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Physiology of Heart
• Heart muscle cells contract,
without nerve impulses, in a
regular, continuous way
• Heart is autorhythmic
• Initiate, conduct and
impulse
• Heart contains special tissue
that produces & sends
electrical impulses to the
heart muscle to contract
Physiology of Heart
• Autorhythmic cardiac
cells are found in the
following areas:
• Sinoatrial (SA) node
• Atrioventricular
(AV)node
• Bundle of His
• Bundle branches
• Purkinje fibers
Physiology of Heart
– Sinoatrial (SA) Node
• Electrical impulse that
causes rhythmic contraction
of heart muscles arises in
the SA node
• Located in R. atrium
• Pacemaker of the heart
• Generates impulses 70 to 80
times a minute
• The electrical impulse from
the SA node spreads over
the right and left atria
• Causes atrial contraction
Physiology of Heart
•
AV Node:
• Then impulses are
conducted to the
atrioventicular (AV) node
• Impulse is delayed at AV
node for 0.1 sec
• Allows completion of atrial
contraction before
venticular contraction
begins
Physiology of Heart
•
Bundle of His (AV bundle)
• Then electrical impulse is
relayed down to Bundle of His
• Bundle of His passes impulse to
right and left bundle branches
•
•
•
Bundle Branches
Right and left Bundle branches
Branch into purkinje fibers
•
Purkinje Fibers
• Enter myocardium of
ventricle walls, and apex of
the heart
• Purkinje fibers transmit
the impulse first to apex of
the heart
• Contraction begins at apex
and pushes the blood to
aorta and pulmonary
trunk
CARDIAC CONDUCTION SYSTEM SUMMARY
Sinoatrial Node
AV Node
AV Bundle
Bundle Branches
Purkinje Fibers
• Conducted cell to cell
• Takes 200-500 ms to complete
• Resting membrane potential is
electronegative
• Unstable resting membrane potential
• Continuously depolarize
• AP takes place in SA node
• When spontaneously changing
potentials, called prepotential
reaches threshold
• Voltage gated Na+ channels
open, Na + influx, K+ channels
close
• Depolarization takes place
• Depolarized to +20 mV
• Repolarization takes place
• Na+
channels close, K+ channels
open
• Conduction
of action potential
produces
electric current that can be
measured
at the surface of the body
• P wave: Atrial
depolarization
• QRS complex: Ventricular
depolarization
• T wave: Ventricular
repolarization
Alterations in an
Electrocardiogram
Normal
SA Node Dysfunction
no P waves
Ventricular Fibrillation
Cardiac Cycle
• Heart is two pumps that work together, right and left
half
•
Each pump consists of
•
Primer pump – Atrium
•
Power pump – Ventricle
Cardiac Cycle
• Cardiac cycle: Is the sequence of
events in one heartbeat
• It is the repetitive pumping
process that begins with onset of
cardiac muscle contraction
and ends with beginning of next
contraction
• Cardiac muscle contraction is
responsible for pressure and
blood movement. How?
• Blood moves from high pressure
to low pressure
Cardiac Cycle
• The length of cardiac cycle is about 0.8 sec
• Interval from end of one contraction to the following contraction
• Consists of Two Phases:
– Systole phase
– Diastole phase
CARDIAC CYCLE
• Systole Phase
semilunar valves
(closed)
– Contraction phase
– Blood ejected
– Atrial Systole (0.1 sec.)
• Following passive filling
with blood
LA
• Atrial pressure rises above
ventricular pressure
• And AV valves open,
semilunar valves closed
• Ventricles fill with blood
tricuspid
(open)
bicuspid
(open)
RA
LV
RV
CARDIAC CYCLE
semilunar valves
(open)
• Systole Phase (cont.)
– Ventricular Systole (0.3
sec.)
• AV and semilunar
valves closed until
pressure opens
semilunar valves
• Blood pushed into
pulmonary trunk and
Aorta
• 120 mm Hg pressure
• Atria in diastole
LA
bicuspid
(closed)
RA
LV
tricuspid
(closed)
RV
CARDIAC CYCLE
semilunar valves
(closed)
• Diastole Phase
– Relaxation phase
– Ventricular Diastole
• Follows ventricular
systole
• AV valves reopen and
filling begins
• 80 mm Hg pressure
LA
RA
LV
tricuspid
(open)
RV
bicuspid
(open)
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
Mean Arterial Blood Pressure
• BP is important for blood movement
• Blood flows from higher to lower pressure
• During one cardiac cycle, blood flows from high pressure in aorta
from contraction of ventricles
• Then towards the lower pressure in relaxed R. atrium
• Mean Arterial Pressure (MAP) = CO x PR
– CO (Cardiac output) is amount of blood pumped by heart per
minute
– PR (Peripheral resistance) is total resistance against which blood
must be pumped
Cardiac Output
• CO = HR x SV
– HR: Heart rate (number of times heart beats per
minute)
– SV: Stroke volume (blood pumped during each heart
beat)
CO = 72 bpm X 70 ml/beat
= 5040ml/min (app. 5L/min)
• Starling’s law of the heart— the more the cardiac muscle is
stretched, the stronger the contraction
• Important factor for stretching the heart muscle is venous
return
• Greater the volume of blood returned to the heart by the veins,
Greater the volume of blood the heart will pump
Regulation of the Heart
• To maintain homeostasis, amount of blood pumped by
heart must vary:
•
Eg. Cardiac output increases more during exercise than
resting
• Intrinsic regulation: Results from normal functional
characteristics of heart, not depend on neural or
hormonal regulation
Regulation of the Heart
• Extrinsic regulation: Involves neural and
hormonal control
• Neural Control
– Parasympathetic stimulation
• Supplied by vagus nerve,
acetylcholine is secreted,
decreases heart rate, maintain
heart beat average of 70 beats/min.
– Sympathetic stimulation
– Supplied by cardiac nerves
– Increases heart rate and force of
contraction.
– Epinephrine and norepinephrine released.
– Increased heart beat causes increased
cardiac output
Regulation of the Heart
– Hormonal Control
– Epinephrine and
norepinephrine from
the adrenal medulla
– Increases rate and
force of heart
contraction
– Occurs in response to
increased physical
activity, emotional
excitement, stress
Heart and Homeostasis
• Effect of blood pressure
– Baroreceptors monitor blood
pressure; in walls of internal
carotids and aorta. This
sensory information goes to
centers in the medulla
oblongata
Baroreceptor Reflex
Heart and Homeostasis
• Effect of pH, carbon dioxide,
oxygen
– Receptors that measure pH
and carbon dioxide levels
found in hypothalamus
– Chemoreceptors monitoring
oxygen levels found in aorta
and internal carotids.
Prolonged lowered oxygen
levels causes increased heart
rate, which increases blood
pressure and can thus deliver
more oxygen to the tissues.
Chemoreceptor Reflex-pH
Heart and Homeostasis
• Effect of extracellular ion
concentration
– Increase or decrease in
extracellular K+ decrease the
heart rate
• Effect of body temperature
– Heart rate increases when body
temperature increases, heart
rate decreases when body
temperature decreases
Effects of Aging on the Heart
• Gradual changes in heart function, minor under resting
condition, more significant during exercise
• Hypertrophy of left ventricle
• Maximum heart rate decreases
• Increased tendency for valves to function abnormally
• Increased oxygen consumption required to pump same
amount of blood
DISORDERS
• Tachycardia
– Abnormally high heart rate (over 100)
• Bradycardia
– Abnormally low heart rate (under 60)
• Fibrillation
– Rapid and out of phase contractions
• Atherosclerosis
– Formation of fatty plaque on artery walls
– Decrease in vessel elasticity and possible blockage
Interrelationships between
– Pressure
– Flow
– Resistance
– And the control mechanisms that
regulate blood pressure and blood flow
Play important role in circulatory system
Laminar flow
– Blood flow in Streamlined fashion
– Interior of blood vessel is smooth
and of equal diameter along its
length
– Outermost layer moving slowest
(move against resistance of
stationary wall)
– And center layer moving fastest
Turbulent flow
– Interrupted
– Fluid passes a constriction, sharp
turn, rough surface
– Partially responsible for heart
sounds
– Sounds due to turbulence not
normal in arteries and is probably
due to some abnormal constriction
– And increases the probability of
thrombosis
• Blood pressure: Measure of force exerted
by blood against the blood vessel wall
• Blood moves through vessels because of
blood pressure
• BP is measured in mm Hg
• Measured by Sphygmomanometer
• Measured by listening for Korotkoff
sounds produced by turbulent flow in
arteries as pressure released from blood
pressure cuff (systolic pressure)
• No sound, continuous laminar flow,
Diastolic pressure
• Pulse Pressure: Difference between systolic and diastolic
pressures
•
Healthy person 120 mm Hg systolic, 80 mm Hg diastolic
•
Pulse pressure is 40 mm Hg
• Pulse pressure increases when stroke volume increases
•
eg. During exercise, stroke volume increases, pulse pressure
also increases
• Pulse pressure can be used to take a pulse to determine heart
rate
• Most frequent site used to measure pulse rate is in the carpus
with the radial artery- the radial pulse
 Rate of flow through a tube is expressed as the volume that passes
a specific point per unit of time. E.g.; cardiac output at rest is
5L/min, thus blood flow through the aorta is 5L/min
 Blood flow = (P1 – P2/R)
 P1 and P2 are pressures in the vessel at points one and two; R is
the resistance to flow
 Blood flow is directly proportional to pressure differences,
inversely proportional to resistance
 Resistance = 8vl/r4
 v is viscosity, l = length of the vessel, r is the radius of the vessel, 8
and  are constant
 radius and viscosity determines resistance
 Blood flow decreases when resistance increases
 Since resistance is proportional to blood vessel diameter,
constriction of a blood vessel increases resistance and thus
decreases flow
 Blood Flow =(P1 –P2)/R
 Poiseuille`s law = (P1 –P2)r4/8vl
 According to Poiseuille`s law, small change in radius dramatically
changes resistance to flow, r is raised to power 4
 During exercise, blood vessels in skeletal muscle vasodilate,
decreases resistance to blood flow
 And blood flow through blood vessels increases dramatically
 Viscosity: Is measure of resistance of liquid to
flow
 As viscosity increases, pressure required to flow
increases
 Viscosity of blood is influenced largely by
hematocrit (percentage of the total blood volume
composed of red blood cells)
 Dehydration and/or uncontrolled production of
RBCs
can increase hematocrit, thus increase viscosity
 Higher viscosity increases the workload on the
heart, heart failure can result
Blood Pressure varies directly with the following:
Blood Volume
Mainly regulated by kidneys
in blood volume =  in B.P.
 in blood vol. =  decrease in B.P.
• By nervous system, kidneys and chemical controls
Nervous System Regulation:
Sympathetic nerve fibers:
Vasoconstriction of blood vessels
diameter,  resistance   B.P.
Vasomotor center in medulla:
Controls cardiac output
Controls degree of vessel constriction
• Epinephrine and Norepinephrine
- Vasoconstriction
-  cardiac output,  B.P.
• ANF (Atrial Natriuretic Factor)
- ANF act on kidney
- Release of more sodium and
water in urine
- Loss of water and sodium in urine
-  blood volume   B.P.
• ADH (Antidiuretic Hormone)
- Stimulates kidneys to reabsorb water
-  blood volume   B.P.
• Renin (Enzyme)
- Released from kidneys in response to low
B.P.
- Stimulates angiotensin/aldosterone system
- Kidneys reabsorb sodium and water 
 blood volume and B.P.
RENIN / ANGIOTENSIN /
ALDOSTERONE SYSTEM
RENAL REGULATION OF B.P.
• Kidneys may alter B.P. directly
- Increased B.P. more blood filtered by kidneys
- More urine produced and released
-  blood volume   B.P.
• Kidneys may alter B.P. indirectly
- Renin/angiotensin system activated with  B.P.
- Vasoconstriction, water reabsorption due to
aldosterone release
-  blood volume   B.P