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Cardiac Physiology
Dr. Meg-angela Christi Amores
The Heart
• separate pumps:
– a right heart that pumps blood through the lungs,
– a left heart that pumps blood through the
peripheral organs
– atrium and ventricle
The Heart
• Cardiac muscle
• Striated
• Myofibrils with actin and myosin
• 3 types of cardiac muscles:
– atrial muscle
– ventricular muscle
– specialized excitatory and conductive muscle fibers
The Cardiac Muscle
• Syncytium
– cardiac muscle fibers are made up of many
individual cells connected in series and in parallel
with one another via intercalated discs
– form permeable "communicating" junctions
– action potentials travel easily from one cardiac
muscle cell to the next
The cardiac muscle
• two syncytiums:
– the atrial syncytium that
constitutes the walls of the two
atria
– the ventricular syncytium that
constitutes the walls of the two
ventricles
– they are conducted only by way of
a specialized conductive system
called the A-V bundle
The cardiac Muscle
• Action Potential
Action Potential
Action potential
• Caused by opening of two types of channels:
– fast sodium channels
• tremendous numbers of sodium ions to enter the skeletal
muscle
• open for only a few thousandths of a second and then abruptly
close
– slow calcium channels
• large quantity of both calcium and sodium ions flows through
these channels to the interior of the cardiac muscle fiber, and
this maintains a prolonged period of depolarization
• Causes PLATEAU of action potential
• are slower to open and remain open for several tenths of a
second
Cardiac muscle Action Potential
• after the onset of the action potential, the
permeability of the cardiac muscle membrane
for potassium ions decreases about fivefold
• greatly decreases the outflux of positively
charged potassium ions during the action
potential plateau
• thereby prevents early return of the action
potential voltage to its resting level
Refractory period
• is the interval of time during which a normal
cardiac impulse cannot re-excite an already
excited area of cardiac muscle
• 0.25 to 0.30 second
• relative refractory period of about 0.05 second
during which the muscle is more difficult than
normal to excite but nevertheless can be
excited by a very strong excitatory signal
excitation-contraction coupling
• the mechanism by which the action potential
causes the myofibrils of muscle to contract
• when an action potential passes over the
cardiac muscle membrane, the action
potential spreads to the interior of the cardiac
muscle fiber along the membranes of the
transverse (T) tubules
excitation-contraction coupling
• T tubule action potentials in turn act on the
membranes of the longitudinal sarcoplasmic
tubules
• release of calcium ions into the muscle
sarcoplasm
excitation-contraction coupling
• calcium ions diffuse into the myofibrils and
catalyze the chemical reactions that promote
sliding of the actin and myosin filaments along
one another
• Producing muscle contraction
excitation-contraction coupling
• a large quantity of extra calcium ions also
diffuses into the sarcoplasm from the T
tubules themselves at the time of the action
potential
Cardiac Cycle
• cardiac events that occur from the beginning
of one heartbeat to the beginning of the next
• Diastole
– during which the heart fills with blood
• Systole
– period of contraction
Diastole
Systole
The atria
• Primer pumps
• 80 per cent of the blood flows
directly through the atria into
the ventricles even before the
atria contract
• atrial contraction usually causes
an additional 20 per cent filling
of the ventricles
• increase the ventricular pumping
effectiveness
The ventricles
• Filling of ventricles
– After systole, AV opens via pressure build up in
atria
• Emptying of ventricles
– Period of Isovolumic (Isometric) Contraction
– Period of Ejection
– Period of Isovolumic (Isometric) Relaxation
Factors for cardiac contractility
• Chemical energy
– Derived mainly from oxidative metabolism of fatty
acids and, to a lesser extent, of other nutrients,
especially lactate and glucose
Regulation of Heart pumping
• heart pumps only 4 to 6 liters of blood each
minute at rest
• severe exercise, the heart may be required to
pump four to seven times
• (1) intrinsic cardiac regulation of pumping in response
to changes in volume of blood flowing into the heart
• (2) control of heart rate and strength of heart pumping
by the autonomic nervous system
Regulation of Heart pump
1. Intrinsic regulation
– the Frank-Starling mechanism of the heart
– intrinsic ability of the heart to adapt to increasing
volumes of inflowing blood
– the greater the heart muscle is stretched during
filling, the greater is the force of contraction and
the greater the quantity of blood pumped into the
aorta
Regulation of Heart pump
2. Control of the Heart by the Sympathetic and
Parasympathetic Nerves
•
•
amount of blood pumped each minute (cardiac
output) often can be increased more than 100 per
cent by sympathetic stimulation
the output can be decreased to as low as zero or
almost zero by vagal (parasympathetic) stimulation
Other factors
• Potassium and Calcium Ions
– Excess potassium in the extracellular fluids causes
the heart to become dilated and flaccid and also
slows the heart rate
– Excess of calcium causes the heart to go toward
spastic contraction
Other Factors
• Temperature
– heat increases the permeability of the cardiac
muscle membrane to ions
– Increased body temperature, as occurs when one
has fever, causes a greatly increased heart rate
Heart Sounds
• lub, dub, lub, dub
• The "lub" is associated with closure of the
atrioventricular (A-V) valves at the beginning
of systole(s1)
• "dub" is associated with closure of the
semilunar (aortic and pulmonary) valves at the
end of systole (s2)
• vibration of the taut valves immediately after
closure
• with vibration of the adjacent walls of the
heart and major vessels around the heart
• Shorter 2nd heart sound: semilunar valves are
more taut than the A-V valves, so that they
vibrate for a shorter time than do the A-V
valves
• Is there a 3rd and 4th heart sound?
• Third Heart Sound
– at the beginning of the middle third of
diastole
– initiated by inrushing blood from the atria
• Atrial Heart Sound (Fourth Heart
Sound)
– occurs when the atria contract, and
presumably, it is caused by the inrush of
blood into the ventricles
Chest areas for auscultation
Heart Murmurs
• Abnormal heart sounds
• Due to abnormalities of valves
• Systolic murmurs
– Occur during systole
– Aortic Stenosis, Mitral Regurgitation
• Diastolic murmurs
– Occur in diastole
– Aortic Regurgitation, Mitral Stenosis