Topic 11: Cardiovascular Physiology

Question 1

Heart

Answer 1

• Dual pump with valves

- Muscle cells connected by gap junctions

Question 2

Conduction System

Answer 2

• Non-contractile cardiac muscle cells - modified to initiate & distribute impulses throughout the heart
• Produces haps spontaneously (no stimulus required) BUT at different rates

Question 3

Conduction System Parts

Answer 3

1) Sinoatrial (SA) Node
2) Atrioventricular (AV) Node
3) Bundle of His (AV bundle)
4) Purkinje Fibres

Question 4

Sinoatrial (SA) Node

Answer 4

• 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

Question 5

Atrioventricular (AV) Node

Answer 5

• In right atrium

- Rate 50 APs/min

Question 6

Bundle of His (AV bundle)

Answer 6

• Originates at AV node
• ONLY route for electrical activity to go from atria to ventricles + bundle branches (right and left)
• 30 Aps/min

Question 7

Purkinje Fibres

Answer 7

• Terminal fibres - stimulate contraction of the ventricular myocardium
• 30 APs/min

Question 8

Pathway of APs in the Heart

Answer 8

• 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

Question 9

APs of SA & AV Nodes

Answer 9

• Cells = non contractile auto rhythmic cardiac muscle cells (self-excitable)
• Threshold = -40 mV

Question 10

Phases of Pacemaker Activity

Answer 10

1) Pacemaker Potential
2) AP Depolarization
3) AP Repolarization
4) Na+ channels open at -50 mV

Question 11

Pacemaker Potential

Answer 11

• Low K+ permeability (K+ voltage gates closed)
• Slow inward leak of Na+ (Na+ voltage gates open)
• Causes slow depolarization toward threshold (-40mV)

Question 12

AP Depolarization

Answer 12

• 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

Question 13

AP Repolarization

Answer 13

• K+ voltage gates open at peak, K+ out –> repolarization

- K+ gates close below threshold

Question 14

Na+ Channels Open at -50mV

Answer 14

• Starts pacemaker potential (1) again, once K+ gates close (continuous cycle)
  Note: no RMP

Question 15

APs in Ventricular Myocardium

Answer 15

• Cells =contractile
• Purkinje fibre AP –> ventricular (contractile) myocardial AP (spread cell to cell by gap junctions)
• Resting MP = -90mV

Question 16

Phases of Ventricular Myocardial APs

Answer 16

1) Depolarization
2) Plateau
3) Depolarization

Question 17

Depolarization

Answer 17

• Na+ voltage gates open (fast) = same gates as neurone, skeleton. muscle
• MP to +30 mV

Question 18

Plateau

Answer 18

• Na+ channels close + inactivate (slight drop in MP)

- Ca2+ slow voltage gates are open (Ca2+ influx maintains depolarization)

Question 19

Depolarization

Answer 19

• Ca2+ channels close

- K+ voltage gates open – increased K+ outflux – MP lower to resting

Question 20

Absolute Refractory Period

Answer 20

• Long - Na+ channels inactivated until MP is close to -70 mV

Question 21

Excitation -Contraction Coupling in Myocardial Cells

Answer 21

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

Question 22

Cardiac Cycle

Answer 22

1) Electrical Activity (ECG)
2) Mechanical Activity
3) Blood Flow through heart

Question 23

Electrical Activity (ECG)

Answer 23

• 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)

Question 24

ECG Waves

Answer 24

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

Question 25

ECG Intervals

Answer 25

1) P-Q = atira contracted, signals passing through AV node
2) S-T = ventricles contracted, atria relaxed
3) T-P = heart at rest

Question 26

Tachycardia

Answer 26

Resting HP more than 100 bpm

Question 27

Bradycardia

Answer 27

Resting HP less than 60 bpm

Question 28

Heart Block

Answer 28

• 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

Question 29

Mechanical Activity

Answer 29

• 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)