Topic 11: Cardiovascular Physiology Flashcards
Heart
- Dual pump with valves
- Muscle cells connected by gap junctions
Conduction System
- Non-contractile cardiac muscle cells - modified to initiate & distribute impulses throughout the heart
- Produces haps spontaneously (no stimulus required) BUT at different rates
Conduction System Parts
1) Sinoatrial (SA) Node
2) Atrioventricular (AV) Node
3) Bundle of His (AV bundle)
4) Purkinje Fibres
Sinoatrial (SA) Node
- In right atrium
- Rate = 100 APs/min (modified by PSNS to be 75 Aps/min at rest)
- Produces APs faster than other areas = is pacemaker
Atrioventricular (AV) Node
- In right atrium
- Rate 50 APs/min
Bundle of His (AV bundle)
- Originates at AV node
- ONLY route for electrical activity to go from atria to ventricles + bundle branches (right and left)
- 30 Aps/min
Purkinje Fibres
- Terminal fibres - stimulate contraction of the ventricular myocardium
- 30 APs/min
Pathway of APs in the Heart
- LOOK at DIAGRAM
- If conduction system damaged, next fastest part becomes pacemaker – i.e. if SA node damaged, AV node takes over (atria may not contract, ventricles 50 beats/minute)
- Artificial pacemakers - stimuli. if SA or AV node damaged
APs of SA & AV Nodes
- Cells = non contractile auto rhythmic cardiac muscle cells (self-excitable)
- Threshold = -40 mV
Phases of Pacemaker Activity
1) Pacemaker Potential
2) AP Depolarization
3) AP Repolarization
4) Na+ channels open at -50 mV
Pacemaker Potential
- Low K+ permeability (K+ voltage gates closed)
- Slow inward leak of Na+ (Na+ voltage gates open)
- Causes slow depolarization toward threshold (-40mV)
AP Depolarization
- At threshold –> AP
- Ca2+ voltage gates open - CA2+ moves in –> depol. (Na+ voltage gates close at threshold – not involved in AP)
- Ca2+ voltage-gates close at peak
AP Repolarization
- K+ voltage gates open at peak, K+ out –> repolarization
- K+ gates close below threshold
Na+ Channels Open at -50mV
- Starts pacemaker potential (1) again, once K+ gates close (continuous cycle)
Note: no RMP
APs in Ventricular Myocardium
- Cells =contractile
- Purkinje fibre AP –> ventricular (contractile) myocardial AP (spread cell to cell by gap junctions)
- Resting MP = -90mV
Phases of Ventricular Myocardial APs
1) Depolarization
2) Plateau
3) Depolarization
Depolarization
- Na+ voltage gates open (fast) = same gates as neurone, skeleton. muscle
- MP to +30 mV
Plateau
- Na+ channels close + inactivate (slight drop in MP)
- Ca2+ slow voltage gates are open (Ca2+ influx maintains depolarization)
Depolarization
- Ca2+ channels close
- K+ voltage gates open – increased K+ outflux – MP lower to resting
Absolute Refractory Period
- Long - Na+ channels inactivated until MP is close to -70 mV
Excitation -Contraction Coupling in Myocardial Cells
1) Open voltage-gated Ca2+ channels of AP = small increase cytosolic Ca2+ (from ECF) -> Not enough to trigger contraction BUT…
2) Opens chemically-gated Ca2+ channels on SR –> cytosolic Ca2+ big increase –> binds to troponin etc –> leads to contraction
3) Contraction
- Sliding filament mechanism
- begins a few msec after AP begins
- Duration of AP ~250 msec and duration of twitch ~300 msec – contraction almost over when AP ends
- Result = no summation – NO tetanus
- Get alternation of contraction-relaxation
Cardiac Cycle
1) Electrical Activity (ECG)
2) Mechanical Activity
3) Blood Flow through heart
Electrical Activity (ECG)
- Small currents due to depolarization/repolarization of heart move through salty body fluids
- Potential difference measured on body surface using electrode pairs: 1 pair = a lead
- Recoding seen as waves = sum of Electrical activity of ALL myocardial cells (NOT an AP)
ECG Waves
1) P wave = atrial depol. –> followed by contraction
2) QRS wave = ventricular depolarization –> followed by contraction - also atrial repol. (–> followed by relaxation) - masked by larger ventricular electrical event (larger muscle mass)
3) T wave = ventricular repol. –> followed by relaxation
ECG Intervals
1) P-Q = atira contracted, signals passing through AV node
2) S-T = ventricles contracted, atria relaxed
3) T-P = heart at rest
Tachycardia
Resting HP more than 100 bpm
Bradycardia
Resting HP less than 60 bpm
Heart Block
- When conduction though the AV node slowed
- Get increased P –> Q interval
- Ventricles may not contract after each atrial contraction e.g. 3rd degree heart block - no conduction through AV node - atria fire at SA node rate (~75 Aps/min), ventricles at bundle/ Purkinje rate ( ~ 30 APs/ min
Mechanical Activity
- 2 amins events:
1) Systole = contraction, emptying
2) Diastole = relaxation, filling - Both events initiated by electrical activity
- 1 complete heartbeat = diastole + systole of atria AND diastole + systole of ventricles
- Timing of mechanical events:
- Average resting Heart rate (HR) = 75 beats/min – 0.8 sec/beat = 1 cardiac cycle (60 sec/min divided 75 beats/min)
