Cardiac Cycle

Question 1

Define convection and describe the purpose of the heart, arteries, capillaries and veins

Answer 1

Convection: Mass movement of fluid caused by pressure difference
Heart - driving force (creates large pressures)
Arteries - distribution (alter blood flow)
Capillaries - exchange (huge numbers - thin)
Veins - reservoir (2/3rd of blood volume)

Question 2

When is diffusion slow and what is it not good for?

Answer 2

Diffusion: Very slow of distance >1 mm, useless for whole body O2 transport

Question 3

Describe the SA node (5)

Answer 3

• SA node is a 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
• 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.
• The atrioventricular node, or 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 right ventricle delaying impulses so that atria have time to eject their blood into ventricles before ventricular contraction.

Question 4

In terms of Sino-atrial node (SA) pacemaker potentials what is the voltage inside the cell?

Answer 4

There is a resting negative voltage in the cell interior as compared to the cell exterior ranging from - 40mV to - 80mV.

Question 5

What ions are pumped into and out of the cell?

Answer 5

Normally high K+ inside high Na+ Cl- outside.

Sodium–potassium pump, uses ATP to transport three sodium ions out of the cell and two potassium ions in.

Question 6

Describe phase 4, 0 and 3

Answer 6

Phase 4: Pacemaker potentials - the ‘ funny current – If ’
This is the resting membrane potential the membrane repolarizes below the If threshold (approx. − 40mV). This is not a genuine resting potential because it is unstable.

At -50mV an Na+ channel is activated, causing Na+ influx and slow depolarisation.

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. Voltage-gated Na+ channels not involved as in normal nerve depolarisation.

Phase 3: Repolarisation
Ca2+ channels switched off
Activation of voltage-gated K+ channels K+ efflux.

Question 7

Describe the phases in atrial and ventricular muscle action potentials

Answer 7

Phase 0: 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
Phase 1: early repolarisation
Na+ channels close cells beginning to repolarise
Phase 2: plateau phase
Voltage gated calcium channels fully open - Ca2+ influx halts the repolarisation.
Voltage-gated K+ channels start to open slowly
Phase 3: rapid repolarisation
Ca2+ channels close & K+ channel open fully so K+ efflux
Phase 4: resting phase
Stable - Na+/K+ pump – 3xNa+ out & 2xK+ in
Membrane slightly impermeable to Na+ slightly permeable to K+

Question 8

Describe the 4 steps of 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 apex

Question 9

Describe what is shown in an ECG and the different segments and intervals.

Answer 9

On image

Question 10

What are the general principles of the cardiac cycle?

Answer 10

• 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 11

Describe the flow of blood through the heart

Answer 11

1. Venae Cavae) - Right atrium
2. Tricuspid valve (AV)
3. Right ventricle
4. Pulmonary (semilunar) valve
5. Pulmonary arteries
6. Lung Circulation
7. Pulmonary veins
8. Left atrium
9. Bicuspid (Mitral) valve (AV)
10. Left ventricle
11. Aortic (semilunar) valve
12. Aorta
13. Systemic circulation

Question 12

Describe the 4 stages in the cardiac cycle

Answer 12

. Ventricular filling/atrial contraction = Diastole
• Blood enters atria and move into ventricles.
• Pressure in atria > ventricles
• mitral/tricuspid valves open
• aided by atria contraction.

2. Isovolumetric contraction = systole

• Pressure in full ventricles > atria.
• mitral/tricuspid valves close.
• Contraction on closed ventricle and so pressure rises.

3. Ejection = systole

• Pressure in ventricles > aorta / pulmonary artery.
• Valves open, ejection of blood.
• Blood enters atria.

4. Isovolumetric relaxation = Diastole
   • Pressure in aorta/pulmonary artery > ventricles.
   • Aortic/pulmonary valves close.
   • Closed ventricle relaxes ready to receive blood.

Question 13

Describe left ventricular pressure changes

Answer 13

1. Contraction of left atrium (during ventricular diastole), ventricular pressure rises slightly. Mitral valve closes ventricle > atrial pressure.
2. Pressure rises during isovolumetric contraction.
3. When ventricle pressure > aorta the aortic valve opens and blood is ejected.
4. Ventricle empties and ventricular pressure < aortic valve so valve closes. We then get isovolumetric relaxation and large pressure drop below that of atrium and mitral valve opens.

Question 14

Describe left volume pressure changes

Answer 14

1. Filling ventricle contraction of atria. EDV 120ml.
2. Full ventricle higher pressure closes mitral valve. Systole begins no change in volume.
3. Ventricular pressure overcomes aortic valve and blood ejected.
4. When ventricular pressure falls the aortic pressure closes aortic valve, isovolumetric ventricular relaxation

Question 15

Describe ventricular pressure - volume loop

Answer 15

• A 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.

Question 16

Right atrial cycle and jugular venous pressures changes

Answer 16

On image

Question 17

Describe the heart sounds (S1, S2, S3 and S4)

Answer 17

• Vibrations induced by closure of cardiac valves
• Vibrations in ventricular chambers
• Turbulent blood flow through valves
S1 – “Lub”
Closure of tricuspid/mitral values at beginning of ventricular systole.
S2 – “Dub”
Closure of aortic/pulmonary valves (semilunar valves) at end of ventricular systole.
S3 – Occasional
Turbulent blood flow into ventricles, detected near 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 18

Give the whole picture overview (7)

Answer 18

1. Atrial contraction
2. Isovolumetric ventricular contraction
3. Rapid ventricular ejection
4. Reduced ventricular ejection
5. Isovolumetric ventricular relaxation
6. Rapid ventricular filling
7. Reduced ventricular filling