Cardiovascular System 3

Question 1

Describe the conducting system of the heart

Answer 1

Muscle cells send signals to the rest of the heart muscle causing a contraction

Impulses from SA node are conducted across the atria from right to left

Pathway

• AV bundle
• Internodal pathways
• Right and left bundle branches (Bundle bunches)
• Purkinje fibres
• Mechanical pathway that should be followed

Question 2

Describe a cardiac action potential

Answer 2

1. Rapid Depolarization
   - Voltage-gated sodium channels open
   - Rapid influx of Na+
2. Plateau
   - Na+ channels close rapidly (~+30mV) → Na+ loss
   - Voltage-gated slow Ca2+ channels open (slowly for long time) → Ca2+ entry
   - Balances slow Na+ outflow (~0mV)
3. Repolarisation
   - Voltage-gated slow Ca2+ channels close
   - Voltage-gated slow K+ channels open → K+ loss

Question 3

Explain the electrical and mechanical events of the cardiac cycle

Answer 3

1. Atrial systole begins: Atrial contraction forces a small amount of additional blood into relaxed ventricles
2. Atrial systole ends, atrial diastole begins
3. Ventricular systole - first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves
4. Ventricular systole - second phase: As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected.
5. Ventricular diastole - early: As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria.
6. Ventricular diastole - late: All chambers are relaxed Ventricles filled passively.

Question 4

Identify the electrical events associated with a normal ECG

Answer 4

P wave - atrial depolarisation

QRS wave - ventricular repolarization

T wave - ventricular repolarization

Question 5

Define cardiodynamic variables

Answer 5

Movements of blood and forces generated during cardiac contractions

End-diastolic volume (EDV) = Volume of blood in each ventricle at end of ventricular diastole

End-systolic volume (ESV) = Volume of blood remaining in each ventricle at the end of ventricular systole (= 40% of EDV)

Stroke volume (SV) = Volume of blood pumped out of each ventricle during a single beat = SV = EDV – ESV = Most important factor in single cardiac cycle

Ejection fraction = The percentage of EDV represented by SV (= 60% of EDV)

Question 6

Explain the factors affecting cardiac output, HR and stroke volume

Answer 6

Affecting cardiac output

- Heart rate (HR)
- Stroke Volume (SV)

Affecting heart rate (HR)

- Autonomic innervation
- Hormones

Affecting stroke volume (SV)

- End-diastolic volume
- End-systolic volume