Cardiac Cycle

Question 1

What does the ‘lub’ and ‘dub’ sounds come from?

Answer 1

Lub= Atrioventricular valve closes (S1)
Dub= Semi lunar valves close (S2)

Question 2

What happens in the aorta and LV when the pressure starts falling in both?

Answer 2

Aorta= Elastic fibres in walls of aorta recoil
LV= Muscular walls start relaxing so heart volume increases

Question 3

What is a dicrotic notch?

Answer 3

This is an increase in pressure in the aorta when the aortic valve shuts and there is recoil of the elastic fibres in the walls of the aorta which increases the pressure so blood can be pumped all around the body.

Question 4

What is the a wave?

Answer 4

This is the bump on the cardiac cycle diagram where the atrium contracts which increases the pressure in the atrium

Question 5

What is the role of chordae tendinae?

Answer 5

They are attached to the atrioventricular valves are prevent the AV valves from buckling back into the atria under the very high pressure of the ventricles

Question 6

What is the c wave?

Answer 6

This is the increase in pressure in the atria as a result of the snapping shut of the AV valves causing a slight recoil of the blood. (This is similar to the dicrotic notch).

Question 7

What does EDV mean?

Answer 7

End diastolic volume- Volume in the ventricle at the end of diastole. About 130ml

Question 8

What does ESV mean?

Answer 8

End systolic volume- the volume in the the ventricle at the end of systole. About 60ml

Question 9

What is Stroke Volume?

Answer 9

The volume ejected per beat from each ventricle, about 70ml

Question 10

What does Inotropy mean?

Answer 10

Relates to the force of muscle contraction

Question 11

What does Chronotropy mean?

Answer 11

Relates to the change of rate

Question 12

What does Lusitropy mean?

Answer 12

Relates to the rate of relaxation

Question 13

When does blood enter the atria?

Answer 13

There are no valves into the atria so blood is always passing into the atria continuously.

Question 14

How do calcium ions affect the force of the muscular contraction?

Answer 14

The more calcium ions that bind to troponin molecules, the more cross bridges that form, and therefore the stronger the muscular contraction.

Question 15

How many phases of the Cardiac Cycle are there?

Answer 15

7

Question 16

What is pre-load?

Answer 16

This is the volume of blood in the ventricle at the end of diastole.

Question 17

What is Isovolumetric contraction?

Answer 17

This is when the ventricles contract with the valves closed in order to create high enough pressure for it to be greater than that of the aorta so blood can flow out of the ventricles.
The ventricular volume does not change because all valves are closed during this phase. Individual myocyte contraction is not necessarily isometric because individual myocytes are undergoing length changes.

Question 18

What is the c wave in a heart chamber pressure graph?

Answer 18

This is due to the bulging of mitral valve leaflets back into the left atrium when the left ventricle contracts and creates a very high pressure in the left ventricle.

Question 19

Why does the atrial pressure rise gradually?

Answer 19

As the blood is continuously flowing into the atria resulting in a gradual increase in pressure.

Question 20

When ventricular pressure falls slightly below that of the tract pressure, why does outward flow still occur?

Answer 20

This is due to the kinetic energy of the blood.

Question 21

What is the S4 sound?

Answer 21

This is ‘a stiff wall’. It is caused by vibration of the ventricular wall during atrial contraction.

Question 22

What is the S3 sound?

Answer 22

May represent tensing of the chordae tendineae and AV ring during ventricular relaxation and filling. (slosh-ing in)

Question 23

Why does the SA node have the highest rate of intrinsic rhythmicity?

Answer 23

It has the shortest action potential and refractory period.

Question 24

Explain the pacemaker action potential curve in the heart?

Answer 24

Starting at around -60mv there is a gradual rise to the threshold potential of -40mv. This is due to an influx of Na+ ions coming into the cell.
At the threshold potential, Calcium ion channels open (and diffuse into the cell) resulting in depolarisation and a steep increase to around +5mv.
At the peak, Calcium ion channels close and Potassium ion channel open resulting (and diffuse out of the cell) resulting in repolarisation of the membrane.
The cycle is then repeated at a constant rate resulting in a continuous heart beat.

Question 25

How does sympathetic stimulation affect the pacemaker potential?

Answer 25

Increases the number of HCN channels in the membrane so the membrane is more leaky to sodium ions so there is more frequent action potentials.

Question 26

What is vagal withdrawal?

Answer 26

This results in withdrawal of the parasympathetic nervous system so the sympathetic activity increases causing SA node to increase to about 100bpm.

Question 27

Why is there a plateau in the membrane potential of the ventricles?

Answer 27

This is due to both calcium ion and potassium ion channels being open so they cancel each other out resulting in a stable membrane potential. This allows for a longer cardiac muscle contraction time and gives time for cardiac muscle cells to depolarize fully.