The Cardiac Cycle

Question 1

As a recap, what is convection and what are the main functions of the heart, arteries, capillaries and veins in the context of circulation?

Answer 1

• Convection is the mass movement of fluid caused by pressure differences.
• HEART: driving force (creates large pressures).
• ARTERIES: distribution (mostly in parallel to alter blood flow).
• CAPILLARIES: exchange (found in huge number, thin for ease of movement).
• VEINS: reservoir (2/3rd of the blood volume stored in veins and venules).

Question 2

Describe the Sinoatrial Node (SAN) and its functions.

Answer 2

• The SA node is a group of cells located in the walls of the right atrium.
• It has the ability to spontaneously produce action potentials that travel through the heart via the electrical conduction system.
• It sets 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 3

Describe the Atrioventricular Node (AVN) and its functions.

Answer 3

• The AV node is a part of the electrical conduction system of the heart that coordinates the top of the heart.
• It electrically connects the right atrium and the right ventricle, delaying impulses so that the atria have time to eject their blood into ventricles before ventricular contraction.

Question 4

Describe the phases of the SAN pacemaker potentials.

Answer 4

1. PHASE 4: PACEMAKER POTENTIALS - “funny current (I_f)”
   
   • As the membrane potential falls below -40mV, the hyperpolarisation causes the funny channels (I_f) to open.
   • At ~ -50 to -60mV the Na+ ion channels open allowing the slow influx of Na+ ions. This causes slow depolarisation of the cell.
2. PHASE 0: VOLTAGE-GATED Ca2+ CHANNELS
   
   • As more Na+ enters the cell it causes the cell to further depolarise.
   • Eventually it reaches a threshold (~ between -30 to -40mV) casuing the voltage-gated Ca2+ channels to open.
   • This leads to a Ca2+ influx (into the cell), causing rapid depolarisation.
3. PHASE 3: REPOLARISATION
   
   • At ~ +10mV the voltage gated Ca2+ channels close and the voltage-gated K+ channels open.
   • This causes K+ ions to leave the cell, and causes the repolarisation of the cell.

Question 5

Describe the phases of atrial/ventricular action potentials.

Answer 5

• PHASE 0: RAPID DEPOLARISATION
  
  • Voltage-gated Na+ channels open at -60 to - 70mV, causing rapid Na+ influx into the cell.
• PHASE 1: EARLY REPOLARISATION
  
  • At ~ +20 to +30mV the voltage gated Na+ channels close.
  • At the same time the voltage-gated K+ channels also open, causing K+ ions to move out of the cell.
  • These two together begin the repolarisation of the cell.
• PHASE 2: PLATEAU PHASE
  
  • The voltage-gated calcium channels open, causing a Ca2+ influx.
  • At the same time the voltage-gated K+ channels are still open allowing K+ to move out of the cell.
  • This causes the membrane potential to not change much.
• PHASE 3: RAPID REPOLARISATION
  
  • The voltage-gated calcium channels close but the voltage-gated K+ channels stay open (and K+ to leave the cell) causing the membrane potential to decrease.
• ​PHASE 4: RESTING PHASE
  
  • The voltage-gated K+ channels close.
  • The Na+/K+ pump works to get Na+ out and K+ in.
  • The membrane is impermeable to Na+ but permeable to K+, which helps establish the equilibrium.

Question 6

Describe the electrical conduction through the heart.

Answer 6

1. Electrical activity generated in the SA node spreads out via the gap junctions into the atria.
2. At the AV node, conduction is delayed to allow the correct filling of the ventricles.
3. Conduction occurs rapidly through the bundle of His into the ventricles.
4. Conduction occurs through the Purkinje fibres and spreads quickly throughout the ventricles.

Ventricular contraction begins at the apex.

Question 7

There are many parts to an ECG. List what is represented by the P wave, PR segment, QRS complex, ST segment, T wave and TP interval.

Answer 7

• P WAVE: atrial depolarisation.
• PR SEGMENT: AV node delay.
• QRS COMPLEX: ventricular depolarisation (atria repolarising simultaneously).
• ST SEGMENT: time during which ventricles are contracting and emptying.
• T WAVE: ventricular repolarisation.
• TP INTERVAL: time during which ventricles are relaxing and filling.

Question 8

What are some general principles of the cardiac cycle regarding electrical conductivity, pressure and valves?

Answer 8

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

Question 9

Describe the movement of blood through the heart by listing all the structures it goes through/past.

Answer 9

1. Venae Cavae to Right Atrium
2. Past the Tricuspid Valve (AV)
3. Into Right Ventricle
4. Past Pulmonary (Semilunar) Valve
5. Into Pulmonary Arteries
6. Goes through Lung Circulation
7. Comes back in Pulmonary Veins
8. Enters Left Atrium
9. Past the Bicuspid (Mitral) Valve
10. Into Left Ventricle
11. Past Aortic (Semilunar) Valve
12. Into Aorta
13. Into Systemic Circulation

Question 10

Describe the cardiac cycle in terms of pressure and valve opening/closing.

Answer 10

1. VENTRICULAR FILLING/ ATRIAL CONTRACTION (diastole)
   
   • Blood enters the atria and moves into the ventricles.
   • The pressure in the atria is greater than in the ventricles, so the mitral/tricuspid valve opens, aided by atrial contraction.
2. ISOVOLUMETRIC CONTRACTION (systole)
   
   • The pressure in the full ventricles is greater than in the atria.
   • This closes the mitral/tricuspid valves.
   • There is a contraction of the closed ventricles, so the pressure rises.
3. EJECTION (systole)
   
   • The pressure in the ventricles is greater than in the aorta/pulmonary artery.
   • This causes the aortic/pulmonary valves to open, ejecting the blood.
   • Blood enters the atria.
4. ISOVOLUMETRIC RELAXATION (diastole)
   
   • The pressure in the aorta/pulmonary artery is greater than in the ventricles, so the aortic/pulmonary valves close.
   • The closed ventricles relax, ready to receive blood.

Question 11

What are the left ventricular pressure changes?

Answer 11

1. Contraction of the left atrium (during ventricular diastole), ventricular pressure rises slightly. Mitral valve closes when the ventricle pressure is greater than the atrial pressure.
2. The pressure rises durimg isovolumetric contraction.
3. When ventricle pressure is greater than the aorta the aortic valve opens and blood is ejected.
4. Ventricle empties and ventriculae pressure decreases to less than that of the aortic valve so th e valve closes. We then get isovolumetric relaxation and large pressure drop below that of the atrium and the mital valve opens.

Isovolumetric meaning - relating to or being an early phase of ventricular systole in which the cardiac muscle exerts increasing pressure on the contents of the ventricle without significant change in the muscle fiber length and the ventricular volume remains constant.

Question 12

What are the left ventricular volume changes?

Answer 12

1. The ventricle fills as the atria comtracts. EDV (End diastolic volume) 120ml.
2. The ventricle fills up and the higher pressure in the ventricle then in the atrium causes the mitral valve to close. Systole begins and there is no change in volume.
3. Ventricular pressure increases and the aortic valve opens and blood is ejected.
4. When ventricular pressure falls the aortic pressure closes the aortic valve causing isovolumetric ventricular relaxation.

Question 13

Describe the ventricular pressure-volume loop, and include the equation learnt for work.

Answer 13

Work = change in ventricle pressure x change in volume.

• The loop relates to the amount of energy consumption during the cardiac cycle.
• The area inside the loop is equal to the amount of stroke work done.

Question 14

Describe heart sounds, and list the four of them.

Answer 14

They are vibrations in ventricular chambers induced by the closure of cardiac valves or by turbulent blood flow through valves.

• S1 - “LUB”
  
  • Closure of tricuspid/mitral valves at the beginning of ventricular systole.
• S2 - “DUB”
  
  • Closure of aortic/pulmonary valves (semilunar valves) at the end of ventricular systole.
• S3 - OCCASIONAL
  
  • Turbulent blood flow into ventricles, detected near the end of first 1/3 diastole, especially in older people.
• S4 - PATHOLOGICAL IN ADULTS
  
  • Forceful atrial contraction against a stiff ventricle, less so in young people

Question 15

Describe the right atrial cycle and jugular venous pressure changes.

Answer 15