Week 7: Chp 28: Physiology of the Heart: Cardiac Conduction System Flashcards
What does the Heart need in order for it to perform its work?
it needs an adequate amount of oxygen and nutrients
What major vessels supply blood to the heart?
the left and right coronary arteries
-the arteries branch off into arterioles and capillaries, then eventually merge into capillary veins that drain the blood into a coronary sinus, which eventually empties into the right atrium
What is needed to be able to facilitate the filling of the the chambers of the heart and expulsion of blood into the systemic circulation?
there needs to be both an electrical impulse conducted across the structures of the heart and a corresponding muscular contraction
The unique characteristics of the cardiac cells that facilitate cardiac electrical conduction are?
- automaticity
- excitability
- contractiliy
Automaticity
allows cardiac cells to generate impulses independently and rhythmically
-heart has several pacemaker cells that generate these impulses
>sinoatrial node (SA)
>atrioventricular node (AV)
Sinoatrial (SA) node
- normal pacemaker of the heart
- located in the Right atrium
- 60 to 100 bpm
- generates automaticity
Atrioventricular (AV) node
in the absence of an impulse from the SA node, can generate impulse at rates of 40 to 60 bpm
generate automaticity in absence of SA node
What happens if both SA and AV nodes fail?
ventricular cells can generate impulses at a rate of 20 to 40 bpm
Cardiac Cycle
circular sequence of events that produces the eventual muscular contraction that cause the ejection of blood from the right ventricle into the pulmonary circulation or from the left ventricle into the systemic circulation
- mechanical events and is regulated by the electrical activity of the myocardium; sequence of mechanical events that occurs during a single heartbeat
- the atria and ventricles alternately contract and relax to fill and empty
- while atria are contracting (emptying), the ventricles are relaxing (filling) vice versa
- both atria contract, followed by a fraction of a second later by contraction of both ventricles
Electrical Conduction
electrical activity that initiates contraction of the myocardium
Conductivity
allows cardiac tissue to transmit the impulses to neighboring connected cells
Other properties unique to cardiac cells
- cardiac cells rely exclusively on aerobic metabolism for adenosine triphosphate production (ATP). without this production of lactic acid that occurs within anaerobic metabolism, cardiac muscle can maintain its regular lifelong rhythm without tiring
- cardiac muscle relies on extracellular calcium to facilitate calcium release from the sarcoplasmic reticulum to produce its muscular contraction. This is referred to as calcium-induced calcium release. it is regulated by the slow inward flow of positively charged calcium ions during the action potential
- cardiac cells sustain a longer contraction, allowing ejection of blood from the atria and ventricles
- cardiac cells have a longer absolute refractory period, decreasing the possibility of repetitive, uncontrolled muscular contraction called tetany
Tetany
repetitive, uncontrolled muscular contraction
-cardiac cells have a longer absolute refractory period, decreasing the possibility of tetany
The Cells of the cardiac electrical conduction system follow this pathway
- the impulse originates in the SA node. (60 to 100 bpm)
- The impulse spreads through the atria through the intermodal pathways, to the
- Atrioventricular node, where the impulse is delayed to allow for atrial contraction and complete ventricular filling
- The impulse leaves the AV node through the bundle of His and branches off into the
- Right and Left bundle branches, which travel down the interventricular septum to end in the..
- Purkinje fibers, which extend the impulse into the ventricular tissue, facilitating ventricular contraction
Cardiac Electrical Conduction System Pathway Simplified
-SA node–> (atria contract)–> AV node—> bundle of His–> bundle branches–> purkinje fibers–>ventricles contract
Electrical Cardiac Conduction is facilitated by?
the movement of ions across the cell membrane that is unique to cardiac cells
Cardiac Action Potential
-a stimulus begins the movement
-a process in which the membrane potential, the difference in charge between the interior and exterior of the cell, changes or goes up and down in a consistent pattern
>depolarization
>repolarization
>absolute refractory period
>refractory period
Depolarization
movement of ions preceding and facilitating cardiac mechanical contraction
-causes cardiac mechanical contraction
Repolarization
movement of ions back to the resting state
-the cardiac resting membrane potential pf -90mV, to allow for the initiation of another action potential
5 Phases of Action Potential
-Phase 0: rapid depolarization caused by opening of sodium (Na+) channels, allowing rapid Na+ influx, moving the membrane potential to +30
-Phase 1: decrease in Na influx, causing a slight movement toward negative of the membrane potential. producing an initial repolarization
phase 2: plateau state caused by an influx of calcium (Ca++) through the opening of Ca++ channels balanced by a slight outflow of potassium (K+)
-Phase 3: final repolarization caused by the closing of the Ca++ channels and the rapid outflow of K+
-Phase 4: Return to resting membrane potential
Absolute Refractory Period
occurs during and immediately following depolarization
- during this time, the cell is unresponsive to any stimulus
- corresponds to phase 0 through the middle of phase 3
Relative Refractory Period
- immediately follows the absolute refractory period
- represents a time when a greater-than-normal stimulus may initiate an impulse
- corresponds to the end of phase 3
What can typically manipulate the movement of ions in some ways?
Medications utilized to control irregular heart rhythms or dysrhythmias
Electrocardiogram (ECG)
electrodes placed on the wrists, ankles, and six locations around the chest produce a comprehensive picture of the cardiac electrical activity
The principle waveforms produced on a ECG are
- P wave
- QRS complex
- T wave
The principle segments produced on an ECG are
- PR segment
- ST segment
The principle intervals produced on an ECG are
- PR interval
- QRS interval
- QT interval
ECG: P wave
corresponds to atrial depolarization produced by the propagation of the impulse from the SA node through the atria
-atrial contraction takes place milliseconds after depolarization
ECG: PR Interval
from the beginning of the P wave to the beginning of the QRS complex
-reflects the time required for atrial depolarization and the delay of the impulse at the AV node
ECG: PR Segment
the time immediately following the P wave to the beginning of the QRS complex
-reflects the delay at the AV node
ECG: QRS complex
corresponds to ventricular depolarization
- ventricular contraction occurs after the QRS complex in the ST segment
- atrial contraction occurs during ventricular contraction (waveform isnt visible but buried in QRS complex)
ECG: QRS Interval
reflects the time required for ventricular depolarization
ECG: T wave
corresponds to ventricular repolarization
ECG: QT interval
reflects the time required for ventricular depolarization and repolarization
ECG: ST Segment
ventricular contraction occurs after QRS complex in ST segment
