The Cardiac Cycle

Question 1

what is convection?

Answer 1

→Convection is the mass movement of fluid caused by pressure differences.

Question 2

what are the main functions of the heart, arteries, capillaries and veins in the context of circulation?

Answer 2

→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 3

describe the Sinoatrial Node (SAN)

Answer 3

The SA node is a group of cells located in the walls of the right atrium.

Question 4

what are the functions of the SAN?

Answer 4

→ action potentials that travel through the heart via the electrical conduction system.

→It sets the rhythm of the heart
→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 5

what are the phases of the SAN pacemaker potentials?

Answer 5

PHASE 4: PACEMAKER POTENTIALS
→ “funny current (If)”
→At the end of an SA action potential, the membrane repolarises below the If threshold ( -40/50 mV)
→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 a Ca2+ influx
→making the inside less negative
→RAPID depolarisation
→Voltage-gated Na+ channels are not involved

PHASE 3: REPOLARISATION 
→The calcium channels switch off. 
→activation of voltage-gated K+ channels 
→ K+ efflux
→ repolarising the cell

Question 6

what are the phases of atrial/ventricular action potentials?

Answer 6

PHASE 0: RAPID DEPOLARISATION
→Voltage-gated Na+ channels open, causing a Na+ influx.
→Voltage-gated Ca2+ channels start to open slowly.

PHASE 1: EARLY REPOLARISATION
→Na+ channels close.

PHASE 2: PLATEAU PHASE
→Voltage-gated calcium channels are now fully open, causing a Ca2+ influx.
→The voltage-gated K+ channels start to open slowly.

PHASE 3: RAPID REPOLARISATION
→Voltage-gated calcium channels close.
→K+ channels open fully, causing a K+ efflux.

PHASE 4: RESTING PHASE
→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 7

describe the electrical conduction through the heart

Answer 7

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 8

P WAVE

Answer 8

atrial depolarisation

Question 9

PR SEGMENT

Answer 9

AV node delay

Question 10

QRS COMPLEX

Answer 10

ventricular depolarisation (atria repolarising simultaneously)

Question 11

ST SEGMENT

Answer 11

time during which ventricles are contracting and emptying

Question 12

T WAVE

Answer 12

ventricular repolarisation

Question 13

TP INTERVAL

Answer 13

time during which ventricles are relaxing and filling

Question 14

where is electrical activity generated and what is it converted into?

Answer 14

→electrical activity is generated at the SA node andconducted throughout the heart

→electrical activity is converted into myocardial contraction which creates pressure changes within chambers

Question 15

where does blood flow from usually (pressure) ?

Answer 15

→blood flows from an area of high pressure to low pressure

→unless the flow is blocked (by a valve, for example)

Question 16

what do valves do?

Answer 16

→valves open and close depending on the pressure changes in the chambers

Question 17

when do events occur in the heart and is there a pressure imbalance?

Answer 17

→events of the right and left sides of the heart are the same
→pressures are lower on the right

Question 18

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

Answer 18

1) Venae Cavae to Right Atrium
2) Past the Tricuspid Valve
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 19

describe VENTRICULAR FILLING/ ATRIAL CONTRACTION

Answer 19

→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

Question 20

describe ISOVOLUMETRIC CONTRACTION

Answer 20

→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.

Question 21

describe EJECTION

Answer 21

→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.

Question 22

describe ISOVOLUMETRIC RELAXATION

Answer 22

→The pressure in the aorta/pulmonary artery is greater than in the ventricle
→aortic/pulmonary valves close
→The closed ventricles relax, ready to receive blood.

Question 23

what is the ventricular pressure-volume loop and what does the area inside the loop represent?

Answer 23

→ 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 24

what is the equation for work?

Answer 24

Work = change in ventricle pressure x change in volume

Question 25

what is the ‘LUB’ sound caused by?

Answer 25

Closure of tricuspid/mitral valves at the beginning of ventricular systole

Question 26

what is the ‘DUB’ sound caused by?

Answer 26

Closure of aortic/pulmonary valves (semilunar valves) at the end of ventricular systole

Question 27

what are the heart sounds caused by?

Answer 27

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

Question 28

what is S3?

Answer 28

Turbulent blood flow into ventricles, detected near the end of first 1/3 diastole, especially in older people

Question 29

what is S4?

Answer 29

Forceful atrial contraction against a stiff ventricle, less so in young people