objective 3 pt 2 Flashcards
The intrinsic cardiac conduction system consists of
noncontractile cardiac cells specialized to initiate and
distribute impulses throughout the heart.
Although the heart depolarizes and contracts without
nervous system stimulation, the rhythm can be altered by
autonomic nervous system.
electrical events of the heart
what is the sequence of excitation?
Cardiac pacemaker cells pass impulses across heart in ∼0.22 seconds
◦ Coordinates depolarization of the heart
◦ Ensures heart beats as a unit
what is the order in which impulses pass across the heart?
- Sinoatrial (SA) node →
- Atrioventricular (AV) node →
- Atrioventricular (AV) bundle →
- Right and left bundle branches →
- Sub-endocardial conducting network
(Purkinje fibers)
sets the pace of the heart as it depolarizes faster than
rest of myocardium
pacemaker
Located in right atrial wall
◦ Generates impulses about 75×/minute (sinus rhythm)
◦ Innate rate of 100×/minute tempered by extrinsic factors
◦ Impulse spreads across atria, and to AV node
sinoatrial node
Located in inferior interatrial septum
◦ Delays impulses approximately 0.1 second
◦ Allows atrial contraction to occur before ventricular contraction
◦ Inherent rate of 50x/minute in absence of SA node input
◦ From the AV node the impulse sweeps to the AV bundle
atrioventricular node
Located in superior interventricular septum
◦ Only electrical connection between atria and ventricles
◦ Atria and ventricles not connected via gap junctions
◦ The AV bundle splits into the rt. and lt. bundle branches
antrioventricular bundle
Found in interventricular septum
◦ Carry impulses toward apex of heart
right and left bundle branches
Extends from interventricular septum into apex and ventricular walls
◦ More elaborate on left side of heart due to size of LV
◦ Process from initiation at SA node to complete contraction takes
~0.22 seconds
◦ Ventricular contraction immediately follows the depolarization wave
sub-endocardial conducting network
sends signals
through sympathetic nervous system to
increase both rate and force of contraction
◦ Stimulates SA and AV nodes, heart muscle,
and coronary arteries
cardio-acceleratory center
sends
parasympathetic signals via vagus nerve to
decrease heart rate
◦ Inhibits SA and AV nodes via vagus nerves
cardio-inhibitory center
is a device that
can detect electrical currents
generated by heart
electrocardiograph
is
a graphic recording of electrical
activity
◦ Combination of all action
potentials at given time; not a
tracing of a single AP
◦ Electrodes are placed at various
points on body to measure
voltage differences
◦ 12 lead ECG is most typical
electrocardiogram
what are the main features of electrocardiography?
P wave
QRS complex
T wave
P-R interval
S-T segment
Q-T interval
depolarization of atria
P wave
ventricular depolarization
QRS complex
ventricular repolarization
T wave
beginning of atrial excitation to beginning of ventricular excitation
P-R interval
entire ventricular myocardium depolarization
S-T segment
beginning of ventricular depolarization through ventricular repolarization
Q-T interval
may be due to enlarged ventricles
enlarged R wave
could be cardia ischemia
elevated or depressed ST segment
could be repolarization abnormality that increases risk of ventricular arrhythmias
prolonged QT interval
period of heart contraction
systole
period of heart relaxation
diastole
blood flow through heart during one heartbeat
Atrial systole and diastole are followed by ventricular
systole and diastole
◦ Cycle represents series of pressure and blood volume
changes
◦ Mechanical events follow electrical events seen on ECG
◦ Cycle lasts ~ 0.8 secs
cardiac cycle
marked by a succession of pressure and blood volume changes in the heart
cardiac cycle
what are the phases of the cardiac cycle?
- Ventricular filling (atrial systole)
- Isovolumetric contraction
- Ventricular ejection
- Isovolumetric relaxation
Contraction of atria completes emptying blood
out of the atria into the ventricles
o AV valves are open; SL valves are closed
o Ventricles are relaxed (diastole) and filling with
blood
o Cycle begins with P wave of ECG
o Takes ~ 0.1 sec
atrial systole
As the atria relax, the ventricles begin contracting
◦ Begins with QRS complex
◦ AV and SL valves are closed
◦ Pressure increases rapidly but volume remains
same
◦ For a split second, the ventricles are completely
closed chambers and the blood volume remains
constant as the ventricles contract
isovolumetric ventricular contraction
Onset coincides with R wave and appearance of 1st heart
sound
◦ AV valves closed
◦ SL valves open
◦ Blood forcefully ejected into pulmonary artery and aorta
◦ T wave occurs just before ventricles relax
◦ Residual volume
◦ End-systolic volume
◦ Takes ~ 0.3 secs
ventricular systole
SL valves close (2nd heart sound)
o AV valves are closed
o All chamber relax simultaneously for ~ 0.4 sec
o Atria passively fill with blood
isovolumetric ventricular relaxation
what are the 2 sounds of the heart?
lub
dub
first sound is the closing of AV valves at beginning of ventricular systole
Lub
second sound is closing of SL valves at beginning of ventricular diastole
Dub
the volume of blood pumped by each ventricle in 1 min
cardiac output
number of beats per min
HR
amount of blood pumped out by one ventricle with each beat
SV
factors increase heart rate
positive chronotropic factors
factors decrease heart rate
negative chronotropic factors
what can HR be regulated by?
autonomic nervous system
chemicals
other factors
can be activated by emotional or physical stressors. upon activation norepinephrine is released
sympathetic nervous system
increases HR
increased contractility
norepinephrine
opposes sympathetic effects. upon activation acetylcholine is released
parasympathetic nervous system
slows HR but has little to no effect on contractility
AcH
The hormones Epinephrine and Thyroxine influence heart rate.
o Epinephrine increases heart rate and contractility
o Thyroxine increases heart rate and enhances effects of epinephrine
and NE
o Intracellular and extracellular ions influence normal heart function
chemical regulation of heart rate
fetus has fastest HR; declines with age
age
females have faster HR than males
gender
increases HR
trained athletes can have slow HR
exercise
HR increases with increased body temp
body temp
