Cardiac Cycle

Question 1

Describe the SA node and it’s function

Answer 1

• group of cells located in the wall of the
  right atrium.
• Ability to spontaneously produce action potential that
  travels through the heart via the electrical conduction
  system
• Set the rhythm of the heart and so is known as the
  heart’s natural pacemaker.
• The rate of action potential production (and therefore
  the heart rate) is influenced by nerves that supply it.

Question 2

Describe the AV node and it’s function

Answer 2

• part of the electrical conduction system of the heart
• electrically connects the right atrium and
  right ventricle delaying impulses so that atria have time to eject their blood into ventricles before ventricular
  contraction.

Question 3

Describe Phase 0 of the atrial & ventricular muscle action potential

Answer 3

• rapid depolarisation
• Receives depolarisation stimulus from SA node causing:
  Voltage-gated Na+ channels open, Na+ influx.
  Voltage-gated Ca2+ channels start to open very slowly.

Question 4

Describe Phase 1 of the atrial & ventricular muscle action potential

Answer 4

• early repolarisation

- Na+ channels close cells beginning to repolarise.

Question 5

Describe Phase 2 of the atrial & ventricular muscle action potential

Answer 5

• plateau phase
• Voltage gated calcium channels fully open - Ca2+ influx halts the repolarisation.
• Voltage-gated K+ channels start to open slowly

Question 6

Describe Phase 3 of the atrial & ventricular muscle action potential

Answer 6

• rapid repolarisation

- Ca2+ channels close & K+ channel open fully so K+ efflux

Question 7

Describe Phase 4 of the atrial & ventricular muscle action potential

Answer 7

• resting phase
• Stable - Na+/K+ pump – 3Na+ out & 2K+ in
• Membrane slightly impermeable to Na+ slightly permeable to K+

Question 8

Describe electrical conduction through the heart

Answer 8

• Electrical activity generated in SA node
  spreads out via gap junctions into atria
• At AV node, conduction is delayed to allow
  correct filling of ventricles
• Conduction occurs rapidly through bundle
  of His into ventricles
• Conduction through Purkinje fibres spreads
  quickly throughout the ventricles
• Ventricular contraction begins at the a

Question 9

Give a general overview of the cardiac cycle

Answer 9

• Electrical activity is generated at SA node and conducted throughout heart.
• Electrical activity is converted into myocardial contraction which creates pressure changes within chambers.
• Blood flows from an area of high pressure to an area of low pressure - unless flow is blocked by a valve.
• Valves open and close depending on pressure changes in chambers.
• Events on the right and left sides of the heart are the same, but pressures are lower on the right.

Question 10

List the different phases of the cardiac cycle

Answer 10

• Atrial Systole
• Ventricular Systole
• Ventricular diastole

Question 11

Describe the process of cardiac diastole

Answer 11

• Relaxation of all the heart muscles.
• Blood returns to the heart and begins to
  fill the atria and ventricles.
• Low pressure in the ventricles allows the
  mitral and tricuspid valves to open and
  the ventricles fill with blood

Question 12

Describe the process of Atrial systole

Answer 12

• Atrial contraction causes blood to move
  into relaxed ventricles.
• As the ventricles fill, the increase in
  pressure in the ventricles forces mitral
  and tricuspid valves to close.

Question 13

Describe the process of Ventricular systole

Answer 13

• The ventricles enter systole and begin
  contracting.
• After a period of isovolumetric contraction
  pressure rises sufficiently to force open aortic
  and pulmonary valves and blood is ejected from
  the ventricles.
• After the atria relax, ventricles contract
  pushing blood out of the heart.

Question 14

Describe how the left ventricular pressure changes during the cardiac cycle

Answer 14

• Contraction of the left atrium pushes blood
  into the relaxed ventricle. Once the ventricle is
  full its pressure rises slightly and forces the
  mitral valve to close.
• Pressure rises during isovolumetric contraction
  of ventricle.
• When ventricle pressure is higher than the
  aorta, the aortic valve is pushed open and
  blood is ejected from the ventricle.
• Ventricle empties and once its pressure is less
  than the aorta the aortic valve closes. This is
  followed by isovolumetric relaxation and large
  pressure drop below that of atrium causing
  mitral valve to open.
• Blood flows into the relaxed heart in
  preparation for another atrial systole.

Question 15

Describe how the left ventricular volume changes during the cardiac cycle

Answer 15

- Filling ventricle contraction of atria.  
EDV ( end diastolic volume ) = 120ml.
- Full ventricle higher pressure closes 
mitral valve.  Systole begins 
isovolumetric contraction.
- Ventricular pressure overcomes aortic 
valve and blood ejected.
- When ventricular pressure falls the 
aortic pressure closes aortic valve, 
isovolumetric ventricular relaxation. End systole volume 
(ESV) = 40 ml

Question 16

How do you calculate the stroke volume ?

Answer 16

Stroke volume (SV) = EDV - ESV

Question 17

Describe how the

Answer 17

Diastole, ventricle relaxed and filling so
the pressure remains low but volume
increases.
B Mitral valve closes and ventricle
contracts, the volume doesn’t change as
both mitral valves and aortic valves are
closed, but pressure increases.
C Pressure becomes high enough to force
aortic valve open and blood begins to leave
the ventricle. The volume decreases as
pressure continues to rise because the heart
is still contracting.
D As the ventricle empties the aortic vavle
closes and the ventricle relaxes so the
pressure falls dramatically.