cardiodynamics

Question 1

describe the conducting system of the heart?

Answer 1

sinoatrial node (pacemaker)

• spontaneously depol, rate determined by neurotransmitters from ANS
• resting HR slower than rate of depol of SA due to parasympathetic tone
• AV node
• spont depol (40-60) times min
• delay occurs at node
• delay allows atria to contract and fill ventricles

Question 2

describe and explain the physiological significance of the cardiac action potential?

Answer 2

excitation occurs, ~75mV 
3 phases 
-rapid depol (Na+ opens, influx) 
-plateau (Na+ channels close rapidly, efflux, Ca2+ channels open slow, influx, balances out) 
-repol (Ca2+ close, K+ open, efflux) 

significance - twitch summation, greater tension before relaxation. ‘tetany’. not possible in cardiac muscle due to refractory period. if we had refractory period, not able pump blood

Question 3

explain the electrical and mechanical events of the cardiac cycle?

Answer 3

electrical

1. SA spont generates an AP
2. stimulus spreads atria and reaches AV node
3. AV nodal delay, atrial contraction begins
4. AP spreads along AV bundle, bundle fibres and purkinje fibres
5. AP relayed across ventricles (contract)

mechanical

1. relax
2. influx,
3. contraction
4. ejection

Question 4

identify the electrical events associated with a normal ECG

Answer 4

P wave - atrial depol
PR seg- AV nodal delay, atria contracted and emptying
QRS wave - ventricular depol
ST seg- ventricles contracting and empyting
T wave - ventricular repol
TP seg - heart at rest and ventricles (atria also) filling

Question 5

define, explain and understand the interrelationship between cardiodynamic variables (EDV, ESV, SV, HR and CO)?

Answer 5

EDV - volume of blood in each ventricle at end of ventricular diastole
ESV - vole of blood remaining in each ventricle at end of ventricular systole (40% of EDV)
SV - volume of blood pumped out of each ventricle during a single bead. MOST IMPORTANT FACTOR. SV = EDV - ESV
HR - heart rate
CO - cardiac output, = HS x SV, adjusted by ANS and hormones

EDV and ESV affect SV, EDV needs to be large as possible and ESV small as possible in order for greatest SV

as HR increases CO increases