3) Cardiac Cycle Flashcards
1
Q
Describe the Sinoatrial node (SAN)
A
- It is a group of cells located in the wall of the right atrium which produces an action potential (due to an unstable resting potential)
- This action potential can travel through the heart through an electrical conduction system
- It sets the rhythm of the heart and is known as the heart’s natural pacemaker.
- The heart rate/ rate of action potentials produced is influenced by the nerves that supply it and chemicals such as hormones
2
Q
Describe the Atrioventricular node (AV)
A
- Part of the electrical conduction system of the heart which (electrically) connects the right atrium and ventricle.
- It delays the impulses between the two so that the atria have time to completely eject their blood into the ventricles before ventricular contraction occurs.
3
Q
What is potential?
A
- The difference in charge from inside and outside of a cell
4
Q
What is the resting voltage?
A
- The voltage at which no action potential is fired
- This ranges from -40 mV to -80 mV
5
Q
Describe the distribution of ions and how they are moved across the membrane?
A
- There is normally a high concentration of K+ and Ca+ inside the cell and a high concentration of Na+ and Cl- outside the cell.
- Na+ and K+ are transported across the membrane via the Sodium-Potassium pump.
- It uses a molecule of ATP to transport 3 Na+ out of the cell and 2 K+ into the cell.
6
Q
What is depolarisation and repolarisation?
A
- Depolarisation: Increasing potential difference
- Repolarisation: Decreasing potential difference
7
Q
Describe the different potentials of the Sino-atrial node (SA) pacemaker
A
- Phase 4: The membrane repolarises to the If threshold (around -40 mV). It is not the resting potential as it is unstable. Around -50 mV the Na+ channels activate causing an influx of Na+ ions leading to slow depolarisation.
- Phase 0: As the cell depolarises it reaches a threshold for voltage gated Ca2+ channels to open causing an influx of Ca2+ leading to rapid depolarisation
- Phase 3: Ca2+ channels switch off at max depolarisation. Voltage-gated K+ channels are activated and K+ leave causing repolarisation.
8
Q
Describe the different action potentials in atrial and ventricular muscles.
A
- Phase 0 (Rapid depolarisation): Depolarisation stimulus is recieved from the SAN causing voltage-gated Na+ channels to open leading to Na+ influx. Voltage-gated Ca2+ channels start to open
- Phase 1 (Early repolarisation): The cell begins to repolarise as Na+ channels start to close
- Phase 2 (Plateau phase): Voltage-gated Ca2+ channels fully open causing repolarisation to stop. Voltage-gated K+ channels start to open.
- Phase 3 (Rapid depolarisation): Ca2+ channels are closed fully and K+ channels are opened fully causing K+ efflux.
- Phase 4 (Resting phase): The Na+/K+ pump stabilises the cell by transporting 3 Na+ out of the cell and 2 K+ into the cell. The membrane is slightly impermeable to Na+ and slightly permeable to K+
9
Q
Describe the electrical conduction through the heart.
A
- First electrical activity generated in SAN and spreads out via gap junctions into atria
- At AV node, conduction is delayed allowing correct filling of ventricles
- Conduction occurs rapidly through bundle of His into ventricles
- Conduction through Purkinjie fibres spreads quickly throughout ventricles
10
Q
Where does ventricular contraction begin?
A
- Near the bottom
11
Q
How is electrical activity generated and what is it converted to?
A
- Electrical activity is generated in the SAN and conducted through the heart.
- Electrical activity is then converted to myocardial contractions which creates pressure changes within the chambers
12
Q
What is the importance of pressure changes in the heart?
A
- Blood flows from an area of high pressure to an area of low pressure (unless blocked by a valve)
- Valves open and close depending on pressure changes in chambers
13
Q
Which side of the heart experiences a higher pressure?
A
- The left side.
14
Q
What is isovolumetric contraction?
A
- Contraction of the ventricles where the pressure inside the ventricles changes but the volume does not.
- This is occurs when the ventricle is contracting but the pulmonary and aortic valves are shut
15
Q
Explain the cardiac cycle.
A
- Cardiac diastole: Heart muscles relax. Blood returns to the heart and fills up the atria. Low pressure in the ventricles allows the mitral and tricuspid valves to open allowing the ventricles to fill with blood.
- Atrial systole: Contraction of atria pumps blood into the ventricles which are relaxed. As ventricles fill up, the pressure inside the ventricles increases causing the mitral and tricuspid valves to close.
- Ventricular systole: The ventricles contract. After the contraction the pressure rises (isovolumetric contraction) which causes the aortic and pulmonary valves to open and blood is ejected from the ventricles
- After this stage the heart goes back to cardiac diastole and the cycle starts again
