Cardiac Cycle

Question 1

Cardiac cycle

Answer 1

1. SA node - located in wall of right atrium.
   * Spontaenously produces action potentials that travel through electrical conduction system.
   * Sets rhythm of heart - known as natural pacemaker.
   * Rate of action potential production/heartrate = influences by nerves that supply it.
2. AV node - Part of electrical conduction system.
   * Electrically connects the right atrium and right ventricle, delaying impulses os that the atria have time to eject blood into the ventricles before ventricular contraction.

Question 2

SA node pacemaker potentials

Answer 2

1. Phase 4 - pacemaker: Membrane repolarises below the lf threshold. Unstable so not actually resting potential. At -50mV, Na+ channels are activated causing Na+ influx and slow depolarisation.
2. Phase 0 - voltage gated Ca2+ channels: As the cell depolarises, it reaches a threshold for voltage gated Ca2+ channels leading to Ca2+ influx –> RAPID depolarisation.
3. Repolarisation - Ca2+ channels switched off at max depolarisation. Activatioj of voltage gates K+ channels – > K+ leaves which causes repolarisation.

Question 3

Atrial and ventricular muscle action potentials

Answer 3

1. Phase 0: Rapid depolarisation - Receives depolarisation stimulus from SA node - causes voltage-gated Na+ channels to open and Na+ influx, and voltage-gated Ca2+ channels start to open.
2. Phase 1: early repolarisation - Na+ channels close, cells beggin to repolarise.
3. Phase 2: plateau phase - Voltage-gated Ca2+ channels open fully, causing influx in Ca2+. Voltage gated K+ channels start to open.
4. Phase 3: rapid repolarisaton - Ca2+ channels close and K+ channels opem fully –> K+ efflux.
5. Phase 4: resting phase - Stable Na+/K+ pump (3xNa+ out and 2xK+ in). The membrane is slightly impearmable to Na+ and permeable to K+

Question 4

Electrical conduction through the heart

Answer 4

Question 5

Electrical activity represented by the electrocardiogram (ECG)

Answer 5

Question 6

Cardiac diastole

Answer 6

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

Question 7

Cardiac cycle - general principles

Answer 7

• Electricall activity is generated at SA node and conducted through 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 rught and left sides of the heart are the same, but pressures are lower on the right.

Question 8

Atrial systole

Answer 8

• 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 9

Ventricular systole

Answer 9

• The ventricles begin contracting.
• After isovolumetric contraction (using energy to contract but not pumping blood), the pressure rises and forces open the aortic and pulmonary valves and blood is ejected from the ventricles.

Question 10

Left ventricular pressure changes

Answer 10

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

Question 11

Left ventricular volume changes

Answer 11

1. Filling ventricle contraction of atria. EDV 120ml.
2. Full ventricle high pressure closes mitral valve. Systole begins isovolumetric contraction.
3. Ventricular pressure overcomes aortic valve and blood ejected.
4. When ventricular pressure falls the aortic pressure closes the aortic valve, isovolumetric ventricular relaxation.

SV = EDV - ESV

Question 12

Ventricular pressure-volume loop

Answer 12

Question 13

Heart sounds

Answer 13

Question 14

Real heart anatomy

Answer 14