Heart Physiology Flashcards
cardiac muscle has
short, thick branching cells (fibers), each one with one central nucleus
sarcoplasmic reticulum is
less developed that in skeletal muscle, but contains larger T-tubes to admit more calcium ions from extracellular fluid
cardiac muscle fibers are
joined end to end by intercalated discs
desmosomes
hold muscle fibers together
gap junctions allow
action potentials to spread from one cardiac fiber to the next fiber
cardiac muscle contains more ___________ than skeletal muscle
myoglobin and more mitochondria than skeletal muscle
-because it utilizes aerobic respiration almost exclusively it is less prone to fatigue
cardiac muscle fibers are autorhythmic and can
depolarize automatically to generate action potentials
excitation of cardiac muscle begins in
the sinoatrial (SA) node
SA node is located
in the right wall of the atrium just below the opening from the superior vena cava
SA node functions as
a pacemaker of the heart to initiate each heartbeat and set sinus rythym for entire heart
cardiac action potential spreads from
SA node throughout both atria by way of gap junctions which causes both atria to contract simultaneously
the nerve siglnal spreads to the
atrioventricular (AV) node in the interatrial septum
`as the nerve signal passes through the AV node there is a
brief time delay (0.1 second)
The nerve signal passes into the atriocentricular bundle (bundle of His), which is
a tract of conducting fibers that is only electrical connection between atria and the ventricles
the atrioventricular bundle divides into
right and left bundle branches, which carry the nerve signal along either side of the interventricular septum toward the heart’s apex
Large diameter conduction myofibers (Purkinje fibers) conduct
the nerve signal from the bundle branches into the ventricular myocardium
cells in the SA node gradually
depolarize due to slow inflow of sodium ions and minimal outflow of potassium ions, generating a pacemaker potential
when this pacemaker potential reaches its threshold of
- 40 mV,voltage regulated “fast calcium channels”open and calcium ions rush in to trigger action potential
- the influx of calcium is what produces the action potential
when nerve signal has concluded
pacemaker potential starts over to produce next heartbeat
it takes about 50 milliseconds for the nerve signal to reach the
AV node wher it is momentarily delayed to allow the ventricles to fill prior to contracting
signals travel fastest through
the Av bundle and the Purkinje fibers and trigger the myocardial contractions that begin at the apex of the heart
cardiac muscle cells depolarize
very rapidly because voltage-regulated Na+ channels open and close very quickly
voltage-regulated calcium channels open and close
slowly to admit calcium ions, prolongs depolarization and creates a plateau when muscle cells contract
voltage-regulated potassium channels open and
potassium ions rush out which returns membrane resting potential
cardiac muscle cells have a very long absolute refractory period which
prevents wave summation and tetany that would halt pumping action of heart
an electrocardiogram (ECG or EKG) is
a graphic recording of the electrical changes that accompany a heartbeat
a P wave is
a small unpward wave produced by depolarization of the atria following the spontaneous initiation of an action potential in the SA node
atrial contraction occurs
about .1 second after P wave begins
the QRS comples is produced when
the AV node fires and the ventricles depolarize as the impulse travels through the Purkinje fibers
complicated shape of the QRS complex is due to
the fact that the left ventricle is larger than the right ventricle and depolarizes at a slightly different rate
ventricle contraction occurs during
the S-T segment
atrial repolarization also occurs during the S-T segment but is
masked by depolarization of ventricles
a T wave is produced when
the ventricles repolarize before they start to relax
both atria contract (atrial systole) while
both ventricles relax (ventricular diastole)
both ventricles contrace (ventricular systole) while
both atria relax (atrial diastole)
heart sounds occur because of
blood turbulence while valves are closing
“lubb” sound of heartbeat occurs when
the AV valves are closing-soon after ventricular systole begins
“dupp” sound of heartbeat occurs when
the semilunar valves snap shut-at begining of ventricular diastole
ventricular filling occurs after
the AV valves open
-begins rapidly, slows down, finishes by time atrial systole occurs
end-diastolic colume (EDV) is
30 mL but only 30% is due to atrial systole
isovolumetric contraction occurs when
the ventricles start to contract but dont eject any blood because all 4 valves are closed
ventricular ejection occurs when
ventricular pressure increases enough to open the semilunar valves and force blood into the aorta and the pulmonary trunk
end-systolic volume (ESV)
after ventricular ejection is about 60 mL
isovolumetric relaxation occurs when
the ventricles start to relax but don’t fill with blood because all 4 valves are closed
cardiac output (CO)
describes the volume of blood ejected from each ventricle each minute and it is calculated from stroke volume and heart rate
stroke volume (SV)
is the amount of blood ejected by each ventricle during ventricular systole (about 70 mL)
heart rate (HR)
averages 75 beats per minute
average cardiac output for a resting adult is
about 5.25 liters/minute
stroke volume is governed by three factors
- preload
- contractility
- afterload
preload is
the degree to which cardiac muscle cells stretch just before they contract
increasing preload is
anything that increases volume of blood that returns to heart (i.e. slow HR) or speed with which blood returns to heart will increase pre-load
Frank-Sterling law of the heart
greater EDV will cause cardiac muscle to stretch more and generate greater contractile force
contractility is
the contractile force that gets developed for a particular preload
increasing contractility will
increase stroke volume
positive inotropic agents
increase contractility by making more calcium available, increases length of plateau and allows more contractile force to develop
negative inotropic agents
(potassium, calcium channel blockers) will produce contractility by reducing amount of calcium that gets released
afterload is
the pressure that is needed to open the semilunar valves
increasing afterload will
decrease stroke volume
heart rate is regulated by
the cardiac center in the medulla oblongata
autonomic nervous system doesn’t
initiate a heartbeat, but it does modulate the heart rate
impulses transmitted along sympathetic cardiac accelerator nerves
release norepinephrine which binds to adrernergic receptors to increase heart rate and exert positive chronotropic effect
iimpulses transmitted along vagus nerve to heart
release acetylcholine which binds to cholinergic receptors to decrease heart rate and exert negative chronotropic effect
cardiac center receives input from
the cerebral cortex, limbic system, hypothalamus, and from various receptors in order to regulate heart rate
proprioceptors in muscles and joints
detect changes in physical activity
baroreceptors in the aorta and carotid arteries
monitor changes in blood pressure
chemoreceptors in the aorta, carotid arteries, and the medulla oblongata
monitor changes in blood pH, carbon dioxide, oxygen
certain chemicals in the body have chronotropic effects on heart rate
epinephrine and norepinephrine, caffeine, nicotine, calcium, potassium
epinephrine and norepinephrine
increase heart rate (positive agent)
caffeine and nicotine
increase heart rate
calcium
prolongs the plateau of a cardiac muscle action potential
potassium
reduces the strength of a cardiac muscle action potential by making myocardium less excitable
