The cardiac cycle

Question 1

The heart is made up of 3 types of muscle cells (cardiomyocytes).
what are they called?

Answer 1

contractile myocytes, autorhythmic pacemaker cells & conducting myocytes

Question 2

describe contractile myocytes

Answer 2

form 99% of heart muscle (myocardium) which is the middle layer of the heart wall
striated like skeletal muscle

Question 3

what is the role of contractile myocytes?

Answer 3

cannot generate an action potential under normal circumstances. Instead, electrical activity in these cells is caused by a change in their membrane potential (the +ve or -ve charge at their cell membrane) as a result of transfer of ions from neighbouring conducting cells.

Question 4

how are contractile myocytes different to skeletal muscle cells?

Answer 4

less nuclei and more mitochondria to meet the high metabolic demands of repeated contraction

Question 5

what is the role of autorhythmic pacemaker cells?

Answer 5

can initiate and transmit action potentials causing electrical depolarisation (‘automaticity’).

Question 6

where are autorhythmic cells found?

Answer 6

Sinoatrial node (SA node) - initiates approximately 100 action potentials per minute
The Atrioventricular node (AV node) and Purkinje fibres have some pacemaking ability if the SA node fails, but the rate is slow at 40-60 and 20-40 bpm respectively.

Question 7

what reduces the activity of the sino atrial node?

Answer 7

The parasympathetic vagus nerve reduces this to 70-80 bpm

Question 8

what can raise heart rate an why would this occur?

Answer 8

The sympathetic nervous system, via the cardiac acceleratory nerve, can raise the rate during stress, exercise or disease

Question 9

describe the role of the conducting myocytes?

Answer 9

these non-contractile cells make up 1% of heart cells and spread the cardiac action potential from the SA node to Pukinje fibres.

Question 10

All heart muscle cells have characteristics which distinguish them from skeletal or smooth muscle cells.
name 3 of these characteristics

Answer 10

desmosome (macula adherens)
Intercalated discs
gap junctions

Question 11

The cardiac myocyte junction is very important for…

Answer 11

mechanical strength and electrical conduction

Question 12

what is an intercalated disc what is it’s role?

Answer 12

This disc forms a junction between adjacent cardiac myocytes and contains the gap junction and the desmosome.

These give mechanical, chemical and electrical connectivity between the individual myocytes so that the entire myocardium can function as a single unit - known as ‘functional syncytium’.

Question 13

why is functional syncytium important?

Answer 13

the atrial muscle contracts as one unit followed by the ventricular muscle contracting as one unit. This permits the coordinated pumping of blood through the heart. The junction as a whole acts to prevent myocardial fatigue

Question 14

what is a desmosome what is it’s role?

Answer 14

anchor and tightly bind adjacent myocytes to prevent separation during myocardial contraction
allow transfer of the force and tension from one contracting myocyte to its neighbour.

Question 15

what is a gap junction and what is it’s role?

Answer 15

channels between adjacent myocytes allow for the transmission of ions involved in depolarisation and propagation of an action potential - such as potassium, sodium and calcium.

This allows for rapid spread of the wave of electrical impulse throughout the heart.

Question 16

Myocardial pacemaker cells have the ability to self-generate action potentials. They do not have a resting membrane potential instead they have a pacemaker potential.
what is a pacemaker potential?

Answer 16

is the slow positive increase in membrane potential between the end of one action potential and the next.

Question 17

describe the generation of an action potential in pacemaker myocytes

Answer 17

Slow depolarisation begins at a voltage of - 60mv which differs from the rapid depolarisation typical of contractile myocytes from - 90mv. Slow channels open allowing passive movement of sodium ions into the cell which gradually make the membrane potential more positive. +ve pottasium ion eflux from the cell gradually stops.

All other channels are closed until - 40mv when the threshold for an action potential is initiated. At this point voltage-gated channels open allowing rapid influx of calcium ions which depolarise the membrane potential to above 0 mv. The process of repolarisation then occurs as calcium channels gradually deactivate and channels open allowing +ve potassium ion efflux out of the cell. The membrane potential becomes more -ve until - 60mv is reached and the cycle repeats.

Question 18

describe the generation of an action potential in contractile myocytes

Answer 18

Phase 4 - Resting membrane phase: the resting potential of a non-pacemaker myocyte is -90mv due to the constant outward leak of K+ out of the cell (true & stable resting pot.)

Phase 0 - Action potential and depolarisation: an action potential in a neighbouring cell causes Na+ to move across the gap junction and stimulate the resting membrane potential of the new cell to rise above -90. this causes slow Na+ channels to open until voltage reaches -70 to -75. then, fast Na+ channels open. this causes depolarisation & membrane potential reaches 0+mv

Phase 1 - Early repolarisation: fast Na+ channels close, no more +tive ions pass into cell. K+ channels begin to close so +tive ions are kept in the cell - causes dip in the membrane potential

Plateau - Phase 2: membrane potential remains steady due to countercurrent movement of Ca2+ ions in & K+ ions out = electrical balance

Repolarisation - Phases 3 + 4: Ca2+ channels close, less +tive ions enter cell. K+ channels reopen, +tive ions lost from cell. voltage returns to resting potential (-90)

Question 19

name the structures involved in electrical conduction of the heart

Answer 19

sino atrial node
atrioventricular node
bundle of his
purkinje fibres
bachmann’s bundle
internodal tracts

Question 20

describe the sino atrial node and its functions

Answer 20

This specialised area of non-contractile myocytes lies just below the superior vena cava in the upper right atrial wall. It is the heart’s main pacemaker which initiates action potentials at a rate of 70 - 80 per minute in the average resting adult. This rate is heavily influenced by the autonomic nervous system, emotional stress, exercise, temperature, hormones and certain drugs.

Question 21

describe the internodal tracts and their functions

Answer 21

The anterior, middle and posterior internodal tracts transmit the wave of electrical impulse rapidly from the SA node to the AV node across the right atrium.

Question 22

describe bachmann’s bundle and it’s functions

Answer 22

This specialised group of conduction cells transmits the cardiac action potential and wave of excitation across to the left atrium ensuring that the right and left atrium depolarise and contract in together to allow coordinated ventricular filling

Question 23

describe the bundle branches and their functions

Answer 23

The right and left Bundle branches emerge from the Bundle of His. These transmit electrical impulses from the AV node to the apex of the heart where they communicate with the Purkinje fibres.

Question 24

what happens if one of the bundles of his gets blocked?

Answer 24

leads to problems with electrical transmission to left or right ventricle.

Question 25

describe the purkinje fibres and their functions

Answer 25

These fibres conduct the electrical impulses from the right and left branches of the Bundle of His to the ventricles.

The rate of action potential firing is usually dictated by the SA node but if this, or the AV node, is dysfunctional then the Purkinje fibres will initiate approximately 20 - 40 action potentials/heart bpm. However, this is unlikely to sustain enough cardiac output to meet the metabolic needs of the body.

Question 26

describe the atrioventricular node and its functions

Answer 26

This area of specialised pacemaker cells lies at the inferior posterior aspect of the right atrium just above the tricuspid valve.

The node receives the electrical impulses form the SA node and internodal tracts but delays the transmission to the Bundle of His in the interventricular septum by approximately 0.09 seconds.

This delay ensures that the atria depolarise and contract before the ventricles so that blood flows in a timely and appropriate way through the heart, allowing ventricular filling

Question 27

Electrical activity of myocytes can be measured via electrodes/leads placed on the surface of the chest and peripheries of the limbs
what does this do?

Answer 27

These measure the voltage across cell membranes of the myocytes and can determine whether the electrical conduction is moving towards the electrodes or away from them. This gives the waveform deflections on an electrocardiograph (ECG).

Question 28

name the components of an ECG

Answer 28

P wave
PR interval
QRS complex
ST segment
T wave

Question 29

what does the p wave represent?

Answer 29

This represents atrial depolarisation. During this stage, the ventricles are passively filling with blood

Question 30

what does the PR interval represent?

Answer 30

This represents the time it takes for the electrical impulse to spread from the SA node to the AV node, including the delay imposed at the AV node. The normal interval is approximately 0.12 seconds.

During this stage the atria contract and complete ventricular filling.

Question 31

what does the QRS complex represent?

Answer 31

This represents ventricular depolarisation as the impulse passes down the Bundle of His and bundle branches to the heart apex and then via the Purkinje fibres to the ventricular muscle. The atria are repolarising during this stage.

Question 32

what does the ST segment represent?

Answer 32

This represents the interval between ventricular depolarisation and repolarisation. During this stage the ventricles are contracting and emptying of blood into the great vessels. The ECG line is flat because no electrical activity is occurring.

Question 33

what does the T wave represent?

Answer 33

This represents ventricular repolarisation. During this stage the ventricles are relaxed (ventricular diastole) and filling with blood

Question 34

name the cardiac cycle stages

Answer 34

1. Isovolumetric relaxation (early diastole)
2. Ventricular filling (late diastole)
3. Atrial systole (contraction)
4. and 5. Isovolumetric contraction and ventricular systole
5. Ventricular systole (contraction)

Question 35

what happens in the Isovolumetric relaxation (early diastole) stage of the cardiac cycle?

Answer 35

the ventricles are relaxed in diastole having just ejected blood into the great vessels. All heart valves are closed. There is no change in ventricular blood volume during this stage (hence the name iso volumetric) but the ventricular pressure reduces as the ventricular chamber muscle relaxes.

The semilunar valves are closed due to greater pressure in the aorta and pulmonary artery than the ventricles, hence shutting the valves. The atrioventricular valves remain closed because there is insufficient blood volume or pressure in the atria to force the valves open.

Question 36

what happens in the Ventricular filling (late diastole) stage of the cardiac cycle?

Answer 36

The ventricles remain relaxed (late - diastole).

Blood flows into the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins.

The atrioventricular valves open. This is because as the atria fill with blood this forces the valves open allowing blood to flow into the ventricles.

Approximately 75 % of ventricular filling occurs by this passive method.

The two semilunar valves, the pulmonary and aortic valves, are closed preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.

Question 37

what happens in the Atrial systole (contraction) stage of the cardiac cycle?

Answer 37

Once the Sinoatrial node has fired, causing atrial depolarisation, these chambers contract in atrial systole actively pushing the final 25% of blood into the ventricles through the open atrioventricular valves.

The ventricles are still relaxed. At the end of ventricular filling, the blood volume in the ventricles is called the end-diastolic volume.

Question 38

what happens in the Isovolumetric contraction and ventricular systole stage of the cardiac cycle?

Answer 38

The atria now relax in atrial diastole and their pressure drops below that of the ventricles whose pressure rises with the received blood volume and the start of ventricular systole/contraction following ventricular depolarisation.

This pressure gradient reversal, where ventricular pressure exceeds atrial pressure, causes the atrioventricular valves to close - the first heart sound S1 (‘Lub’). The ventricular pressure is not high enough to open the semilunar valves, as aortic and pulmonary artery pressures still exceed ventricular pressure. This is phase is known as the ‘Isovolumetric contraction’ because the ventricular blood volume remains unchanged but pressure increases due to the start of ventricular muscle contraction.

Question 39

what happens in the Ventricular systole (contraction) stage of the cardiac cycle?

Answer 39

As the ventricles fully contract, the pressure increases to above that in the aorta and pulmonary arteries which forces the semilunar valves open - causing 2nd heart sound S2 (‘dub’). Ventricular blood is ejected into the aorta and pulmonary artery. The blood ejected from each ventricle is known as the stroke volume.

Some blood always remains in the ventricle - called the end-systolic volume. It is approximately 70 mls in health in adults.

This marks the end of the cardiac cycle.

Question 40

The first phase of the cardiac cycle is Early diastole (isovolumetric relaxation). Chose the statement which best describes this phase:

a) ventricular blood volume remains constant. Ventricular pressure reduces as the muscle relaxes. All valves are closed.

b) ventricular blood volume increases. Ventricular pressure remains the same. All valves are closed.

c) ventricular volume and ventricular pressure both reduce as the muscle relaxes. All valves are open

Answer 40

a) ventricular volume remains constant wheres the ventricular pressure reduces as the muscle relaxes. All valves are closed.

During the first phase of the cardiac cycle - early diastole/relaxation (isovolumetric relaxation) the blood volume in the ventricle remains constant. This is because all heart valves are closed so that no blood can enter or leave.

The ventricle chamber pressure reduces as the ventricular muscle relaxes during diastole.

Venous blood starts filling the atria during this stage.

Question 41

Chose which statement best describes the second phase of the cardiac cycle - ventricular filling, late diastole:

a) the atrioventricular valves are closed and blood fills the atria

b) the atrioventricular valves are closed following atrial contraction and ejection of blood into the ventricles

c) the atrioventricular valves are open and blood flows passively into the ventricles from the atria

Answer 41

c) the atrioventricular valves are open and blood flows passively into the ventricles from the atria

During this phase blood continues to fill the atria and fall into the ventricles below causing 75% of filling.

Question 42

Which of these statements about Atrial systole are true?

a) both atria contract together to force the final 25% of blood into the ventricles

b) the ventricles remain relaxed because there is a delay in the electrical conduction from the atria at the AV node. This prevents the ventricles from depolarising and contracting at the same time as the atria

c) the volume of blood in the ventricles at the end of this stage is called the end diastolic volume

d) the atrioventricular valves are open but the semilunar valves are closed

Answer 42

All of the statements a) - d) are true.

During this stage the semilunar valves are closed because the pressure in the ventricles is still low as the muscle remains relaxed.

Question 43

The final stage of the cardiac cycle is Ventricular contraction/systole. What causes the second heart sound S2 during this phase?

a) the semilunar valves opening as the ventricular muscle contracts forcing the cusps to release. This allows blood ejection to the aorta and pulmonary artery.

b) the semilunar valves closing - this occurs after blood has been ejected from the ventricles into the great vessels. The higher pressure in these vessels and backflow of blood onto the semilunar valve cusps, cause the valves to shut. This prevents blood backflow to the ventricles.

Answer 43

b) the semilunar valves closing - this occurs after blood has been ejected from the ventricles into the great vessels. The higher pressure in these vessels and backflow of blood onto the semilunar valve cusps, cause the valves to shut. This prevents blood backflow to the ventricles.