating an
  increase in urinary production, causing a
  decrease in blood volume and blood pressure

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Chemoreceptor Reflex Control
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Shock

• Inadequate blood flow throughout body
• Three stages
• Compensated Blood pressure decreases only a
  moderate amount and mechanisms able to
  reestablish normal blood pressure and flow
• Progressive Compensatory mechanisms inadequate
  and positive feedback cycle develops cycle
  proceeds to next stage or medical treatment
  reestablishes adequate blood flow to tissues
• Irreversible Leads to death, regardless of
  medical treatment

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Fetal circulation
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