LP #104 The Cardiac Cycle Flashcards
2 mechanisms control heart rate
- intrinsic
- extrinsic
- mediates heart fxn to meet changing needs of the body
- responds to external & internal stimuli
- located in medulla oblongata (regulates vital autonomic fxns)
- fxn: CSN control of heart
Extrinsic mechanism
ANS centres (medulla oblongata):
- CV
- respiratory
- coughing
- swallowing
- vomiting
CV centres (medulla oblongata):
- Cardiac centre
2. Vasomotor centre
- 2 centres;
- cardioaccelatory
- cardiohibitory
- Controls contraction force & HR to meet body’s demands
Cardiac centre
- regulates contraction of vascular smooth m (helps control BP)
- vasoconstriction:
- decrease lumen size= increase BP
- vasodilation:
- increase lumen size= decrease BP
Vasomotor centre
Medulla oblongata sends signals to heart via:
Cardioaccelatory centre
Cardioinhibitory centre
- sympathetic
- fibres projects from medulla to motor neurone at T1-T5 SC level
- innervates: SA & AV nodes, heart m, & coronary aa
Cardioaccelatory centre
- parasympathetic
- fibres project from medulla to heart via vagus n
- innervates mainly SA & AV nodes
- damage to vagus n is life threatening
- if parasympathetic fibers damaged, sympathetic nn are unopposed
Cardioinhibitory centre
- mediates ongoing control of HR, independent of NS
- found in myocardium
- fxn: excites heart m cells at a continuous/ constant pace
Intrinsic mechanism
Myocardial cells:
- contractile?
- non-contractile (pacemaker cells)?
*responsible for contraction of heart m as a syncytium
- autorythmic
- found strategic sites in heart
Depolarization?
Excites cell membrane
Hyperpolarization?
Inhibits cell membrane
Restoration of the RMP?
Returning ions to their place: NA+ is pumped out of cell & K+ brought into cell via ATPase pump (energy dependent)
2 events in pacemaker cells during an AP?
- returning to pacemaker potential
- after hyperpolarization of the cell membrane, once an AP has be initiated
- initiating an AP
- rapid influx of calcium ions
After AP event:
- cell membrane is hyperpolarized
- cycle begins again
*decrease permeability to K+ & slow Na+ influx?
- depolarization
- pacemaker potential
*increase per ability to Ca2+?
AP
*decrease permeability to Ca2+ & K+, & increase permeability to Na+
Polarization
Intrinsic mechanism locations:
- SA nodes?
- AV nodes?
- AV bundle (bundle of his)?
- Bundle branches?
- Purkinje fibres?
- R atrium
- R atrium
- W/in interventricular septum
- Ventricular myocardium
- Ventricular myocardium
Cardiac cycle: Events associated w/BF through heart during 1 heartbeat (2 processes)?
- electrical
2. mechanical
Excitation of pacemaker cells
Electrical
- contraction of cardiac m cells (entire heart)
- atrial contraction of cardiac m cells (entire heart)
- ventricular contraction forces blood out of ventricles into aorta & pulmonary trunk
Mechanical
Heart alternately contracts & relaxes?
Diastole & systole
Diastolic phase (diastole) ?
Relaxation
Systolic phase (systole) ?
Contraction
- cardiac m is relaxed
- blood is flowing passively through atria, open AV valves & into ventricles (~80% of ventricular filling)
- when atria contracts, remaining 20% pumped into ventricles
- ventricles in axiom diastole & blood capacity= end diastolic volume (EDV)
Ventricular filling (mid-late diastole)
- as atria relax, ventricles begin contracting
- ventricular P increase rapidly & AV valves close
- increase ventricular P opens Sl valves & blood is expelled into aorta & pulmonary trunk
- ventricular ejection phase (peak P): P in aorta~ 120mm Hg
- atria is diastole begins to fill w/blood (from venae cavae & pulmonary vv)
Ventricular systole (atria diastole)
-blood remaining in ventricles= end systolic volume (ESV)
-rapid decrease in ventricular P
-increase P in aorta & pulmonary trunks closes SL valves
-atria continue to fill w/blood
*atrial P > ventricular P
*AV valves open & blood flows into ventricles
…. & the cycle repeats….
Early ventricular diastol
