Chapter 20 Lecture 4 Flashcards
Conduction System of Heart
Coordinates contraction of heart muscle.
Autorhythmic Cells
Cells fire spontaneously, act as pacemaker and form conduction system for the heart
SA node
cluster of cells in wall of Rt. Atria
begins heart activity that spreads to both atria
excitation spreads to AV node
AV node
in atrial septum, transmits signal to bundle of His
AV bundle of His
the connection between atria and ventricles
divides into bundle branches & purkinje fibers, large diameter fibers that conduct signals quickly
Timing of Atrial & Ventricular Excitation
SA node setting pace since is the fastest
In 50 msec excitation spreads through both atria and down to AV node
100 msec delay at AV node due to smaller diameter fibers- allows atria to fully contract filling ventricles before ventricles contract
In 50 msec excitation spreads through both ventricles simultaneously
Physiology of Contraction
Depolarization, plateau, repolarization
Depolarization
Cardiac cell resting membrane potential is -90mv
excitation spreads through gap junctions
fast Na+ channels open for rapid depolarization
Plateau Phase:
250 msec (only 1msec in neuron) slow Ca+2 channels open, let Ca +2 enter from outside cell and from storage in sarcoplasmic reticulum, while K+ channels close Ca +2 binds to troponin to allow for actin-myosin cross-bridge formation & tension development
Repolarization:
Ca+2 channels close and K+ channels open & -90mv is restored as potassium leaves the cell
Refractory period:
very long so heart can fill
action potential in cardiac muscle:
changes in cell membrane permeability
EKG (Electrocardiogram)
Action potentials of all active cells can be detected and recorded
P wave
atrial depolarization
P to Q interval:
conduction time from atrial to ventricular excitation
T wave
ventricular repolarization
Cardiac Cycle:
At 75 beats/min, one cycle requires 0.8 sec
systole (contraction) and diastole (relaxation) of both atria, plus the systole and diastole of both ventricles
End diastolic volume (EDV)
volume in ventricle at end of diastole, about 130ml
End systolic volume (ESV)
volume in ventricle at end of systole, about 60ml
Stroke volume (SV)
the volume ejected per beat from each ventricle, about 70ml
SV = EDV - ESV
Phases of Cardiac Cycle:
Isovolumetric relaxation
Ventricular filling
Ventricular systole
Isovolumetric:
brief period when volume in ventricles does not change– ventricles relax, pressure drops and AV valves open
Ventricular filling
as blood flows from full atria
diastasis: as blood flows from atria in smaller volume
atrial systole pushes final 20-25 ml blood into ventricle
Ventricular systole
isovolumetric contraction
ventricular ejection: as SL valves open and blood is ejected
Fibrillation
erratic heartbeat, uncoordinated contractions.
Atrial (AF, A-fib) =
interference with AV node erratic, incomplete atrial contractions. Often secondary to other heart problem, and rarely life-threatening.
- Treatment: drug/electro-cardioconversion; anti-coagulants
- Symptoms: none, palpitations, fainting, nausea, chest pain; can lead to stroke and congestive heart failure.
Ventricular fibrillation (VF, V-fib) =
erratic, incomplete ventricular contractions. Immediately life-threatening due to lack of somatic, pulmonary, and cardiac circulation; often secondary to other heart disease.
- Symptoms: sudden collapse; death often first “symptom.”
- Treatment: electric cardioconversion, then treat underlying issue
Tachycardia:
rapid (especially ventricular) heartbeat.
- Symptoms: range from faintness, short-of-breath to sudden death, depending location and cause.
- Treatment: drug or electric cardioconversion, then treat underlying issue
bradycardia:
slow heartbeat
Blood pressure in aorta
is 120mm Hg
Blood pressure in pulmonary trunk is
30mm Hg
The volume of blood ejected from each ventricle is
70ml (stroke volume)
To increase:
More blood/beat
Upper limit = size of ventricle and time allowed to fill.
To increase:More beats/min
Upper limit = need to reinitiate beat (refractory period)
- Nervous System -Sensors:
: limbic system, baroreceptors, chemoreceptors, proprioreceptors. Feed into medulla’s cardiovascular center (medulla). Control through ANS.
- Nervous System - Sympathetic NS:
nerves connect with SA & AV nodes and myocardium. increases rate of autorhythmic SA firing and uptake of Ca++ ( contractility) by all muscle cells.
- Nervous System - Parasympathetic NS:
nerves that connect with SA & AV nodes and myocardium of atria. Decreases rate of autorhythmic firing with ACh.
- Endocrine System Control:
Adrenal medulla (responding to hypothalamus) releases epinephrine and norepinephrine = increase of autorhythmic rate and contractility.
- Cation Control:
Why is KCl included in lethal injection cocktails?
Increased K+ prevents action potential
Increased Na+ blocks Ca++ entry
Increased Ca++ boost contractility, rate
- Metabolic Control
Increased H+ (acidosis) or OH– decrease heart rate
